O2 concentration caused an increase (P < 0.05) in dry mass per needle, tree height and the concentration of the monoterpene alpha-pinene, but there were no direct effects of CO2 concentration on any of the other chemical measurements made. High nutrient availability increased cellulase digestibility of pine needles. There was a significant negative effect of the OTCs on protein precipitating capacity of the needle extracts in comparison to the open-air controls. Results suggest that predicted changes in atmospheric CO2 concentration will be insufficient to produce large changes in the concentration of condensed tannins and monoterpenes in Scots pine. Processes which are influenced by these compounds, such as decomposition and herbivore food selection; along with their effects on ecosystem functioning, are therefore unlikely to be directly affected through changes in these secondary metabolites.
tance in ecological interactions between pine and other organisms including herbivores and decomposers. Elevated C2032^2^Hodge,A^Millard,P^1998^1^Effect of elevated CO2 on carbon partitioning and exudate release from Plantago lanceolata seedlings^37^103^2^280-286^^^^^Jun^^^^^6830
1146^1334^1803^2364^2508^344^374^376^537^92^chemical measurements made. High nutrient availability increased cellulase digestibility of pine needles. There was a significant negative effect of the OTCs on protein precipitating capacity of the needle extracts in comparison to the open-air controls. Results suggest that predicted changes in atmospheric CO2 concentration will be insufficient to produce large changes in the concentration of condensed tannins and monoterpenes in Scots pine. Processes which are influenced by these compounds, such as decomposition and herbivore food selection; along with their effects on ecosystem functioning, are therefore unlikely to be directly affected through changes in these secondary metabolites.
tance in ecological interactions between pine and other organisms including herbivores and decomposers. Elevated CA^6829^Plantago lanceolata L. seedlings were grown in sand microcosm units over a 43-day experimental period under two CO2 regimes (800 or 400 mu mol mol(-1)) to investigate the effect of elevated atmospheric CO2 concentration on carbon partitioning and exudate release. Total organic carbon (TOC) content of the collected exudate material was measured throughout the experimental period. After 42 days growth the seedlings were labelled with [C-14]-CO2 and the fate of the label within the plant and its release by the roots monitored. Elevated CO2 significantly (P less than or equal to 0.001) enhanced shoot, root and total dry matter production although the R:S ratio was unaltered, suggesting no alteration in press carbon partitioning. The cumulative release of TOC (in mg C) over 0-42 days was unaltered by CO2 treatment however, when expressed as a percentage of net assimilated C, ambient-grown plants released a significantly (P less than or equal to 0.001) higher percentage from their roots compared to elevated CO2-grown plants (i.e. 8 vs 3%). The distribution of C-14-label was markedly altered by CO2 treatment with significantly (P less than or equal to 0.001) greater per cent label partitioned to the roots under elevated CO2. This indicates increased partitioning of recent assimilate belowground under elevated CO2 treatment although there was no significant difference in the percentage of C-14-label released by the roots. Comparison of plant C budgets based on C-14-pulse-chase methodology and TOC measurements is discussed.

2033^2^Jongen,M^Jones,MB^1998^1^Effects of elevated carbon dioxide on plant biomass production and competition in a simulated neutral grassland community^52^82^1^111-123^^^^^Jul^^^^^6832
1065^1684^1960^3107^344^349^436^506^507^92^release of TOC (in mg C) over 0-42 days was unaltered by CO2 treatment however, when expressed as a percentage of net assimilated C, ambient-grown plants released a significantly (P less than or equal to 0.001) higher percentage from their roots compared to elevatedA^6831^Using open-top chambers, four prominent species (Lolium perenne, Cynosurus cristatus, Holcus lanatus and Agrostis capillaris) of Irish neutral grasslands were grown at ambient and elevated (700 mu mol mol(-1)) atmospheric CO2 for a period of 8 months. The effects of interspecific competition on plant responses to CO2 enrichment were investigated by growing the species in a four-species mixture. The results indicate that the species differ in their ability to respond to elevated CO2. CO2-enrichment had the largest effect on the biomass production of H. lanatus, but substantial stimulations in biomass production were also found for the other three species. The CO2-stimulation of biomass production for H. lanatus was accompanied by increased tillering. In addition, reductions in specific leaf area were found for all species. Exposure to elevated CO2 increased the community biomass of the four- species mixture. This increase can be mainly attributed to a significant increase in the biomass of H. lanatus at elevated CO2. No statistically-significant changes in species composition of community biomass were found. However, H. lanatus did increase its share of community biomass at each of the harvests, with the other three species, mainly L. perenne, suffering losses in their shares at elevated CO2. The results show that: (1) the species varied in their response to elevated CO2; and (2) species composition in natural plant communities is likely to change at elevated CO2, but these changes may occur rather slowly. Much longer periods of exposure to elevated atmospheric CO2 may be required to permit detection of significant changes in species composition. (C) 1998 Annals of Botany Company.

2034^3^Knapp,AK^Conard,SL^Blair,JM^1998^1^Determinants of soil CO2 flux from a sub-humid grassland: Effect of fire and fire history^56^8^3^760-770^^^^^Aug^^^^^6834
1445^2512^2917^310^3108^3109^372^427^668^91^e four- species mixture. This increase can be mainly attributed to a significant increase in the biomass of H. lanatus aA^6833^Soil CO2 flux (J(CO2)) was measured at midday over a 2-yr period in undisturbed tallgrass prairie (Konza Prairie, Kansas, USA) to quantify seasonal and annual budgets, to evaluate temperature and moisture as determinants of soil CO2 flux, and to assess the effect of a common land management tool, spring fire, and fire history on soil respiration. We hypothesized that: (1) maximum rates and annual estimates of soil J(CO2) would be greater in more productive burned sites than in unburned sites, (2) soil J(CO2) would be greater in newly burned sites with a history of fire exclusion than in annually burned sites (consistent with differences in aboveground production), and (3) soil temperature and water availability would be primary abiotic determinants of soil J(CO2) in tallgrass prairie. A preliminary assessment of the effects of large herbivores on soil J(CO2) was included to evaluate the hypothesis that removal of aboveground biomass would reduce soil J(CO2). Results indicated that spring fire increased maximum monthly soil J(CO2) by 20-55% relative to unburned tallgrass prairie, with greatest monthly differences measured in April (fourfold higher in burned sites). In burned sites that differed in fire history, maximum monthly J(CO2) in annually burned prairie was 33% greater than in burned sites with a history of fire exclusion. Soil J(CO2) in these latter sites was still significantly higher than in unburned sites. Soil J(CO2) in sites grazed by bison was reduced by as much as 30% relative to adjacent ungrazed areas. Reduced root biomass and activity in grazed areas, unburned sites, and sites with a history of fire exclusion suggest that plants play a major role in determining soil J(CO2) in this grassland. Soil temperature at 5 cm was related strongly to midday J(CO2) in both annually burned sites (r(2) = 0.58) and unburned sites (r(2) = 0.71). In contrast, differences in soil moisture among sites, enhanced by comparing irrigated grassland to control areas, increased maximum monthly J(CO2) by only 8%. Thus, soil temperature was the primary abiotic determinant of soil J(CO2) during this study. Maximum monthly estimates of soil J(CO2) in tallgrass prairie ranged from 10.3 mu mol CO2 . m(-2) . s(-1) in unburned sites to 15.1 mu mol . m(-2) . s(-1) in annually burned irrigated sites, whereas annual estimates varied from 4.7 to 7.8 kg CO2/m(2). Over the 2-yr period, spring fire increased estimated annual soil J(CO2) by 38-51% relative to unburned sites, while irrigation increased annual soil J(CO2) by 13%. These estimates for tallgrass prairie are much higher than those reported for most temperate ecosystems but are similar to estimates for tropical forests. Characteristics of undisturbed tallgrass prairie that may lead to high levels of soil J(CO2) include: high above- and belowground productivity; a relatively high proportion of C stored belowground; levels of soil microbial biomass and activity that are among the highest in native ecosystems in the United States; and the lack of a single dominant factor such as temperature, moisture, or nutrient availability, that consistently limits biotic processes during the growing season. The sensitivity of soil J(CO2) in tallgrass prairie to different land use practices (fire and grazing) suggests that it is critical to include these factors in the development of grassland C budgets, as well as in regional models that estimate biogeochemical responses to land use change.

2035^6^Lutze,JL^Roden,JS^Holly,CJ^Wolfe,J^Egerton,JJG^Ball,MC^1998^1^Elevated atmospheric [CO2] promotes frost damage in evergreen tree seedlings^9^21^6^631-635^^^^^Jun^^^^^6836
130^perate ecosystems but are similar to estimates for tropical forests. Characteristics of undisturbed tallgrass prairie that may lead to high levels of soil J(CO2) include: high above- and belowground productivity; a relatively high proportion of C stored belowground; levels of soil microbial biomass and activity that are among the highest in native ecosystems in the United States; and the lack of a single dominant factor sucA^6835^Growth under elevated [CO2] promoted spring frost damage in field grown seedlings of snow gum (Eucalyptus pauciflora Sieb, ex Spreng,), one of the most frost tolerant of eucalypts, Freezing began in the leaf midvein, consistent with it being a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO2] (- 5.7 degrees C versus - 4.3 degrees C), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO2 fertilization and may affect predictions of vegetation change based on increasing temperature.
latively high proportion of C stored belowground; levels of soil microbial biomass and activity that are among the highest in native ecosystems in the United States; and the lack of a single dominant factor suc2036^4^Moya,TB^Ziska,LH^Namuco,OS^Olszyk,D^1998^1^Growth dynamics and genotypic variation in tropical, field- grown paddy rice (Oryza sativa L.) in response to increasing carbon dioxide and temperature^127^4^6^645-656^^^^^Aug^^^^^6838
349^434^eing a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO2] (- 5.7 degrees C versus - 4.3 degrees C), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO2 fertilization and may affect predictions of vegetation change based on increasing temperature.
latively high proportion of C stored belowground; levels of soil microbial biomass and activity that are among the highest in native ecosystems in the United States; and the lack of a single dominant factor sucA^6837^While previous studies have examined the growth and yield response of rice to continued increases in CO2 concentration and potential increases in air temperature, little work has focused on the long-term response of tropical paddy rice (i.e. the bulk of world rice production) in situ, or genotypic differences among cultivars in response to increasing CO2 and/or temperature. At the International Rice Research Institute, rice (cv IR72) was grown from germination until maturity for 4 field seasons, the 1994 and 1995 wet and the 1995 and 1996 dry seasons at three different CO2 concentrations (ambient, ambient + 200 and ambient + 300 mu L L-1 CO2) and two air temperatures (ambient and ambient + 4 degrees C) using open-top field chambers placed within a paddy site. Overall, enhanced levels of CO2 alone resulted in significant increases in total biomass at maturity and increased seed yield with the relative degree of enhancement consistent over growing seasons across both temperatures. Enhanced levels of temperature alone resulted in decreases or no change in total biomass and decreased seed yield at maturity across both CO2 levels. In general, simultaneous increases in air temperature as well as CO2 concentration offset the stimulation of biomass and grain yield compared to the effect of CO2 concentration alone. For either the 1995 wet and 1996 dry seasons, additional cultivars (N-22, NPT1 and NPT2) were grown in conjunction with IR72 at the same CO2 and temperature treatments. Among the cultivars tested, N-22 showed the greatest relative response of both yield and biomass to increasing CO2, while NPT2 showed no response and IR72 was intermediate. For all cultivars, however, the combination of increasing CO2 concentration and air temperature resulted in reduced grain yield and declining harvest index compared to increased CO2 alone. Data from these experiments indicate that (a) rice growth and yield can respond positively under tropical paddy conditions to elevated CO2, but that simultaneous exposure to elevated temperature may negate the CO2 response to grain yield; and, (b) sufficient intraspecific variation exists among cultivars for future selection of rice cultivars which may, potentially, convert greater amounts of CO2 into harvestable yield.

2037^2^Neilson,RP^Drapek,RJ^1998^1^Potentially complex biosphere responses to transient global warming^127^4^5^505-521^^^^^Jun^^^^^6840
1234^1637^174^3110^3111^3112^314^55^633^673^ with IR72 at the same CO2 and temperature treatments. Among the cultivars tested, N-22 showed the greatest relative response of both yield and biomass to increasing CO2, while NPT2 showed no response and IR72 was intermediate. For all cultivars, however, the combination of increasing CO2 concentration and air temperature resulted in reduced grain yield and declining harvest index compared to increased CO2 alone. Data from these experiments indicate that (a) rice growth and yield can respond positively under tropical paddy conditions to elevated CO2, but that simultaneous exposure to elevatA^6839^Feedback interactions between terrestrial vegetation and climate could alter predictions of the responses of both systems to a doubling of atmospheric CO2. Most previous analyses of biosphere responses to global warming have used output from equilibrium simulations of current and future climate, as compared to more recently available transient GCM simulations. We compared the vegetation responses to these two different classes of GCM simulation (equilibrium and transient) using an equilibrium vegetation distribution model, MAPSS. Average climatologies were extracted from the transient GCM simulations for current and doubled (2 x) CO2 concentrations (taken to be 2070-2099) for use by the equilibrium vegetation model. However, the 2 x CO2 climates extracted from the transient GCM simulations were not in equilibrium, having attained only about 65% of their eventual 2 x CO2 equilibrium temperature change. Most of the differences in global vegetation response appeared to be related to a very different simulated change in the pole to tropic temperature gradient. Also, the transient scenarios produced much larger increases of precipitation in temperate latitudes, commensurate with a minimum in the latitudinal temperature change. Thus, the (equilibrium) global vegetation response, under the transient scenarios, tends more to a greening than a decline in vegetation density, as often previously simulated. It may be that much of the world could become greener during the early phases of global warming, only to reverse in later, more equilibrial stages. However, whether or not the world's vegetation experiences large drought-induced declines or perhaps large vegetation expansions in early stages could be determined by the degree to which elevated CO2 will actually benefit natural vegetation, an issue still under debate. There may occur oscillations, perhaps on long timescales, between greener and drier phases, due to different frequency responses of the coupled ocean-atmosphere-biosphere interactions. Such oscillations would likely, of themselves, impart further reverberations to the coupled Earth System.

2038^3^Peisker,M^Heinemann,I^Pfeffer,M^1998^1^A study on the relationship between leaf conductance, CO2 concentration and carboxylation rate in various species^91^56^1^35-43^^^^^Apr^^^^^6842
1754^1918^243^256^348^372^374^384^665^923^reening than a decline in vegetation density, as often previously simulated. It may be that much of the world could become greener during the early phases of global warming, only to reverse in later, more equilibrial stages. However, whether or not the world's vegetation experiences large drought-induced declines or perhaps large vegetation expansions in early stages could be determined by the degree to which elevated CO2 will actually benefit natural vegetation, an issue still under debate. There may occur oscillations, perhaps on long timescales, between greener and drier phases, due to different frequency responses of the coupled ocean-atmosphere-biosphere interactions. Such oscillatioA^6841^Leaf conductance g(L) is strongly influenced by environmental factors like CO2, irradiance and air humidity. According to Ball et al. (1987), g(L) is correlated with an index calculated as the product of net CO2 exchange rate A and ambient water vapour concentration W-a, divided by ambient CO2 concentration c(a). However, this empirical model does not apply to high values of g(L) observed at c(a) below CO2 compensation concentration Gamma. Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P-1 and P-2, were determined by adding to A the quotient of Gamma and the sum of gas phase resistance r(g) and intracellular resistance for CO2 exchange r(i), P-1 = A+Gamma/(r(g) + r(i)). or the quotient of Gamma and r(i), P-2 = A + Gamma/r(i). If P-2 is chosen, c(a) in the Ball index has to be replaced by the intercellular CO2 concentration c(i). By using the modified indices P-1.W-a/c(a) and P-2.W-a/c(i), we analysed data from the C-3 species Nicotiana tabacum and Nicotiana plumbaginifolia, the C-3-C-4 intermediate species Diplotaxis tenuifolia, and the C-4 species Zea mays. The data were collected at widely varying levels of irradiance and CO2 concentration. For all species uniform relationships between g(L) and the new indices were found for the whole range of CO2 concentrations below and above Gamma. Correlations between g(L) and P-1.W-a/c(a) were closer than those between g(L) and P-2.W- a/c(i) because P-1/c(a) implicitly contains g(L). Highly significant correlations were also obtained for the relationships between g(L) and the ratios P-1/c(a) and P- 2/c(i).

2039^1^Poorter,H^1998^1^Do slow-growing species and nutrient-stressed plants respond relatively strongly to elevated CO2?^127^4^6^693-697^^^^^Aug^^^^^6844
224^229^243^341^344^361^437^57^682^975^chosen, c(a) in the Ball index has to be replaced by the intercellular CO2 concentration c(i). By using the modified indices P-1.W-a/c(a) and P-2.W-a/c(i), we analysed data from the C-3 species NA^6843^Mainly based on a simulation model, Lloyd & Farquhar (1996; Functional Ecology, 10, 4-32) predict that inherently slow- growing species and nutrient-stressed plants show a relatively strong growth response to an increased atmospheric CO2 concentration. Compiling published experiments, I conclude that these predictions are not supported by the available data. On average, inherently fast-growing species are stimulated proportionately more in biomass than slow-growing species and plants grown at a high nutrient supply respond more strongly than nutrient-stressed plants.

2040^6^Prior,SA^Torbert,HA^Runion,GB^Mullins,GL^Rogers,HH^Mauney,JR^1998^1^Effects of carbon dioxide enrichment on cotton nutrient dynamics^166^21^7^1407-1426^^^^^^^^^^6846
1334^361^374^434^456^685^881^977^24^229^243^341^344^361^437^57^682^975^chosen, c(a) in the Ball index has to be replaced by the intercellular CO2 concentration c(i). By using the modified indices P-1.W-a/c(a) and P-2.W-a/c(i), we analysed data from the C-3 species NA^6845^The rise in atmospheric carbon dioxide (CO2) concentration is predicted to have positive effects on agro-ecosystem productivity. However, an area which requires further study centers on nutrient dynamics of crops grown under elevated CO2 in the field. In 1989 and 1990, cotton [Gossypium hirsutum (L.) Deltapine 77'] was grown under two CO2 levels [370 mu mol mol(- 1)=ambient and 550 mu mol mol(-1)=free-air CO2 enrichment(FACE)]. At physiological maturity, nutrient concentration and content of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) were determined for whole plant and individual plant organs. While the effects of added CO2 on whole plant nutrient concentrations and contents were consistent, some differences among plant organs were observed between years. FACE often decreased tissue nutrient concentration, but increased total nutrient accumulation. Results indicate that under elevated CO2, field grown cotton was more nutrient efficient in terms of nutrient retrieval from the soil and nutrient utilization in the plant. This implies more efficient fertilizer utilization, better economic return for fertilizer expenditures, and reduced environmental impact from agricultural fertilization practices in the future.

2041^1^Rosenthal,Y^1998^1^Variations of ecosystem gas exchange in the rain forest mesocosm at Biosphere 2 in response to elevated CO2^127^4^5^539-547^^^^^Jun^^^^^6848
312^314^360^entration and content of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) were determined for whole plant and individual plant organs. While the effects of added CO2 on whole plant nutrient concentrations and contents were consistent, some differences among plant organs were observed between years. FACE often decreased tissue nutrient concentration, but increased total nutrient accumulation. Results indicate that under elevated CO2, field grown cotton was moreA^6847^The effects of elevated CO2 on tropical ecosystems were studied in the artificial rain forest mesocosm at Biosphere 2, a large- scale and ecologically diverse experimental facility located in Oracle, Arizona. The ecosystem responses were assessed by comparing the whole-system net gas exchange (NEE) upon changing CO2 levels from 900 to 450 ppmV. The day-NEE was significantly higher in the elevated CO2 treatment. In both experiments, the NEE rates were similar to values observed in natural analogue systems. Variations in night-NEE, reflecting both soil CO2 efflux and plants respiration, covaried with temperature but showed no clear correlation with atmospheric CO2 levels. After correcting for changes in CO2 efflux we show that the rain forest net photosynthesis increased in response to increasing atmospheric CO2. The photosynthetic enhancement was expressed in higher quantum yields, maximum assimilation rates and radiation use efficiency. The results suggest that photosynthesis in large tropical trees is CO2 sensitive, at least following short exposures of days to weeks. Taken at face value, the data suggest that as a result of anthropogenic emissions of CO2, tropical rain forests may shift out of steady state, and become a carbon sink at least for short periods. However, a better understanding of the unique conditions and phenomena in Biosphere 2 is necessary before these results are broadly useful.

2042^4^Saccardy,K^Pineau,B^Roche,O^Cornic,G^1998^1^Photochemical efficiency of Photosystem II and xanthophyll cycle components in Zea mays leaves exposed to water stress and high light^91^56^1^57-66^^^^^Apr^^^^^6850
164^2092^2885^3113^3114^3115^3116^3117^3118^713^c CO2 levels. After correcting for changes in CO2 efflux we show that the rain forest net photosynthesis increased in response to increasing atmospheric CO2. The photosynthetic enhancement was expressed in higher quantum yields, maximum assimilation rates and radiation use efficiency. The results suggest that photosynthesis in large tropical trees iA^6849^The effects of two light treatments (photosynthetically active photon flux density of either 650 or 1950 mu mol m(-2) s(-1)) on the photochemical efficiency of Photosystem II (PS II) (measured as variable to maximum fluorescence ratio) and on the xanthophyll cycle components was studied in wilted Zea mays leaves. For comparison, these parameters were followed under the same light conditions in well-hydrated leaves maintained either in normal or CO2-free air. The net CO2 assimilation of dehydrated leaves declined rapidly as their relative water content (RWC) decreased from 100 to 60% while the PS II efficiency measured after a prolonged dark period of 16 h declined only when RWC leaves was lower than 60%. Furthermore, drought caused an increase in the pool size of the xanthophyll cycle pigments and the presence of a sustained elevated level of zeaxanthin and antheraxanthin at the end of the long dark period. The leaf water deficit enhanced the sensitivity of PS II efficiency to light exposure. During illumination, strong inhibition of PS II efficiency and large violaxanthin deepoxidation was observed in wilted leaves even under moderate photon flux density compared to control leaves in the same conditions. After 2 h of darkness following the light treatment, the PS II efficiency that is dependent on the previous PPFD, decreased with leaf water deficit. Moreover, zeaxanthin epoxidation led to an accumulation of antheraxanthin in dehydrated leaves, All these drought effects on PS II efficiency and xanthophyll cycle components were also obtained in well-hydrated leaves by short-term CO2 deprivation during illumination. We conclude that the increased susceptibility of PS II efficiency to light in wilted maize leaves is mainly explained by the decrease of CO2 availability and the resulting low net CO2 assimilation.

2043^3^Saxe,H^Ellsworth,DS^Heath,J^1998^1^Tree and forest functioning in an enriched CO2 atmosphere^84^139^3^395-436^^^^^Jul^^^^^6852
1344^1633^3119^3120^3121^3122^3123^343^398^861^osure. During iA^6851^Forests exchange large amounts of CO2 with the atmosphere and can influence and be influenced by atmospheric CO2. There has been a recent proliferation of literature on the effects of atmospheric CO2 on forest trees. More than 300 studies of trees on five different continents have been published in the last five years. These include an increasing number of field studies with a long-term focus and involving CO2 x stress or environment interactions. The recent data on long-term effects of elevated atmospheric CO2 on trees indicate a potential for a persistent enhancement of tree growth for several years, although the only relevant long-term datasets currently available are for juvenile trees. The current literature indicates a significantly larger average long-term biomass increment under elevated CO2 for conifers (130%) than for deciduous trees (49%) in studies not involving stress components. However, stimulation of photosynthesis by elevated CO2 in long-term studies was similar for conifers (62 %) and deciduous trees (53 %). Recent studies indicate that elevated CO2 causes a more persistent stimulation of biomass increment and photosynthesis than previously expected. Results of seedling studies, however, might not be applicable to other stages of tree development because of complications of age- dependent and size-dependent shifts in physiology and carbon allocation, which are accelerated by elevated CO2. In addition, there are many possible avenues to down-regulation, making the predicted canopy CO2 exchange and growth of mature trees and forests in a CO2-rich atmosphere uncertain. Although, physiological down-regulation of photosynthetic rates has been documented in field situations, it is rarely large enough to offset entirely photosynthetic gains in elevated CO2. A persistent growth stimulation of individual mature trees has been demonstrated although this effect is more uncertain in trees in natural stands. Resource interactions can both constrain tree responses to elevated CO2 and be altered by them. Although drought can reduce gas-exchange rates and offset the benefits of elevated CO2, even in well watered trees, stomatal conductance is remarkably less responsive to elevated CO2 than in herbaceous species. Stomata of a number of tree species have been demonstrated to be unresponsive to elevated CO2. We conclude that positive effects of CO2 on leaf area can be at least as important in determining canopy transpiration as negative, direct effects of CO2 on stomatal aperture. With respect to nutrition, elevated CO2 has the potential to alter tree-soil interactions that might influence future changes in ecosystem productivity. There is continued evidence that in most cases nutrient limitations diminish growth and photosynthetic responses to elevated CO2 at least to some degree, and that elevated CO2 can accelerate the appearance of nutrient limitations with increasing time of treatment. In many studies, tree biomass responses to CO2 are artefacts in the sense that they are merely responses to CO2-induced changes in internal nutritional status of the tree. There are numerous interactions between CO2 and factors of the biotic and abiotic environment. The importance of increasing atmospheric CO2 concentrations for productivity is likely to be overestimated if these are not taken into account. Many interactions, however, are simply additive rather than synergistic or antagonistic. This appears to hold true for many parameters under elevated CO2 in combination with temperature, elevated O- 3, and other atmospheric pollutants. However, there is currently little evidence that elevated CO2 will counteract O-3 damage. When the foliage content of C, mineral nutrients and secondary metabolites is altered by elevated CO2, tree x insect interactions are modified. In most trees, mycorrhizal interactions might be less important for direct effects of CO2 than for alleviating general nutrient deficiencies. Since many responses to elevated CO2 and their interactions with stress show considerable variability among species/genotypes, one principal research need is for comparative studies of a large variety of woody species and ecosystems under realistic conditions. We still need more long-term experiments on mature trees and stands to address critical scaling issues likely to advance our understanding of responses to elevated CO2 at different stages of forest development and their interactions with climate and environment. The only tools available at present for coping with the consequences of rising CO2 are management of resources and selection of genotypes suitable for the future climate and environment.

2044^3^Serraj,R^Sinclair,TR^Allen,LH^1998^1^Soybean nodulation and N-2 fixation response to drought under carbon dioxide enrichment^9^21^5^491-500^^^^^May^^^^^6854
1530^2843^3124^3125^3126^384^386^506^859^92^might be less important for direct effects of CO2 than for alleviating general nutrient deficiencies. Since many responses to elevated CO2 and their interactions with stress show considerable variability among species/genA^6853^The combined effects of carbon dioxide (CO2) enrichment and water deficits on nodulation and N-2 fixation were analysed in soybean [Glycine max (L.) Merr.]. Two shortterm experiments were conducted in greenhouses with plants subjected to soil drying, while exposed to CO2 atmospheres of either 360 or 700 mu mol CO2 mol(-1). Under drought-stressed conditions, elevated [CO2] resulted in a delay in the decrease in Nz fixation rates associated,vith drying of the soil used in these experiments. The elevated [CO2] also allowed the plants under drought to sustain significant increases in nodule number and mass relative to those under ambient [CO2], The total non-structural carbohydrate (TNC) concentration was lower in the shoots of the plants exposed to drought; however, plants under elevated CO2 had much higher TNC levels than those under ambient CO2. For both [CO2] treatments, drought stress induced a substantial accumulation of TNC in the nodules that paralleled N-2 fixation decline, which indicates that nodule activity under drought may not be carbon limited. Under drought stress, ureide concentration increased in all plant tissues. However, exposure to elevated [CO2] resulted in substantially less drought- induced ureide accumulation in leaf and petiole tissues. A strong negative correlation was found between ureide accumulation and TNC levels in the leaves. This relationship, together with the large effect of elevated [CO2] on the decrease of ureide accumulation in the leaves, indicated the importance of ureide breakdown in the response of N-2 fixation to drought and of feedback inhibition by ureides on nodule activity. It is concluded that an important effect of CO2 enrichment on soybean under drought conditions is an enhancement of photoassimilation, an increased partitioning of carbon to nodules and a decrease of leaf ureide levels, which is associated with sustained nodule growth and N-2 rates under soil water deficits. We suggest that future [CO2] increases are likely to benefit soybean production by increasing the drought tolerance of N-2 fixation.

2045^1^Sicher,RC^1998^1^Yellowing and photosynthetic decline of barley primary leaves in response to atmospheric CO2 enrichment^37^103^2^193-200^^^^^Jun^^^^^6856
1672^2423^341^343^385^409^434^493^618^92^tissues. A strong negative correlation was found between ureide accumulation and TNC levels in the leaves. This relationship, together with the large effect of elevated [CO2] on the decrease of ureide accumulation in the leaves, indicated the importance of ureide breakdown in the response of N-2 fixation to drought and of feedback inhibition by ureides on nodule activity. It is concluded that an important effect of CO2 enrichment on soybean under drought conditions is an enhancement of photoassimilation, an increased partitioning of carbon to nodules and a decrease of leaf ureide levels, which is associated with sustained nodule growth and N-2 rates under soil water deficits. We suggest that future [CO2] increases are likely to benefit soybean production by A^6855^The photosynthetic response of barley (Hordeum vulgare L. cv. Brant) primary leaves was studied as a function of chlorosis induced by CO2 enrichment. Leaf yellowing; measured as changes of chlorophyll a and b, was more extensive in controlled environments at elevated (680 +/- 17 mu l l(-1)) than at ambient (380 +/- 21 mu l l(-1)) CO2. Stomatal conductance of primary leaves was decreased by growth in elevated CO2 between 11 and 18 days after sowing (DAS) when measured at both 380 and 680 mu l l(-1) CO2. Internal leaf CO2 concentration (C-i) was also lower for elevated- compared to ambient-CO2-grown primary leaves between 11 and 14 DAS. Results suggest that non-stomatal factors were responsible for the decreased photosynthetic rates of elevated- compared to ambient-CO2-grown primary leaves 18 DAS. Various photochemical measurements, including quantum absorptance (alpha), minimal (F-0), maximal (F-m), and variable (F-v) chlorophyll fluorescence, as well as the F-v/F-m ratio, were significantly decreased 18 DAS in the elevated- compared to ambient-CO2 treatment. Photochemical (q(P)) and nonphotochemical (q(N)) chlorophyll fluorescence quenching: coefficients of 18-day-old primary leaves did not differ between CO2 treatments. Photosynthetic electron transport rates of photosystem II were slightly lower for elevated- compared to ambient-CO2-grown primary leaves 18 DAS. Concentrations of alpha-amino N (i.e. free amino acids) in barley primary leaves were increased by CO2 enrichment 10 DAS, but subsequently, alpha-amino N decreased in association with photosynthetic decline. Total acid protease activity was greater in elevated- than in ambient-CO2-grown leaves 18 DAS. The above findings suggest that photoinhibition and premature senescence were factors in the CO2-dependent yellowing of barley primary leaves.

2046^3^Staddon,PL^Graves,JD^Fitter,AH^1998^1^Effect of enhanced atmospheric CO2 on mycorrhizal colonization by Glomus mosseae in Plantago lanceolata and Trifolium repens^84^139^3^571-580^^^^^Jul^^^^^6858d 1096^137^2137^372^374^376^392^419^474^92^t-CO2 treatment. Photochemical (q(P)) and nonphotochemical (q(N)) chlorophyll fluorescence quenching: coefficients of 18-day-old primary leaves did not differ between CO2 treatments. Photosynthetic electron transport rates of photosystem II were slightly lower for elevated- compared to ambient-CO2-grown primary leaves 18 DAS. Concentrations of alpha-amino N (i.e. free amino acids) in barley primary leaves were increased by CO2 enrichment 10 DAS, but subsequently, alpha-amino N decreased in association with photosynthetic decline. Total acid protease activity was greater in elevated- than in ambient-CO2-grown leaves 18 DAS. The above findings suggest that photoinhibition and premature senescence were factors in the CO2-dependent yellowing of barley primary leaves.

2046^3^Staddon,PL^Graves,JD^Fitter,AH^1998^1^Effect of enhanced atmospheric CO2 on mycorrhizal colonization by Glomus mosseae in Plantago lanceolata and Trifolium repens^84^139^3^571-580^^^^^Jul^^^^^6858d A^6857^Plantage lanceolata L. and Trifolium repens L. were grown for 16 wk in ambient (360 mu mol mol(-1)) and elevated (610 mu mol mol(-1)) atmospheric CO2. Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and given a phosphorus supply in the form of bonemeal, which would not be immediately available to the plants. Seven sequential harvests were taken to determine whether the effect of elevated CO2 on mycorrhizal colonization was independent of the effect of CO2 on plant growth. Plant growth analysis showed that both species grew faster in elevated CO2 and that P. lanceolata had increased carbon allocation towards the roots. Elevated CO, did not affect the percentage of root length colonized (RLC); although total colonized root length was greater, when plant size was taken into account this effect disappeared. This finding was also true for root length colonized by arbuscules. No CO2 effect was found on hyphal density (colonization intensity) in roots. The P content of plants was increased at elevated CO2, although both shoot and root tissue P concentration were unchanged. This was again as a result of bigger plants at elevated CO2. Phosphorus inflow was unaffected by CO2 concentrations. It is concluded that there is no direct permanent effect of elevated CO2, on mycorrhizal functioning, as internal mycorrhizal development and the mycorrhizal P uptake mechanism are unaffected. The importance of sequential harvests in experiments is discussed. The direction for future research is highlighted, especially in relation to C storage in the soil.

2047^4^Tognetti,R^Longobucco,A^Miglietta,F^Raschi,A^1998^1^Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring^9^21^6^613-622^^^^^Jun^^^^^6860
1285^2712^3127^3128^3129^343^344^372^384^91^is effect disappeared. This finding was also true for root length colonized by arbuscules. No CO2 effect was found on hyphal density (colonization intensiA^6859^Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994), We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994, At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured, A distinct environmental gradient exists between the two sites with higher atmospheric CO2 concentrations in the proximity of a natural CO, spring. Trees at the spring site have been growing for generations in elevated atmospheric CO2 concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole- plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid- summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid 'runaway embolism'. Trees at the CO2 spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO2 was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean land diurnal) sap flux were consistently higher in the control site trees than in the CO2 spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area, Larger trees displayed smaller differences than smaller trees, between the control and the CO2 spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO2 spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross- sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO2 on plant water use at the organ and whole-tree scale.

2048^2^Winter,K^Virgo,A^1998^1^Elevated CO2 enhances growth in the rain forest understory plant, Piper cordulatum, at extremely low light intensities^54^193^3^323-326^^^^^Jul^^^^^6862
2297^312^344^374^376^389^494^713^ site trees than in the CO2 spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area, Larger trees displayed smaller differences than smaller trees, between the control and thA^6861^Seedlings of the rain forest understory shrub Piper cordulatum were grown for several months at ambient and elevated concentrations of atmospheric CO2. Photon flux density (PFD) during 12-h photoperiods was maintained at extremely low levels (< 10 mu mol m(-2) s(-1)). Despite these low PFDs, dry matter accumulation and leaf area production were increased in air containing elevated (approximately twice-ambient) CO2 concentrations compared to ambient air. In leaves that had developed in deep shade and at ambient CO2, rates of net CO2 uptake, measured at 6 mu mol photons m(-2)s(-1), were 30% higher at elevated than at ambient CO2. Rates of net CO2 loss in the dark were 10% lower at elevated than at ambient CO2.

2049^3^Zanetti,S^Hartwig,UA^Nosberger,J^1998^1^Elevated atmospheric CO2 does not affect per se the preference for symbiotic nitrogen as opposed to mineral nitrogen of Trifolium repens L^9^21^6^623-630^^^^^Jun^^^^^6864
1292^1751^3065^3130^346^376^417^427^704^92^r trees, between the control and thA^6863^The objective of this investigation was to examine the effect of an elevated atmospheric CO2 partial pressure (pCO(2)) on the N-sink strength and performance of symbiotic N-2 fixation in Trifolium repens L. cv. Milkanova, After initial growth under ambient pCO(2) in a nitrogen-free nutrient solution, T. repens in the exponential growth stage was exposed to ambient and elevated pCO(2) (35 and 60 Pa) and two levels of mineral N (N- free and 7.5 mol m(-3) N) for 36 d in single pots filled with silica sand in growth chambers. Elevated pCO(2) evoked a significant increase in biomass production from day 12 after the start of CO2 enrichment. For plants supplied with 7.5 mol m(-3) N, the relative contribution of symbiotically fixed N (%N-sym) as opposed to N assimilated from mineral sources (N- 15-isotope-dilution method), dropped to 40%, However, in the presence of this high level of mineral N, %N-sym was unaffected by atmospheric pCO(2) over the entire experimental period. In plants fully dependent on N-2 fixation, the increase in N yield reflects a stimulation of symbiotic N-2 fixation that was the result of the formation of more nodules rather than of higher specific N-2 fixation. These results are discussed with regard to physiological processes governing symbiotic N-2 fixation and to the response of symbiotic N-2 fixation to elevated pCO(2) in field-grown T. repens.

2050^10^Ziska,LH^Moya,TB^Wassmann,R^Namuco,OS^Lantin,RS^Aduna,JB^Abao,E^Bronson,KF^Neue,HU^Olszyk,D^1998^1^Long-term growth at elevated carbon dioxide stimulates methane emission in tropical paddy rice^127^4^6^657-665^^^^^Aug^^^^^6866
1334^349^374^434^673^685^ CO2 enrichment. For plants supplied with 7.5 mol m(-3) N, the relative contribution of symbiotically fixed N (%N-sym) as opposed to N assimilated from mineral sources (N- 15-isotope-dilution method), dropped to 40%, However, in the presence of this high level of mineral N, %N-sym was unaffected by atmospheric pCO(2) over the entire experimental period. In plants fully dependent on N-2 fA^6865^Recent anthropogenic emissions of key atmospheric trace gases (e.g. CO2 and CH4) which absorb infra-red radiation may lead to an increase in mean surface temperatures and potential changes in climate. Although sources of each gas have been evaluated independently, little attention has focused on potential interactions between gases which could influence emission rates. In the current experiment, the effect of enhanced CO2 (300 mu L L-1 above ambient) and/or air temperature (4 degrees C above ambient) on methane generation and emission were determined for the irrigated tropical paddy rice system over 3 consecutive field seasons (1995 wet and dry seasons 1996 dry season). For all three seasons, elevated CO2 concentration resulted in a significant increase in dissolved soil methane relative to the ambient control. Consistent with the observed increases in soil methane, measurements of methane flux per unit surface area during the 1995 wet and 1996 dry seasons also showed a significant increase at elevated carbon dioxide concentration relative to the ambient CO2 condition (+49 and 60% for each season, respectively). Growth of rice at both increasing CO2 concentration and air temperature did not result in additional stimulation of either dissolved or emitted methane compared to growth at elevated CO2 alone. The observed increase in methane emissions were associated with a large, consistent, CO2-induced stimulation of root growth. Results from this experiment suggest that as atmospheric CO2 concentration increases, methane emissions from tropical paddy rice could increase above current projections.

2051^6^Tuba,Z^Jones,MB^Szente,K^Nagy,Z^Garvey,L^Baxter,R^9UNKNOWN YEAR^1^Some ecophysiological and production responses of grasslands to long-term elevated CO2 under continental and atlantic climates^357^^^241-250^
312^344^348^399^5^641^665^674^778^92^962^
2052^4^Allen,LH^Valle,RR^Jones,JW^Jones,PH^1998^1^Soybean leaf water potential responses to carbon dioxide and drought^48^90^3^375-383^^^^^May-Jun^^^^^6869evate1337^230^264^348^386^431^434^537^57^642^o the ambient CO2 condition (+49 and 60% for each season, respectively). Growth of rice at both increasing CO2 concentration and air temperature did not result in additional stimulation of either dissolved or emitted methane compared to growth at elevated CO2 alone. The observed increase in methane emissions were associated with a large, consistent, CO2-induced stimulation of root growth. Results from this experiment suggest that as atmospheric CO2 concentration increases, methane emissions from tropical paddy rice could increase above current projections.

2051^6^Tuba,Z^Jones,MB^Szente,K^Nagy,Z^Garvey,L^Baxter,R^9UNKNOWN YEAR^1^Some ecophysiological and production responses of grasslands to long-term elevated CO2 under continental and atlantic climates^357^^^241-250^
312^344^348^399^5^641^665^674^778^92^962^
2052^4^Allen,LH^Valle,RR^Jones,JW^Jones,PH^1998^1^Soybean leaf water potential responses to carbon dioxide and drought^48^90^3^375-383^^^^^May-Jun^^^^^6869evateA^6868^Rising CO2 can have direct effects on crop water relations and indirect effects on water available for growth. We studied the effects of elevated CO2 and drought on leaf water relations of soybean [Glycine max (L.) Merr. cv. Bragg] and considered the hypothesis of osmotic adjustment mediated by increased photosynthesis (Hypothesis 1) vs. the hypothesis of water conservation mediated by decreased stomatal conductance (Hypothesis 2) to explain improved water relations of plants growing under elevated CO2. In Exp. 1, soybean was grown at 330, 450, 660, and 800 mu mol mol(-1) CO2 in sunlit, closed- circulation, controlled-environment chambers under well-watered conditions. Leaf total water potential (WP), osmotic potential (OP), and turgor potential (TP) were measured at midday during V4 to R6 stages of development. In Exp. 2 (well-watered, R1-R3) and Exp. 3 (13-d drying cycle, R6 seed filling), soybean was grown at 330 and 660 mu mol mol(-1) CO2 and WP, OP, and TP were measured five times per day on sunlit and shaded leaves. In Exp. 3, stomatal conductance (g(s)) and transpiration rate (TR) of leaves were also measured. Experiments 1 and 2 showed that elevated CO2 increased TP and decreased OP, but did not affect leaf WP, thus favoring Hypothesis 1. In Exp. 3, leaf WP was higher in elevated than ambient CO2. Diurnal TP was higher in elevated than ambient CO2 at the beginning of drought, and was maintained longer each day as drought progressed. At the end of drought, TP and WP was higher in elevated than ambient CO2. Elevated CO2 leaves had lower TR because of lower g(s) than ambient CO2 counterparts. Thus, Exp. 3 supported Hypothesis 2, that both stressed and nonstressed plants in elevated CO2 have a better water status (e.g., higher TP) than plants in ambient CO2 due to water conservation mediated by decreased g(s). Remobilization of leaf nutrients during seed filling may limit the capability for osmotic adjustment. Regardless of the mechanisms, growth of plants in elevated CO2 should be less affected by drought than plants in ambient CO2.

2053^4^Arisi,ACM^Cornic,G^Jouanin,L^Foyer,CH^1998^1^Overexpression of iron superoxide dismutase in transformed poplar modifies the regulation of photosynthesis at low CO2 partial pressures or following exposure to the prooxidant herbicide methyl viologen^8^117^2^565-574^^^^^Jun^^^^^6871
1092^130^1569^1677^2369^2410^3131^440^493^713^ginning of drought, and was maintained longer each day as drought progressed. At the end of drought, TP and WP was higher in elevated than ambient CO2. Elevated CO2 leaves had lower TR because of lower g(s) than ambient CO2 counterparts. Thus, Exp. 3 supported Hypothesis 2, that both stressed and nonstressed plants in elevated CO2 have a better water status (e.g., higher TP) than plants in ambient CO2 due to water conservation mediated by decreased g(s). Remobilization of leaf nutrients during seed filling may limit the capability for osmotic adjustment. Regardless of the mechanisms, growth of plants in elevated CO2 should be less affected by dA^6870^Chloroplast-targeted overexpression of an Fe superoxide dismutase (SOD) from Arabidopsis thaliana resulted in substantially increased foliar SOD activities. Ascorbate peroxidase, glutathione reductase, and monodehydroascorbate reductase activities were similar in the leaves from all of the lines, but dehydroascorbate reductase activity was increased in the leaves of the FeSOD transformants relative to untransformed controls. Foliar H2O2, ascorbate, and glutathione contents were comparable in all lines of plants. Irradiance-dependent changes in net CO, assimilation and chlorophyll a fluorescence quenching parameters were similar in all lines both in air (21% O-2) and at low (1%) O-2. CO2-response curves for photosynthesis showed similar net CO2-exchange characteristics in all lines. In contrast, values of photochemical quenching declined in leaves from untransformed controls at intercellular CO2 (Ci) values below 200 mu L L-1 but remained constant with decreasing Ci in leaves of FeSOD transformants. When the O-2 concentration was decreased from 21 to 1%, the effect of FeSOD overexpression on photochemical quenching at limiting Ci was abolished. At high light (1000 mu mol m(-2) s(-1)) a progressive decrease in the ratio of variable (F-v) to maximal (F-m) fluorescence was observed with decreasing temperature. At 6 degrees C the high-light-induced decrease in the F-v/F-m ratio was partially prevented by low O-2 but values were comparable in all lines. Methyl viologen caused decreased F- v/F-m ratios, but this was less marked in the FeSOD transformants than in the untransformed controls. These observations suggest that the rate of superoxide dismutation limits flux through the Mehler-peroxidase cycle in certain conditions.

2054^2^Ball,AS^Drake,BG^1998^1^Stimulation of soil respiration by carbon dioxide enrichment of marsh vegetation^130^30^8-9^1203-1205^^^^^Aug
417^57^733^818^ols at intercellular CO2 (Ci) values below 200 mu L L-1 but remained constant with decreasing Ci in leaves of FeSOD transformants. Wh2055^5^Bandara,DC^Nobuyasu,H^Ofosu-Budu,KG^Ando,T^Fujita,K^1998^1^Effect of CO2 enrichment on biomass production, photosynthesis, and sink activity in soybean cv. Bragg and its supernodulating mutant nts 1007^316^44^2^179-186^^^^^Jun^^^^^6874
1096^2612^3132^3133^3134^3135^417^427^57^92^decreasing temperature. At 6 degrees C the high-light-induced decrease in the F-v/F-m ratio was partially prevented by low O-2 but values were comparable in all lines. Methyl viologen caused decreased F- v/F-m ratios, but this was less marked in the FeSOD transformants than in the untransformed controls. These observations suggest that the rate of superoxide dismutation limits flux through the Mehler-peroxidase cycle in certain conditions.

2054^2^Ball,AS^Drake,BG^1998^1^Stimulation of soil respiration by carbon dioxide enrichment of marsh vegetation^130^30^8-9^1203-1205^^^^^Aug
417^57^733^818^ols at intercellular CO2 (Ci) values below 200 mu L L-1 but remained constant with decreasing Ci in leaves of FeSOD transformants. WhA^6873^Soybean (Glycine max L. Merr.) cv. Bragg and its supernodulating mutant nts 1007 were grown in pots containing vermiculite with a N-free nutrient solution in order to examine the effect of elevated CO2 concentration (100+20 Pa CO2) on biomass production, photosynthesis, and biological nitrogen fixation. The whole plant weight increase in Bragg was higher than in the mutant at a high CO2 concentration. Apparent photosynthetic activities of the upper leaves in both Bragg and the mutant increased up to 14 d after treatment initiation by the CO2 enrichment and thereafter decreased to some extent. Both leaf area and leaf thickness of Bragg increased more than in nts 1007. With the elevated CO2 concentration, biological nitrogen fixation (BNF) also responded in the same manner as biomass production in both Bragg and nts 1007. The increase of BNF in Bragg was largely due to an increase in nodule weight. Starch contents in the leaves of both Bragg and the mutant increased significantly by CO2 enrichment, with a higher increase in Bragg than in its mutant. Sugar content in leaf differed only slightly in both Bragg and the mutant. N content in leaf decreased in both Bragg and its mutant, with the decrease being more pronounced in Bragg. However, in other plant parts (roots, stem, and petiole + pods), N content increased in the mutant while in Bragg, it decreased in the pod. N accumulation rate was higher in Bragg than in the mutant and increased more in Bragg than in the mutant by CO2 enrichment. The ureide content in leaf decreased in Bragg but increased in the mutant by elevated CO2 concentration. In the nodules, ureide content increased in both Bragg and the mutant by CO2 enrichment. Based on these results, it is suggested that in terms of biomass production and photosynthetic rate, Bragg responded more to elevated CO2 concentration than its mutant nts 1007. The alleviation of the stunted vegetative growth of the mutant by CO2 enrichment was limited despite the significant increase in the photosynthetic activity, presumably due to the limitation of sink activity in the growing parts and not to insufficient supply of N through BNF.

2056^1^Colinvaux,PA^1998^1^A new vicariance model for Amazonian endemics^175^7^2^95-96^^^^^Mar^^^^^6876
3136^
A^6875^It is unlikely that ice age climates of the Amazon were sufficiently arid to fragment the forest as required by the Haffer refugial hypothesis. However, glacial Amazon climates were colder and had reduced CO2 concentrations that would have had their strongest effects on the biota in the elevated areas stipulated to have been refugia. If local endemicity of butterflies or birds records Pleistocene speciation, this is because glacial climates provided cool, CO2 starved islands in a sea of continuous forest.

2057^4^Cook,AC^Tissue,DT^Roberts,SW^Oechel,WC^1998^1^Effects of long-term elevated [CO2] from natural CO2 springs on Nardus stricta: Photosynthesis, biochemistry, growth and phenology^9^21^4^417-425^^^^^Apr^^^^^6878
1121^2347^30^360^361^374^376^377^417^691^etic activitA^6877^Plants of Nardus stricta growing near a cold, naturally emitting CO2 spring in Iceland were used to investigate the long-term (> 100 years) effects of elevated [CO2] on photosynthesis, biochemistry, growth and phenology in a northern grassland ecosystem. Comparisons were made between plants growing in an atmosphere naturally enriched with CO2 (approximate to 790 mu mol mol(-1)) near the CO2 spring and plants of the same species growing in adjacent areas exposed to ambient CO2 concentrations (approximate to 360 mu mol mol(-1)). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (approximate to 25%), Rubisco content (approximate to 26%), Rubisco activity (approximate to 40%), Rubisco activation state (approximate to 23%), chlorophyll content (approximate to 33%) and leaf area index (approximate to 22%) compared,vith plants growing away from the spring. The potential positive effects of elevated [CO2] on grassland ecosystems in Iceland are likely to be reduced by strong down-regulation in the photosynthetic apparatus of the abundant N, stricta species.

2058^3^Cotrufo,MF^Briones,MJI^Ineson,P^1998^1^Elevated CO2 affects field decomposition rate and palatability of tree leaf litter: Importance of changes in substrate quality^130^30^12^1565-1571^^^^^Oct^^^^^6880
2364^3137^374^377^417^57^750^92^approximate to 790 mu mol mol(-1)) near the CO2 spring and plants of the same species growing in adjacent areas exposed to ambient CO2 concentrations (approximate to 360 mu mol mol(-1)). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (approximate to 25%), Rubisco content (approximate to 26%), Rubisco activity (approximate to 40%), Rubisco activation state (approximate to 23%), chlorophyll content (approximate to 33%) and leaf area index (approximate to 22%) compared,vith plants growing away from the spring. The potential positive effects of elevated [CO2] on grassland ecosystems in Iceland are likelyA^6879^Field decomposition rates of ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.) leaf litters were measured for litters grown at ambient and elevated concentration of atmospheric CO2 inside solar domes. Litter raised at 600 mu l l(-1) CO2 retained significantly more mass at the end of the first year of field decomposition than material raised at 350 mu l l(-1). This reduction in decomposition could be related to changes in tissue quality resulting from growing the plants at higher CO2 concentrations, with C-to-N ratios and lignin contents being significantly increased. The elevated CO2 treatment also affected the rate of consumption of ash leaf litter by Oniscus asellus L. (Isopoda: Oniscoidea), with significantly less (-16%) material being consumed for litter derived from the high CO2 regime. Our results indicate that changes in litter quality, which we may expect under elevated CO2, may affect litter palatability for soil fauna. (C) 1998 Elsevier Science Ltd. All rights reserved.
kely2059^2^Cowling,SA^Sage,RF^1998^1^Interactive effects of low atmospheric CO2 and elevated temperature on growth, photosynthesis and respiration in Phaseolus vulgaris^9^21^4^427-435^^^^^Apr^^^^^6882
1260^1980^2575^312^348^360^384^417^423^914^600 mu l l(-1) CO2 retained significantly more mass at the end of the first year of field decomposition than material raised at 350 mu l l(-1). This reduction in decomposition could be related to changes in tissue quality resulting from growing the plants at higher CO2 concentrations, with C-to-N ratios and lignin contents being significantly increased. The elevated CO2 treatment also affected the rate of consumption of ash leaf litter by Oniscus asellus L. (Isopoda: Oniscoidea), with significantly less (-16%) material being consumed for litter derived from the high CO2 regime. Our results indicate that changes in litter quality, which we may expect under elevated CO2, may affect litter palatability for soil fauna. (C) 1998 Elsevier Science Ltd. All rights reserved.
kelyA^6881^For most of the past 250 000 years, atmospheric CO, has been 30-50% lower than the current level of 360 mu mol CO2 mol-l air. Although the effects of CO2 on plant performance are well recognized, the effects of low CO2 in combination with abiotic stress remain poorly understood. In this study, a growth chamber experiment using a two-by-two factorial design of CO2 (380 mu mol mol(-1), 200 mu mol mol(-1)) and temperature (25/20 degrees C day/night, 36/29 degrees C) was conducted to evaluate the interactive effects of CO2 and temperature variation on growth, tissue chemistry and leaf gas exchange of Phaseolus vulgaris. Relative to plants grown at 380 pmol mol(-1) and 25/20 degrees C, whole plant biomass was 36% less at 380 mu mol mol(-1) x 36/29 degrees C, and 37% less at 200 mu mol mol(-1) x 25/20 degrees C, Most significantly, growth at 200 mu mol mol(- 1) x 36/29 degrees C resulted in 77% less biomass relative to plants grown at 380 pmol mol(-1) x 25/20 degrees C, The net CO2 assimilation rate of leaves grown in 200 mu mol mol(-1) x 25/20 degrees C was 40% lower than in leaves from 380 pmol mol(-1) x 25/20 degrees C, but similar to leaves in 200 mu mol mol(-1) x 36/29 degrees C. The leaves produced in low CO2 and high temperature respired at a rate that was double that of leaves from the 380 mu mol mol(-1) x 25/20 degrees C treatment. Despite this, there was little evidence that leaves at low CO2 and high temperature were carbohydrate deficient, because soluble sugars, starch and total non-structural carbohydrates of leaves from the 200 mu mol mol(-1) x 36/29 degrees C treatment were not significantly different in leaves from the 380 mu mol mol(-1) x 25/20 degrees C treatment. Similarly, there was no significant difference in percentage root carbon, leaf chlorophyll and leaf/root nitrogen between the low CO2 x high temperature treatment and ambient CO2 controls. Decreased plant growth was correlated with neither leaf gas exchange nor tissue chemistry. Rather, leaf and root growth were the most affected responses, declining in equivalent proportions as total biomass production. Because of this close association, the mechanisms controlling leaf and root growth appear to have the greatest control over the response to heat stress and CO2 reduction in P. vulgaris.

2060^2^Gallardo,A^Merino,J^1998^1^Soil nitrogen dynamics in response to carbon increase in a Mediterranean shrubland of SW Spain^130^30^10-11^1349-1358^^^^^Sep^^^^^6884
1298^20^2102^3138^3139^362^417^535^672^733^s, starch and total non-structural carbohydrates of leaves from the 200 mu mol mol(-1) x 36/29 degrees C treatment were not significantly different in leaves from the 380 mu mol mol(-1) x 25/20 degrees C treatment. Similarly, there was no significant difference in percentage root carbon, leaf chlorophyll and leaf/root nitrogen between the low CO2 x high temperature treatment and ambient CO2 controls. Decreased plant growth was correlated with neither leaf gas exchange nor tissue chemistry. Rather, leaf and root growth were the most affected rA^6883^Most models predict that high atmospheric CO2 concentrations will lead to an increase in the C-to-N ratio of litter production in terrestrial ecosystems. The effect of an increase in the soil C-to-N ratio on the nitrogen dynamics in a Mediterranean shrubland was simulated by mixing with the lifter layer wood shavings with a high C-to-N ratio. Samples of mineral soil, taken subsequently eight times during 404 d, were analyzed for total C, total N, total soil carbohydrates, potential net N mineralization, potential net nitrification and microbial biomass-N. We found significant increases in the concentration of total carbohydrates, C-to-N ratio and microbial biomass N in amended soils during the experiment, while potential net N mineralization rate and net nitrification rate significantly decreased; amounts of available nitrogen (NH4+-N + NO3-N) were unaffected by the amendment treatment. However, by the end of the experiment, no significant differences between amended and control soil samples were found. The total carbohydrates-to-K2SO4-extractable total-N ratio was the best predictor of both net mineralization rate and microbial biomass N, showing that the available C-to- available-N ratio is a better indicator of N dynamics than the total C to total N ratio. Our results support the hypothesis that increasing C availability in soils leads to a decrease in N availability for plants through the immobilization of N in microbial biomass and to an increase in the temporal heterogeneity of soil properties in a Mediterranean shrubland. (C) 1998 Elsevier Science Ltd. All rights reserved.

2061^2^Gao,Q^Yu,M^1998^1^A model of regional vegetation dynamics and its application to the study of Northeast China Transect (NECT) responses to global change^137^12^2^329-344^^^^^Jun^^^^^6886
227^243^2589^2762^3140^51^633^696^700^738^ailable nitrogen (NH4+-N + NO3-N) were unaffected by the amendment treatment. However, by the end of the experiment, no significant differences between amended and control soil samples were founA^6885^We developed a dynamic regional vegetation model to address problems of responses of regional vegetation to elevated ambient CO2 and climatic change. The model takes into consideration both local ecosystem processes within a patch or grid cell, such as plant growth and death, and mass and energy flow, such as plant migration, across adjacent grid cells. The model is able to couple vegetation structure dynamics and primary production processes. The normalized differential vegetation index from meteorological satellite AVHRR was used to parameterize the model. Plant migration rates were derived based on effective seedling distribution around parent plants. The model was applied to Northeast China Transect at a spatial resolution of 10 min latitude by 10 min longitude per grid cell and a temporal resolution of 1 month. The results indicated that with doubled CO2 concentration, a 20% increase in precipitation and a 4 degrees C increase in temperature, the model predicted that net primary productivity (NPP) of Larix forests, conifer-broadleaf mixed forests, Aneurolepidium chinense steppes, Stipa grandis steppes, and wetland and salty meadows would decrease by 15% to 20%. However, NPP of deciduous broadleaf forests, woodland and shrubs, Stipa baicalensis meadow steppes, and desert grasslands would increase by 20% to 115%, as predicted by the model for the same climatic scenario. The average NPP of natural vegetation over the whole transect would decrease slightly, largely because of the compensation between the positive effects of increased CO2 and precipitation and the negative effect of increased evapotranspiration induced by increased temperature.

2062^9^Geiger,M^Walch-Liu,P^Engels,C^Harnecker,J^Schulze,ED^Ludewig,F^Sonnewald,U^Scheible,WR^Stitt,M^1998^1^Enhanced carbon dioxide leads to a modified diurnal rhythm of nitrate reductase activity in older plants, and a large stimulation of nitrate reductase activity and higher levels of amino acids in young tobacco plants^9^21^3^253-268^^^^^Mar^^^^^6888ty (NPP1437^2249^229^3141^372^376^448^504^57^92^d forests, Aneurolepidium chinense steppes, Stipa grandis steppes, and wetland and salty meadows would decrease by 15% to 20%. However, NPP of deciduous broadleaf forests, woodland and shrubs, Stipa baicalensis meadow steppes, and desert grasslands would increase by 20% to 115%, as predicted by the model for the same climatic scenario. The average NPP of natural vegetation over the whole transect would decrease slightly, largely because of the compensation between the positive effects of increased CO2 and precipitation and the negative effect of increased evapotranspiration induced by increased temperature.

2062^9^Geiger,M^Walch-Liu,P^Engels,C^Harnecker,J^Schulze,ED^Ludewig,F^Sonnewald,U^Scheible,WR^Stitt,M^1998^1^Enhanced carbon dioxide leads to a modified diurnal rhythm of nitrate reductase activity in older plants, and a large stimulation of nitrate reductase activity and higher levels of amino acids in young tobacco plants^9^21^3^253-268^^^^^Mar^^^^^6888ty (NPPA^6887^Higher rates of nitrate assimilation are required to support faster growth in enhanced carbon dioxide. To investigate how this is achieved, tobacco plants were grown on high nitrate and high light in ambient and enhanced (700 mu mol mol(-1)) carbon dioxide. Surprisingly, enhanced carbon dioxide did not increase leaf nitrate reductase (MR) activity in the middle of the photoperiod. Possible reasons for this anomalous result were investigated. (a) Measurements of biomass, nitrate, amino acids and glutamine in plants fertilized once and twice daily with 12 mol m(-3) nitrate showed that enhanced carbon dioxide did not lead to a nitrate limitation in these plants. (b) Enhanced carbon dioxide modified the diurnal regulation of NR activity in source leaves. The transcript for nia declined during the light period in a similar manner in ambient and enhanced carbon dioxide. The decline of the transcript correlated with a decrease of nitrate in the leaf, and was temporarily reversed after re-irrigating with nitrate in the second part of the photoperiod. The decline of the transcript was not correlated with changes of sugars or glutamine. NR activity and protein decline in the second part of the photoperiod, and NR is inactivated in the dark in ambient carbon dioxide. The decline of NR activity was smaller and dark inactivation was partially reversed in enhanced carbon dioxide, indicating that post- transcriptional or post-translational regulation of NR has been modified. The increased activation and stability of NR in enhanced carbon dioxide was correlated with higher sugars and lower glutamine in the leaves. (c) Enhanced carbon dioxide led to increased levels of the minor amino acids in leaves. (d) Enhanced carbon dioxide led to a large decrease of glycine and a small decrease of serine in leaves of mature plants. The glycine:serine ratio decreased in source leaves of older plants and seedlings. The consequences of a lower rate of photorespiration for the levels of glutamine and the regulation of nitrogen metabolism are discussed. (e) Enhanced carbon dioxide also modified the diurnal regulation of NR in roots. The nia transcript increased after nitrate fertilization in the early and the second part of the photoperiod. The response of the transcript was not accentuated in enhanced carbon dioxide. NR activity declined slightly during the photoperiod in ambient carbon dioxide, whereas it increased 2-fold in enhanced carbon dioxide. The increase of root NR activity in enhanced carbon dioxide was preceded by a transient increase of sugars, and was followed by a decline of sugars, a faster decrease of nitrate than in ambient carbon dioxide, and an increase of nitrite in the roots. (f) To interpret the physiological significance of these changes-in nitrate metabolism, they were compared with the current growth rate of the plants. (g) In 4-5-week-old plants, the current rate of growth was similar in ambient and enhanced carbon dioxide (approximate to 0.4 g(-1) d(-1)). Enhanced carbon dioxide only led to small changes of NR activity, nitrate decreased, and overall amino acids were not significantly increased. (h) Young seedlings had a high growth rate (0.5 g(-1) d(-1)) in ambient carbon dioxide, that was increased by another 20% in enhanced carbon dioxide. Enhanced carbon dioxide led to larger increases of NR activity and NR activation, a 2-3-fold increase of glutamine, a 50% increase of glutamate, and a 2-3-fold increase in minor amino acids. It also led to a higher nitrate level. It is argued that enhanced carbon dioxide leads to a very effective stimulation of nitrate uptake, nitrate assimilation and amino acid synthesis in seedlings. This will play an important role in allowing faster growth rates in enhanced carbon dioxide at this stage.

2063^6^Hodge,A^Paterson,E^Grayston,SJ^Campbell,CD^Ord,BG^Killham,K^1998^1^Characterisation and microbial utilisation of exudate material from the rhizosphere of Lolium perenne grown under CO2 enrichment^130^30^8-9^1033-1043^^^^^Aug^^^^^6890
1096^1262^1803^2364^312^361^57^733^738^778^ NR aA^6889^The effects of elevated atmospheric CO2 concentration on alterations, both qualitatively and quantitatively, of exuded compounds from the roots of Lolium perenne seedlings were investigated by growing plants in a sterilised sand microcosm unit. In addition, the effect of CO2 treatment on carbon substrate utilisation of microbial populations extracted from the rhizosphere of L. perenne seedlings grown in soil microcosm units was examined and alterations on microbial activity and diversity assessed using a commercially-available redox-based sole C source utilisation test (Biolog(R)) including additional exudate compounds. Both types of microcosm units (sand and soil) were maintained at specific growth conditions under two CO2 regimes (450 and 720 mu mol mol(-1)). Growth of L. perenne seedlings from both types of microcosm units was enhanced under elevated atmospheric CO2 although the root-to-shoot ratios were not significantly altered, indicating no gross change in dry matter partitioning. Cumulative total organic carbon (TOC) release in the exudate material over the duration of the experiment was significantly (P less than or equal to 0.05) higher from ambient-grown seedlings despite a significant (P less than or equal to 0.05) increase in the dry weight of roots of the elevated CO2 grown seedlings as determined at harvest. Over the individual sampling periods TOC release was significantly (P less than or equal to 0.05) higher from elevated CO2 grown seedlings on only one occasion (21 d). Qualitative differences, measured between d 1-6 and 14-18, also occurred with elevated CO2 treatment decreasing the amount of phenolic acids and total sugars at the latter sampling period compared to ambient CO2 seedlings. Total numbers of bacteria were significantly (P less than or equal to 0.05) decreased under elevated CO2 although culturable numbers significantly (P less than or equal to 0.05) increased. This increase in culturable microorganisms may explain the faster carbon source utilisation rates of the elevated CO2 treatment. No change in morphotypes of microbial colonies were observed suggesting a quantitative difference due to elevated CO2 treatment only. (C) 1998 Elsevier Science Ltd. All rights reserved.

2064^3^Hu,SJ^Firestone,MK^Chapin,FS^1998^1^Elevated atmospheric CO2 and soil biota^32^281^5376^518^^^^^24 Jul

2065^2^Kellomaki,S^Wang,KY^1998^1^Daily and seasonal CO2 exchange in Scots pine grown under elevated O-3 and CO2: experiment and simulation^331^136^2^229-248^^^^^Jun^^^^^6893
2035^273^312^3142^3874^395^444^446^493^602^etween d 1-6 and 14-18, also occurred with elevated CO2 treatment decreasing the amount of phenolic acids and total sugars at the latter sampling period compared to ambient CO2 seedlings. Total numbers of bacteria were significantly (P less than or equal to 0.05) decreased under elevated CO2 although culturable numbers significantly (P less than or equal to 0.05) increased. This increase in culturable microorganisms may explain the faster carbon source utilisation rates of the elevated A^6892^Starting in early spring of 1994, naturally regenerated, 30- year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers and exposed in situ to doubled ambient O-3, doubled ambient CO2 and a combination of O-3 and CO2 from 15 April to 15 September. To investigate daily and seasonal responses of CO2 exchange to elevated O-3 and CO2, the CO2 exchange of shoots was measured continuously by an automatic system for measuring gas exchange during the course of one year (from 1 Januray to 31 December 1996). A process-based model of shoot photosynthesis was constructed to quantify modifications in the intrinsic capacity of photosynthesis and stomatal conductance by simulating the daily CO2 exchange data from the field. Results showed that on most days of the year the model simulated well the daily course of shoot photosynthesis. Elevated O-3 significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O-3 also led to a delay in onset of photosynthetic recovery in early spring and an increase in the sensitivity of photosynthesis to environmental stress conditions. The combination of elevated O-3 and CO2 had an effect on photosynthesis and stomatal conductance similar to that of elevated O-3 alone, but significantly reduced the O-3 induced depression of photosynthesis. Elevated CO2 significantly increased the photosynthetic capacity of Scots pine during the main growing season but slightly decreased it in early spring and late autumn. The model calculation showed that, compared to the control treatment, elevated O-3 alone and the combination of elevated O-3 and CO2 decreased the annual total of net photosynthesis per unit leaf area by 55% and 38%, respectively. Elevated CO2 increased the annual total of net photosynthesis by 13%.
l the daily course of shoot photosynthesis. Elevated O-3 significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O-3 also led to a delay in on2066^4^Kurschner,WM^Stulen,I^Wagner,F^Kuiper,PJC^1998^1^Comparison of palaeobotanical observations with experimental data on the leaf anatomy of durmast oak [Quercus petraea (Fagaceae)] in response to environmental change^52^81^5^657-664^^^^^May^^^^^6895
1627^2460^312^344^372^376^708^745^746^92^ the O-3 induced depression of photosynthesis. Elevated CO2 significantly increased the photosynthetic capacity of Scots pine during the main growing season but slightly decreased it in early spring and late autumn. The model calculation showed that, compared to the control treatment, elevated O-3 alone and the combination of elevated O-3 and CO2 decreased the annual total of net photosynthesis per unit leaf area by 55% and 38%, respectively. Elevated CO2 increased the annual total of net photosynthesis by 13%.
l the daily course of shoot photosynthesis. Elevated O-3 significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O-3 also led to a delay in onA^6894^To test whether stomatal density measurements on oak leaf remains are reliable tools for assessing palaeoatmospheric carbon dioxide concentration [CO2], under changing Late Miocene palaeoenvironmental conditions, young seedings of oak (Quercus petraea, Liebl.) were grown at elevated vs. ambient atmospheric [CO2] and at high humidity combined with an increased air temperature. The leaf anatomy of the young oaks was compared with that of fossil leaves of the same species. In the experiments, stomatal density and stomatal index were significantly decreased at elevated [CO2] in comparison to ambient [CO2]. Elevated [CO2] induced leaf cell expansion and reduced the intercellular air space by 35%. Leaf cell size or length were also stimulated at high air humidity and temperature. Regardless of a temperate or subtropical palaeoclimate, leaf cell size in fossil oak was not enhanced, since neither epidermal cell density nor length of the stomatal apparatus changed. The absence of these effects may be attributed to the phenological response of trees to climatic changes that balanced temporal changes in environmental variables to maintain leaf growth under optimal and stable conditions. Quercus petraea, which evolved under recurring depletions in the palaeoatmospheric [CO2], may possess sufficient phenotypic plasticity to alter stomatal frequency in hypostomatous leaves allowing high maximum stomatal conductance and high assimilation rates during these phases of low [CO2]. (C) 1998 Annals of Botany Company.

2067^3^Liakatas,A^Roussopoulos,D^Whittington,WJ^1998^1^Controlled-temperature effects on cotton yield and fibre properties^178^130^^463-471^^^^^Jun^^^^^6897
348^361^372^685^cellular air space by 35%. Leaf cell size or length were also stimulated at high air humidity and temperature. Regardless of a temperate or subtropical palaeoclimate, leaf cell size in fossil oak was not enhanced, since neither epidermal cell density nor length of the stomatal apparatus changed. The absence of these effects may be attributeA^6896^Temperature effects on cotton yield and fibre properties of three cotton cultivars were determined. Plants were grown in pots maintained in growth rooms at varying day and night temperatures representing seasonally constant or varying (C) or daily varying (V) regimes. Yield and fibre characters responded to variation of daily mean and amplitude of temperature. Mean temperature reduction improved yield components, but fibre length, uniformity, strength and micronaire were increased by high, particularly high day, temperatures. A large daily temperature amplitude produced an intermediate number of flowers and the lowest retention percentage. Fruiting and yield were increased by reduction in temperature down to the threshold mean temperature of 22 degrees C. However, V-regimes with a low minimum temperature acted as a further drop (below 22 degrees C) of temperature and adversely affected these characters, An adverse effect of low minimum temperature combined with a moderate day temperature was observed also on lint percentage and fibre properties. Varietal differences were more pronounced for highly heritable characters such as fibre properties, for which significant interactions between varieties and temperature also occurred. Differences in reproductive development were not sufficient to be of much practical importance.

2068^10^Ludewig,F^Sonnewald,U^Kauder,F^Heineke,D^Geiger,M^Stitt,M^Muller-Rober,BT^Gillissen,B^Kuhn,C^Frommer,WB^1998^1^The role of transient starch in acclimation to elevated atmospheric CO2^358^429^2^147-151^^^^^12 Jun^^^^^6899
1351^1584^1871^243^312^3144^3145^441^788^845^mber of flowers and the lowest retention percentage. Fruiting and yield were increased by reduction in temperature down to the threshold mean temperature of 22 degrees C. However, V-regimes with a low minimum temperature acted as a further drop (below 22 degrees C) of temperature and adversely affected these characters, An adverse effect of low minimum temperature combined with a moderate day temperature was observed A^6898^Although increased concentrations of CO2 stimulate photosynthesis, this stimulation is often lost during prolonged exposure to elevated carbon dioxide, leading to an attenuation of the potential gain in yield. Under these conditions, a wide variety of species accumulates non-structural carbohydrates in leaves, It has been proposed that starch accumulation directly inhibits photosynthesis, that the rate of sucrose and starch synthesis limits photosynthesis, or that accumulation of sugars triggers changes in gene expression resulting in loser activities of Rubisco and inhibition of photosynthesis. To distinguish these explanations, transgenic plants unable to accumulate transient starch due to leaf mesophyll-specific antisense expression of AGP B were grown at ambient and elevated carbon dioxide. There was a positive correlation between the capacity for starch synthesis and the rate of photosynthesis at elevated CO2 concentrations, showing that the capability to synthesize leaf starch is essential for photosynthesis in elevated carbon dioxide, The results show that in elevated carbon dioxide, photosynthesis is restricted by the rate of end product synthesis, Accumulation of starch is not responsible for inhibition of photosynthesis, Although transgenic plants contained increased levels of hexoses, transcripts of photosynthetic genes were not downregulated and Rubisco activity was not decreased arguing against a role of sugar sensing in acclimation to high CO2, (C) 1998 Federation of European Biochemical Societies.

2069^2^Monje,O^Bugbee,B^1998^1^Adaptation to high CO2 concentration in an optimal environment: radiation capture, canopy quantum yield and carbon use efficiency^9^21^3^315-324^^^^^Mar^^^^^6901
1290^130^348^372^374^389^409^422^685^692^P B were grown at ambient and elevated carbon dioxide. There was a positive correlation between the capacity for starch synthesis and the rate of photosynthesis at elevated CO2 concentrations, showing that the capability to synthesize leaf starch is essential for pA^6900^The effect of elevated [CO2] on wheat (Triticum aestivum L, Veery 10) productivity was examined by analysing radiation capture, canopy quantum yield, canopy carbon use efficiency, harvest index and daily C gain, Canopies were grown at either 330 or 1200 mu mol mol(-1) [CO2] in controlled environments, where root and shoot C fluxes were monitored continuously from emergence to harvest. A rapidly circulating hydroponic solution supplied nutrients, water and root zone oxygen, At harvest, dry mass predicted from gas exchange data was 102.8 +/- 4.7% of the observed dry mass in six trials. Neither radiation capture efficiency nor carbon use efficiency were affected by elevated [CO2], but yield increased by 13% due to a sustained increase in canopy quantum yield. CO2 enrichment increased root mass, tiller number and seed mass. Harvest index and chlorophyll concentration were unchanged, but CO2 enrichment increased average life cycle net photosynthesis (13%, P < 0.05) and root respiration (24%, P < 0 05). These data indicate that plant communities adapt to CO2 enrichment through changes in C allocation. Elevated [CO2] increases sink strength in optimal environments, resulting in sustained increases in photosynthetic capacity, canopy quantum yield and daily C gain throughout the life cycle.

2070^1^Mortensen,LM^1998^1^Effects of elevated CO2 concentration on growth of Betula pubescens Ehrh. in different climatic conditions^108^13^2^197-203^^^^^^^^^^6903
1262^243^3146^344^348^361^376^402^430^528^ dry mass predicted from gas exchange data was 102.8 +/- 4.7% of the observed dry mass in six trials. Neither radiation capture efficiency nor carbon use efficiency were affected by elevated [CO2], but yield increased by 13% due to a sustained increase in canopy quantum yield. CO2 enrichment increased root mass, tiller number and seed mass. Harvest index and chlorophyll concentration were unchanged, but CO2 enrichment increased average life cycle net photosynthesis (13%, P < 0.05) and root respiration (24%, P < 0 05). TheA^6902^Seedlings of Betula pubescens Ehrh. (mountain birch) were grown at ambient and elevated CO2 concentrations in environment- controlled growth chambers, and in chambers or wind tunnels in the field. In the two preliminary experiments in a controlled environment, CO2 enrichment increased the dry weights of six birch provenances grown at a daily mean temperature (MT) of 17 degrees C and 15 provenances grown at 12.5 degrees C MT by 27 and 7%. respectively. In more realistic conditions in field chambers (13.9 degrees C MT), the shoot dry weight of plants grown for 65 days was not significantly affected by the elevated CO2 concentration. In a parallel experiment, CO2 enrichment increased the shoot dry weight by 36% in both unheated (14.7 degrees C MT) and heated (18.1 degrees C MT) wind tunnels. In a final experiment over two seasons in open- top chambers at 850 m a.s.l., elevated CO2 concentrations increased the root (42%) but not the shoot dry weight. The results are discussed in relation to variable climatic conditions.

2071^1^Norby,RJ^1998^1^Nitrogen deposition: a component of global change analyses^84^139^1^189-200^^^^^May^^^^^6905
1103^243^372^374^377^400^483^58^715^741^or wind tunnels in the field. In the two preliminary experiments in a controlled environment, CO2 enrichment increased the dry weights of six birch provenances grown at a daily mean temperature (MT) of 17 degrees C and 15 provenances grown at 12.5 degrees C MT by 27 and 7%. respectively. In more realistic conditions in field chambers (13.9 degrees C MT), the shoot dry weight of plants grown for 65 days was not significantly affected by the elevated CO2 concentration. In a parallel experiment, CO2 enrichment increased the shoot dry weight by 36% in both unheated (14.7 degrees C MT) and heated (18.1 degrees C MT) wind tunnels. In a final experiment over two seasons in open- top chambers at 850 m a.s.l., elevated CO2 concentrations increased the root (42%) but not the shoot dry weight. The results are discussed in relation to variable climaA^6904^The global cycles of carbon and nitrogen are being perturbed by human activities that increase the transfer from large pools of non-reactive forms of the elements to reactive forms that are essential to the functioning of the terrestrial biosphere. The cycles are closely linked at all scales, and global change analyses must consider C and N cycles together. The increasing amount of N originating from fossil fuel combustion and deposited to terrestrial ecosystems as nitrogen oxides could increase the capacity of ecosystems to sequester C, thereby removing some of the excess carbon dioxide from the atmosphere and slowing the development of greenhouse warming. Several global and ecosystem models have calculated the amount of C sequestration that can be attributed to N deposition, based on assumptions about the allocation of N among ecosystem components with different C:N ratios. They support the premise that, since industrialization began, N deposition has been responsible for an increasing terrestrial C sink, but there is great uncertainty whether ecosystems will continue to retain exogenous N. Whether terrestrial ecosystems continue to sequester additional C will depend in part on their response to increasing concentrations of atmospheric carbon dioxide, widely thought to be constrained by limited N availability. Ecosystem models generally support the conclusion that responses to increasing concentrations of carbon dioxide will be greater, and the range of possible responses will be wider, in ecosystems where increased N inputs originate as atmospheric deposition. The interactions between N deposition and increasing carbon dioxide concentrations could be altered considerably, however, by additional factors, including N saturation of ecosystems, changes in community composition, and climate change, Nitrogen deposition is also linked to global change issues through the volatile losses of nitrous oxide, which is a potent greenhouse gas, and the role of nitrogen oxides in the production of tropospheric ozone, which could interact with plant responses to elevated carbon dioxide. Any consideration of the role of N deposition in global change issues must also balance the projected responses against the serious detrimental impact of excess N on the environment.

2072^9^Osborne,CP^LaRoche,J^Garcia,RL^Kimball,BA^Wall,GW^Pinter,PJ^LaMorte,RL^Hendrey,GR^Long,SP^1998^1^Does leaf position within a canopy affect acclimation of photosynthesis to elevated CO2? Analysis of a wheat crop under free-air CO2 enrichment^8^117^3^1037-1045^^^^^Jul^^^^^6907
1093^2455^264^3147^341^344^384^417^57^92^teractions between N deposition and increasing carbon dioxide concentrations could be altered considerably, however, by additional factors, including N saturation of ecosystems, changes in community composition, and climate change, Nitrogen deposition is also linked to global change issues through the volatile losses of nitrous oxide, which is a potent greenhouse gas, and the role of nitrogen oxides in the production of tropospheric ozone,A^6906^Previous studies of photosynthetic acclimation to elevated CO2 have focused on the most recently expanded, sunlit leaves in the canopy. We examined acclimation in a vertical profile of leaves through a canopy of wheat (Triticum aestivum L.). The crop was grown at an elevated CO2 partial pressure of 55 Pa within a replicated field experiment using free-air CO2 enrichment. Cas exchange was used to estimate in vivo carboxylation capacity and the maximum rate of ribulose-1,5- bisphosphate-limited photosynthesis. Net photosynthetic CO2 uptake was measured for leaves in situ within the canopy. Leaf contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO2 did not affect carboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite this acclimation, in situ photosynthetic CO2 uptake remained higher under elevated CO2. Acclimation at elevated CO2 was accompanied by decreases in both Rubisco and total leaf N contents and an increase in LHC content. Elevated CO2 led to a larger increase in LHC/Rubisco in lower canopy leaves than in the uppermost leaf. Acclimation of leaf photosynthesis to elevated CO2 therefore depended on both vertical position within the canopy and the developmental stage.

2073^2^Peng,CH^Apps,MJ^1998^1^Simulating carbon dynamics along the Boreal Forest Transect Case Study (BFTCS) in central Canada - 2. Sensitivity to climate change^137^12^2^393-402^^^^^Jun^^^^^6909
1106^178^3050^3148^3149^372^374^697^700^715^ Leaf contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO2 did not affect carboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite this acclimation, in situ photosynthetic CO2 uptake remained higher under elevated CO2. Acclimation at elevA^6908^The effects of climate change and doubling atmospheric CO2 on carbon dynamics of the boreal forest in the area of the Boreal Forest Transect Case Study in central Canada were investigated using the process-based plant-soil model CENTURY 4.0. The results presented here suggest that (1) across the transect climate change would result in increased total carbon in vegetation biomass but decreased overall carbon in soil; (2) increased atmospheric CO2 concentration under current climatic patterns would result in increased total carbon in vegetation and in soil organic matter; and (3) combined climate change and elevated CO2 would increase both net primary productivity and decomposition rates relative to the current climate condition, but their combined action would be a reduction of soil carbon losses relative to those due to climate change alone. The interactive effects of climate change and elevated CO2, however, are not a simple additive combination of the individual responses. The responses to climate change and elevated CO2 vary across the climate gradient from southern to northern sites on the transect. The present simulations indicate that the northern sites are more sensitive to climate change than the southern sites are, but these simulations do not consider likely changes in the disturbance regime or changes in forest species distribution.

2074^2^Rey,A^Jarvis,PG^1998^1^Long-term photosynthetic acclimation to increased atmospheric CO2 concentration in young birch (Betula pendula) trees^13^18^7^441-450^^^^^Jul^^^^^6911
1121^1437^2453^256^2965^374^385^398^417^92^matter; and (3) combined climate change and elevated CO2 would increase both net primary productivity and decomposition rates relative to the current climate condition, but their combined action would be a reduction of soil carbon losses relative to those due to climate change alone. The interactive effects of climate change and elevated CO2, however, are not a simple additive combination of the individual responses. The responses to climate cA^6910^To study the long-term response of photosynthesis to elevated atmospheric CO2 concentration in silver birch (Betula pendula Roth.), 18 trees were grown in the field in open-top chambers supplied with 350 or 700 mu mol mol(-1) CO2 for four consecutive growing seasons. Maximum photosynthetic rates, stomatal conductance and CO2 response curves were measured over the fourth growing season with a portable photosynthesis system. The photosynthesis model developed by Farquhar et al. (1980) was fitted to the CO2 response curves. Chlorophyll, soluble proteins, total nonstructural carbohydrates, nitrogen and Rubisco activity were determined monthly. Elevated CO2 concentration stimulated photosynthesis by 33% on average over the fourth growing season. However, comparison of maximum photosynthetic rates at the same CO2 concentration (350 or 700 mu mol mol(-1)) revealed that the photosynthetic capacity of trees grown in an elevated CO2 concentration was reduced. Analysis of the response curves showed that acclimation to elevated CO2 concentration involved decreases in carboxylation efficiency and RuBP regeneration capacity. No clear evidence for a redistribution of nitrogen within the leaf was observed. Down-regulation of photosynthesis increased as the growing season progressed and appeared to be related to the source-sink balance of the trees. Analysis of the main leaf components revealed that the reduction in photosynthetic capacity was accompanied by an accumulation of starch in leaves (100%), which was probably responsible for the reduction in Rubisco activity (27%) and to a lesser extent for reductions in other photosynthetic components: chlorophyll (10%), soluble protein (9%), and N concentrations (12%) expressed on an area basis. Despite a 21% reduction in stomatal conductance in response to the elevated CO2 treatment, stomatal limitation was significantly less in the elevated, than in the ambient, CO2 treatment. Thus, after four growing seasons exposed to an elevated CO2 concentration in the field, the trees maintained increased photosynthetic rates, although their photosynthetic capacity was reduced compared With trees grown in ambient CO2.

2075^2^Rouhier,H^Read,DJ^1998^1^The role of mycorrhiza in determining the response of Plantago lanceolata to CO2 enrichment^84^139^2^367-373^^^^^Jun^^^^^6913
1096^1334^1850^1983^2070^2467^312^474^672^733^ysis of the main leaf components revealed that the reduction in photosynthetic capacity was accompanied by an accumulation of starch in leaves (100%), which was probably responsible for the reduction in Rubisco activity (27%) and to a lesser extent for reductions in other photosynthetic components: chlorophyll (10%), soluble protein (9%), and N concentrations (12%) expressed on an area basis. Despite a 21% reduction in stomatal conductance in response to the elevated CO2 treatment, stomatal limitation was significantly less in the elevated, than in the ambient, CO2 treatment. Thus, after four growing seasons exposed to an elevated CO2 concentration in the field, the treesA^6912^Plantago lanceolata L. was grown for 104 d with (M) or without (NM) arbuscular mycorrhizal colonization under conditions of ambient (C-AMB = 350 mu l l(-1)) and elevated (C-ELEV = 540 mu l l(-1)) CO2. Sequential harvests (H) were taken at 41 (H-1), 76 (H-2) and 104 d (H-3) to determine the time-course of mycorrhizal influence on the response of the plant to CO2 enrichment. Total yields of M plants were greater than those of NM from H-2 onwards. Plants in the M-ELEV treatment were significantly larger than those in the M-AMB at 104 d. There were significant but much smaller differences in yield between NMELEV and NMAMB. The differences in total yield arose through impact of C-ELEV on both shoots and roots. Total root length was greater in M-ELEV than in M-AMB only at H-3, but total length of mycorrhizal root was greater at H-2 and H-3. The percentages of root length colonized and that occupied by arbuscules and vesicles were greater in M-ELEV than in M-AMB at the last two harvests, indicating increased sequestration of carbon in internal fungal structures. Though extraradical hyphal lengths were greater in M-ELEV than in M-AMB at H-2 and H-3, the differences were not significant. Phosphorus inflow and P content of M plants were higher than those of NM plants at H-2 and H-3, and were higher in M-ELEV than in M-AMB at H-3. ANOVA revealed no significant interactions between CO2 and mycorrhizal treatment. The results are discussed in relation to carbon sequestration in mycorrhizal systems and likely impacts of CO2 enrichment on P. lanceolata grown under field conditions. The importance of sequential harvesting for realistic determination of responses to CO2 is stressed.

2076^5^Spunda,V^Kalina,J^Cajanek,M^Pavlickova,H^Marek,MV^1998^1^Long-term exposure of Norway spruce to elevated CO2 concentration induces changes in photosystem II mimicking an adaptation to increased irradiance^4^152^4-5^413-419^^^^^May^^^^^6915
1092^1871^243^312^344^348^360^384^493^92^in M-AMB at the last two harvests, indicating increasedA^6914^Fifteen-year-old Norway spruces (Picea abies [L.] Karst.) were grown in open top chambers (OTC) at ambient (A) and elevated (i.e. ambient + 350 mu mol(CO2)mol(-1)) concentrations of CO2 (E) for four growing seasons (1992-1995). During this time period several examples of the depression of photosynthetic activities were observed for E needles. In order to better characterize the nature of this depression the gas exchange and fluorescence parameters were analyzed on current year needles during the last season (July 1995). The photon flux density response curves of CO2 uptake (P-N) revealed a significantly reduced stimulation of P-N for E needles as compared with short-term exposure to doubled CO2. Moreover, the sudden exposure of E shoots to 350 mu mol(CO2)mol(-1) at saturating irradiance revealed a depression of both P-Nmax (by 20 %) and quantum yield of PS II (by 32 %) compared with A shoots measured at 350 mu mol(CO2)mol(-1). The data supporting the diminished light harvesting system of photosystem II (PS II) in E shoots compared with A shoots were obtained from pigment analysis, low temperature fluorescence spectra and Chl a fluorescence induction kinetics. The relative proportion of inactive reaction centres of PS II determined from F-pl of the fluorescence induction was 20 % higher for E needles. These changes found for E needles mimicked an adaptation of PS II to increased irradiance compared with A needles. As the irradiance exposure was the same for the examined needles from both E and A spruces we suggest that these changes reported for E needles resulted from the feed-back limitation of photochemical reactions due to suppressed electron transport through the plastoquinone pool.

2077^4^Tognetti,R^Johnson,JD^Michelozzi,M^Raschi,A^1998^1^Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2^173^39^3^233-245^^^^^Jun^^^^^6917
1120^1144^130^1342^243^3150^345^376^377^384^The data supporting the diminished light harvesting system of photosystem IA^6916^Long-term effects on and adaptations of the carbon physiology of long-lived trees exposed to increasing atmospheric levels of CO2 are unknown. We compared two indigenous Quercus species, Q. ilex and Q. pubescens, growing in a natural CO2 spring located in central Italy and at a nearby control site. In May, 1995 photosynthetic rate at least doubled when measured with supplemental CO2 in both species and sites. Dark respiration was much higher at the CO2 spring site in both species. Foliar sugar and starch concentrations in Q. ilex exhibited significant site and diurnal differences (May and September). In July, 1995 there was little difference in the water potential values of the measured trees at the different sites over the diurnal period. Photosynthetic rate was higher for both species in the CO2 spring, particularly in the early morning and late afternoon. Mid-day stomatal closure reduced photosynthesis to similar levels. In the morning leaf conductance and transpiration were generally lower in the CO2 spring trees, contributing to higher instantaneous water use efficiency for both species. Isoprene emission rates were higher in Q. pubescens trees growing in the CO2 spring. The maximum difference between control and CO2 spring trees occurred in late afternoon. In contrast, Q. ilex exhibited isoprene emission near background level. Foliage and branch carbon and nitrogen status showed increased concentrations of starch and tannins in Q. ilex and of soluble sugars in Q. pubescens in the elevated CO2 environment, while nitrogen concentration decreased in both species. Wood gravity increased 6 and 3% in Q. ilex and Q. pubescens, respectively, growing in the CO2 spring. Q. ilex exhibited afternoon recovery of water potential compared to Q. pubescens which had better night-time recovery. Q. ilex and e. pubescens exposed to elevated CO2 for prolonged periods exhibit different mechanisms for dealing with additional reduced carbon and do maintain an altered carbon physiology, even in midst of the region's characteristic summer drought. (C) 1998 Elsevier Science B.V. All rights reserved.

2078^1^Uprety,DC^1998^1^Carbon dioxide enrichment technology: Open top chambers a new tool for global climate research^202^57^5^266-270^^^^^May^^^^^6919
 trees occurred in late afternoon. In contrast, Q. ilex exhibited isoprene emission near background level. Foliage and branch carbon and nitrogen status showed increased concentrations of starch and tannins in Q. ilex and of soluble sugars in Q. pubescens in the elevated CO2 environment, while nitrogen concentration decreased in both species. Wood gravity increased 6 and 3% in Q. ilex and Q. pubescens, respectively, growing in the CO2 spring. Q. ilex exhibited afternoon recovery of water potential compared to Q. pubescens which had better night-time recovery. Q. ilex and e. pubescens exposed to elevated CO2 for prolonged periods exhibit different mechanisms for dealing with additional reduced carbon and do maintain an altered carbon physiology, even in midst of the region's characA^6918^There are many technical difficulties in conducting crop response studies for elevated carbon dioxide. Available facilities include green house, leaf cuvettes, phytotron, and air exclusion systems. The environmental control on these systems induces uncertainity in the extrapolation of results to the variable natural environments. However, open top chamber technology does not modify the micro-climate and induces realistic natural conditions. Open top chambers are cylindrical, aluminium frames with clear flexible covering and frustrum to reduce the incursion of external air. CO2 enriched air is introduced into the chamber through a perforated spurt with the help of a blower to distribute CO2 uniformly. A relatively simpler design and construction of open top chambers make them the most likely method to be used in the near future for long-term elevated CO2 exposures of crops and other ecosystems.
additional reduced carbon and do maintain an altered carbon physiology, even in midst of the region's charac2079^3^Verburg,PSJ^Gorissen,A^Arp,WJ^1998^1^Carbon allocation and decomposition of root-derived organic matter in a plant-soil system of Calluna vulgaris as affected by elevated CO2^130^30^10-11^1251-1258^^^^^Sep^^^^^6921
1334^310^344^362^374^376^538^57^672^92^ainity in the extrapolation of results to the variable natural environments. However, open top chamber technology does not modify the micro-climate and induces realistic natural conditions. Open top chambers are cylindrical, aluminium frames with clear flexible covering and frustrum to reduce the incursion of external air. CO2 enriched air is introduced into the chamber through a perforated spurt with the help of a blower to distribute CO2 uniformly. A relatively simpler design and construction of open top chambers make them the most likely method to be used in the near future for long-term elevated CO2 exposures of crops and other ecosystems.
additional reduced carbon and do maintain an altered carbon physiology, even in midst of the region's characA^6920^The effect of elevated CO2 on C allocation in plant and soil was assessed using soil cores planted with 1-y-old heather (Calluna vulgaris (L.) Hull). Plants were pulse-labeled with (CO2)-C-14 at ambient and elevated CO2 and two nitrogen regimes (low and high). After harvesting the plants, the soil was incubated to monitor total respiration and decomposition of C- 14-labeled rhizodeposits. Total and shoot biomass increased at high N but were not affected by CO2. Root biomass was not affected by either N or CO2 treatments. Total C-14 uptake and shoot-C-14 increased upon adding N and elevating CO2 but the N effect was strongest. Total C-14 uptake per unit shoot mass decreased with N, but increased with CO2. Root-C-14 content was not significantly affected by the N or CO2 treatment. Total soil-C-14 slightly increased at elevated CO2 whereas microbial C-14 increased due to high N. C allocation to shoots increased at the expense of roots, soil and respiration at high N but was not affected by the CO2 treatment. Variation in C-14 distribution within each treatment was small compared to variation in total C-14 amounts in each plant-soil compartment. Initially, C-14 respiration from rhizodeposits correlated well with root-C-14, total soil-C-14, soil solution-C-14 and microbial C-14, at harvest time and was increased by elevated CO2. By the end of the incubation, however, decomposition of labeled organic matter was not affected by the treatments whereas total (=C-12+C-14) respiration was lowest for the elevated-CO2 soils. We speculate that initially, respiration is dominated by decomposition of fresh root exudates whereas in the longer term, respiration originates from decomposition of more recalcitrant root material that had been formed during the entire experiment. The increased net C-14 uptake and unchanged distribution pattern, combined with an increased decomposition of easily-decomposable compounds and a decreased decomposition of more recalcitrant root-derived material indicated a small sink function of a Calluna plant-soil system under elevated CO2. (C) 1998 Elsevier Science Ltd. All rights reserved.

2080^1^Ziska,LH^1998^1^The influence of root zone temperature on photosynthetic acclimation to elevated carbon dioxide concentrations^52^81^6^717-721^^^^^Jun^^^^^6923
344^348^374^376^417^92^ time and was increased by elevated CO2. By the end of the incubation, however, decomposition of labeled organic matter was not affected by the treatments whereas total (=C-12+C-14) respiration was lowest for the elevated-CO2 soils. We speculate that initially, respiration is dominated by decomposition of fresh root exudates whereas in the longer term, respiration originates from decomposition of more recalcitrant root material that had been formed during the entire experiment. The increased net C-14 uptake and unchanged distribution pattern, combined with an increased decomposition of easily-decomposable compounds and a decreased decomposition of more recalcitrant root-derived material indicated a small sink function of a CA^6922^Soybean (Glycine max 'Clark') was grown from germination to 21 d after sowing (DAS) at ambient (similar to 360 mu mol mol(-1)) or elevated (similar to 720 mu mol mol(-1)) carbon dioxide (CO2) at either one of two soil temperatures, 25 or 30 degrees C to determine the influence of root zone temperature on root growth and photosynthetic stimulation at ambient and elevated concentrations of carbon dioxide. Although the photosynthetic rate became less stimulate over time, a significant stimulation of whole plant photosynthesis and plant dry weight was observed at the elevated CO2 concentration during the experimental period irrespective of soil temperature. At neither carbon dioxide concentration did the warmer soil temperature (30 degrees C) stimulate whole plant growth compared to a soil temperature of 25 degrees C, but it did increase root growth relative to shoot (top) growth with a subsequent increase in root/shoot ratio. Increasing soil temperature at either carbon dioxide concentration also significantly stimulated whole plant photosynthetic rate. However, the degree of stimulation was reduced with time irrespective of carbon dioxide concentration so that at 21 DAS no difference in photosynthesis between ambient and elevated soil temperatures was observed. Data from this experiment indicate that for soybean, a higher soil temperature stimulates root/shoot ratio and enhances photosynthetic response to elevated carbon dioxide in the short-term (i.e. days), but increasing root/shoot ratios does not provide a satisfactory explanation of long-term stimulation of photosynthesis at elevated levels of carbon dioxide. (C) 1988 Annals of Botany Company.

2081^3^Bransby,DI^McLaughlin,SB^Parrish,DJ^1998^1^A review of carbon and nitrogen balances in switchgrass grown for energy^310^14^4^379-384^^^^^^^^^^6925
 of 25 degrees C, but it did increase root growth relative to shoot (top) growth with a subsequent increase in root/shoot ratio. Increasing soil temperature at either carbon dioxide concentration also signifiA^6924^Increased atmospheric CO2, caused partly by burning fossil fuels, is assumed to elevate the risk of global warming, while nitrate contamination of surface runoff and groundwater from fertilizer and agricultural wastes constitutes a serious environmental hazard on a regional scale. Switchgrass (Panicum virgatum L.) grown as an energy crop could reduce atmospheric CO2 accumulation by replacing fossil fuels and sequestering C. It could also improve soil productivity by C sequestration, and reduce NO3-1 contamination of water by absorbing N lost from fertilizer and agricultural waste if planted in filter strips on adjacent land. The objective of this study was to assess potential impacts of switchgrass on C and N balances by reviewing and synthesizing information from current literature, unpublished data and on-going research. Replacing fossil fuels with switchgrass, or any other biomass, will have a much greater effect on atmospheric CO2 than C sequestration. This is because replacing fossil fuels provides a cumulative effect, while C sequestration offers only a one-time benefit. Furthermore, switchgrass will provide net gains in C sequestration only if it replaces annual row crops, but not if it replaces grazed pasture. Nitrogen recovery by switchgrass in an Alabama study was 65.6%, which compares favorably with the 50% recovery frequently quoted as the norm for wheal (Triticum aestivum L.) and corn (Zea mays L). (C) 1998 Elsevier Science Ltd. All rights reserved.

2082^3^Bryant,J^Taylor,G^Frehner,M^1998^1^Photosynthetic acclimation to elevated CO2 is modified by source : sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment^9^21^2^159-168^^^^^Feb^^^^^6927
230^244^245^2575^344^348^384^745^from current literature, unpublished data and on-going research. Replacing fossil fuels with switchgrass, or any other biomass, will have a much greater effect on atmospheric CO2 than C sequestration. This is because replacing fossil fuels providA^6926^Artificial chalk grassland swards were exposed to either ambient air or air enriched to 600 mu mol mol(-1) CO2, using free-air CO2 enrichment technology, and subjected to an 8 week simulated grazing regime. After 14 months of treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco) activity (V- c,V-max) and electron transport mediated ribulose-1,5- bisphosphate (RuBP) regeneration capacity (J(max)), estimated from leaf gas exchange, were significantly lower in fully expanded leaves of Anthyllis vulneraria L. (a legume) and Sanguisorba minor Scop, grown in elevated CO2. After a change in source:sink balance brought about by defoliation, photosynthetic capacity was fully restored in A. vulneraria and S. minor, but acclimation continued in the grass Bromopsis erecta (Hudson) Fourr. Changes in net photosynthesis (P-n) with growth at elevated CO2 ranged from a 1.6% reduction in precut leaves of A. vulneraria to a 47.1% stimulation in postcut leaves of S. minor. Stomatal acclimation was observed in leaves of A. vulneraria (reduced stomatal density) and B. erecta (reduced stomatal conductance). The results are discussed in terms of whole-plant resource-use optimization and chalk grassland community competitive interactions at elevated CO2.

2083^2^Cannell,MGR^Thornley,JHM^1998^1^N-poor ecosystems may respond more to elevated [CO2] than N- rich ones in the long term. A model analysis of grassland^127^4^4^431-442^^^^^Apr^^^^^6929
1103^1262^344^362^376^384^392^512^715^977^fully expanded leaves of Anthyllis vulneraria L. (a legume) and Sanguisorba minor Scop, grown in elevated CO2. After a change in source:sink balance brought about by defoliation, photosynthetic capacity was fully restored in A. vulneraria and S. minor, but acclimation continued in the grass Bromopsis erecta (Hudson) Fourr. Changes in net photosynthesis (P-n) with growth at elevated CO2 ranged from a 1.6% reduction in precut leaves of A. vulneraria to a 47.1% stimulation in postcut leaves of S. minor. Stomatal acclimation was observed in leavesA^6928^The Hurley Pasture Model was used to examine the short and long-term responses of grazed grasslands in the British uplands to a step increase from 350 to 700 mu mol mol(-1) CO2 concentration ([CO2]) with inputs of 5 or 100 kg N ha(-1) y(- 1). In N-rich grassland, [CO2] doubling quickly increased net primary productivity (NPP), total carbon (C-sys) and plant biomass by about 30%. By contrast, the N-poor grassland underwent a prolonged 'transient', when there was little response, but eventually NPP, C-sys and plant biomass more than doubled. The 'transient' was due to N immobilization and severe depletion of the soil mineral N pool. The large long-term response was due to slow N accumulation, as a result of decreased leaching, decreased gaseous N losses and increased N- 2-fixation, which amplified the CO2 response much more in the N-poor than in the N-rich grassland. It was concluded that (i) ecosystems use extra carbon fixed at high [CO2] to acquire and retain nutrients, supporting the contention of Gifford et al. (1996), (ii) in the long term, and perhaps on the real timescale of increasing [CO2], the response (in NPP, C-sys and plant biomass) of nutrient-poor ecosystems may be proportionately greater than that of nutrient-rich ones, (iii) short-term experiments on nutrient-poor ecosystems may observe only the transient responses, (iv) the speed of ecosystem responses may be limited by the rate of nutrient accumulation rather than by internal rate constants, and (v) ecosystem models must represent processes affecting nutrient acquisition and retention to be able to simulate likely real-world CO2 responses.

2084^5^Entry,JA^Runion,GB^Prior,SA^Mitchell,RJ^Rogers,HH^1998^1^Influence of CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation in longleaf pine seedlings^206^200^1^3-11^^^^^Mar^^^^^6931
1086^1096^1262^3151^347^362^407^58^733^92^d. It was concluded that (i) ecosystems use extra carbon fixed at high [CO2] to acquire and retain nutrients, supporting the contention of GiA^6930^One-year old, nursery-grown longleaf pine (Pinus palustris Mill.) seedlings were grown in 45-L pots containing a coarse sandy medium and were exposed to two concentrations of atmospheric CO2 (365 or 720 mu mol(-1)) and two levels of nitrogen (N) fertility (40 or 400 kg N ha(-1) yr(-1)) within open top chambers for 20 months. At harvest, needles, stems, coarse roots, and fine roots were separated and weighed. Subsamples of each tissue were frozen in liquid N, lyophilized at -50 degrees C, and ground to pass a 0.2 mm sieve. Tissue samples were analyzed for carbon (C), N, nonpolar extractives (fats, waxes, and oils = FWO), nonstructural carbohydrates (total sugars and starch), and structural carbohydrates (cellulose, lignin, and tannins). Increased dry weights of each tissue were observed under elevated CO2 and with high N; however, main effects of CO2 were significant only on belowground tissues. The high N fertility tended to result in increased partitioning of biomass aboveground, resulting in significantly lower root to shoot ratios. Elevated CO2 did not affect biomass allocation among tissues. Both atmospheric CO2 and N fertility tended to affect concentration of C compounds in belowground, more than aboveground, tissues. Elevated CO2 resulted in lower concentrations of starch, cellulose, and lignin, but increased concentrations of FWO in root tissues. High N fertility increased the concentration of starch, cellulose, and tannins, but resulted in lower concentrations of lignin and FWO in roots. Differences between CO2 concentrations tended to occur only with high N fertility. Atmospheric CO2 did not affect allocation patterns for any compound; however the high N treatment tended to result in a lower percentage of sugars, cellulose, and lignin belowground.
s of each tissue were observed under elevated CO2 and with high N; however, main effects of CO2 were significant only on belowground tissues. The high N fertility tended to result in increased partitioning of biomass aboveground, resulting in signifi2085^8^Fernandez,MD^Pieters,A^Donoso,C^Tezara,W^Azkue,M^Herrera,C^Rengifo,E^Herrera,A^1998^1^Effects of a natural source of very high CO2 concentration on the leaf gas exchange, xylem water potential and stomatal characteristics of plants of Spatiphylum cannifolium and Bauhinia multinervia^84^138^4^689-697^^^^^Apr^^^^^6933
1208^2060^243^344^348^376^398^431^92^968^ fertility increased the concentration of starch, cellulose, and tannins, but resulted in lower concentrations of lignin and FWO in roots. Differences between CO2 concentrations tended to occur only with high N fertility. Atmospheric CO2 did not affect allocation patterns for any compound; however the high N treatment tended to result in a lower percentage of sugars, cellulose, and lignin belowground.
s of each tissue were observed under elevated CO2 and with high N; however, main effects of CO2 were significant only on belowground tissues. The high N fertility tended to result in increased partitioning of biomass aboveground, resulting in signifiA^6932^The effect of a very high CO2 mole fraction (27 000-35 000 mu mol mol(-1)) on photosynthesis and water relations was studied during the dry and the rainy season in plants of Spatiphylum cannifolium (Dryand.) Schott and Bauhinia multinervia (H.B.K.) DC. growing near natural cold CO2 springs. Xylem water potential in plants of both species was lowered by drought, high CO2 growth-concentration decreasing it further in S. cannifolium. In plants of both species growing under high CO2 concentration photosynthetic rates measured at a CO2 mole fraction of 1000 mu mol mol(-1) were higher than in plants growing at ambient CO2 mole fraction and measured at 350 mu mol mol(-1). The response was the result of a direct effect of CO2 on the photosynthetic machinery. Changes in carboxylation efficiency in response to high CO2 were found during the rainy season, with an increase in S. cannifolium and a decrease in B. multinervia; a significant interaction between growth CO2 concentration and season in B. multinervia resulted from significant effects of both factors. An increase in intrinsic water-use efficiency due to high CO2 was determined in both species by an increase in photosynthetic rate as well as a decrease in leaf conductance. In high-CO2 plants of S. cannifolium a 71 % decrease in stomatal density and 73 % in stomatal index suggested that CO2 affected stomatal initiation, whereas in B. multinervia an 85 % decrease in stomatal index and a 72 % decrease in stomatal density indicated that CO2 influenced stomatal initiation as well as epidermal cell expansion. Our results indicate that very high CO2 concentrations did not inhibit photosynthesis in these species, and that growth under high CO2 allowed plants to attain carbon balances higher than those of plants growing under low CO2. This was particularly so during the dry season, since the photosynthetic rates at the corresponding ambient concentration were higher in plants nearer the springs, and carboxylation efficiency and some stomatal characteristics of both species apparently acclimated to high CO2, but patterns were not consistent and bore no obvious relationship to photosynthetic capacity.

2086^2^Garcia-Ibilcieta,D^Pushnik,JC^1997^1^Differential gene displays from Pinus ponderosa seedlings experiencing elevated CO2 stress^359^11^9^A1104^^^^^31 Jul

2087^3^Grodzinski,B^Jiao,JR^Leonardos,ED^1998^1^Estimating photosynthesis and concurrent export rates in C-3 and C-4 species at ambient and elevated CO2^8^117^1^207-215^^^^^May^^^^^6936
130^1538^2600^3152^348^385^632^788^n as well as epidermal cell expansion. Our results indicate that very high CO2 concentrations did not inhibit photosynthesis in these species, and that growth under high CO2 allowed plants to attain carbon balances higher than those of plants growing under low CO2. This was particularly so during the dry season, since the photosynthetic rates at the corresponding ambient concentration were higher in plants nearer the springs, and carboxylation efficiency and some stomatal characteristics of both sA^6935^The ability of 21 C-3 and C-4 monocot and dicot species to rapidly export newly fixed C in the light at both ambient and enriched CO2 levels was compared. Photosynthesis and concurrent export rates were estimated during isotopic equilibrium of the transport sugars using a steady-state (CO2)-C-14-labeling procedure. At ambient CO2 photosynthesis and export rates for C-3 species were 5 to 15 and 1 to 10 mu mol C m(-2) s(-1), respectively, and 20 to 30 and 15 to 22 mu mol C m(-2) s(-1), respectively, for C-4 species. A linear regression plot of export on photosynthesis rate of all species had a correlation coefficient of 0.87. When concurrent export was expressed as a percentage of photosynthesis, several C-3 dicots that produced transport sugars other than Suc had high efflux rates relative to photosynthesis, comparable to those of C-4 species. At high CO2 photosynthetic and export rates were only slightly altered in C, species, and photosynthesis increased but export rates did not in all C(3)species. The C-3 species that had high efflux rates relative to photosynthesis at ambient CO2 exported at rates comparable to those of C-4 species on both an absolute basis and as a percentage of photosynthesis. At ambient CO2 there were strong linear relationships between photosynthesis, sugar synthesis, and concurrent export. However, at high CO2 the relationships between photosynthesis and export rate and between sugar synthesis and export rate were not as strong because sugars and starch were accumulated.

2088^3^Hunt,HW^Morgan,JA^Read,JJ^1998^1^Simulating growth and root-shoot partitioning in prairie grasses under elevated atmospheric CO2 and water stress^52^81^4^489-501^^^^^Apr^^^^^6938
137^2139^230^314^374^407^423^427^494^57^t produced transport sugars other th