Water stress substantially alters plant metabolism, decreasing plant growth and photosynthesis 1 , 2 , 3 , 4 and profoundly affecting ecosystems and agriculture, and thus human societies 5 . There is controversy over the mechanisms by which stress decreases photosynthetic assimilation of CO 2 . Two principal effects are invoked 2 , 4 : restricted diffusion of CO 2 into the leaf, caused by stomatal closure 6 , 7 , 8 , and inhibition of CO 2 metabolism 9 , 10 , 11 . Here we show, in leaves of sunflower ( Helianthus annuus L.), that stress decreases CO 2 assimilation more than it slows O 2 evolution, and that the effects are not reversed by high concentrations of CO 2 12 , 13 . Stress decreases the amounts of ATP 9 , 11 and ribulose bisphosphate found in the leaves, correlating with reduced CO 2 assimilation 11 , but the amount and activity of ribulose bisphosphate carboxylase-oxygenase (Rubisco) do not correlate. We show that ATP-synthase (coupling factor) decreases with stress and conclude that photosynthetic assimilation of CO 2 by stressed leaves is not limited by CO 2 diffusion but by inhibition of ribulose biphosphate synthesis, related to lower ATP content resulting from loss of ATP synthase.

The effects of micropropagation conditions on avocado (Persea americana Mill.) have been measured in leaves and plants cultured in vitro. The consequences of the type and concentration of sugar in the medium and of carbon dioxide concentration in the atmosphere on the rates of photosynthesis and amounts of ribulose 1,5-biphosphate carboxylase-oxygenase (EC 4.1.1.39; Rubisco) and total soluble protein (TSP) were measured. At the highest sucrose supply (87.6 mM), Rubisco content was substantially decreased in leaves, and even more when elevated CO2 (1 000 μmol·mol−1) was supplied. Maximum photosynthetic rate (Pmax) was significantly decreased when plants developed in high sucrose and elevated CO2. However, Rubisco concentration was significantly greater when glucose was supplied at the same molar concentration or when the concentration of sucrose was small (14.6 mM), and no differences were observed due to the CO2 concentration in the air in these treatments. The ratio of Rubisco to total soluble protein (Rubisco/TSP) was dramatically decreased in plants grown in the highest concentration of sucrose and with elevated CO2. Leaf area and ratio of leaf fresh weight/(stem + root) fresh weight, were greater in plants grown with CO2 enriched air. However, upon transplanting, survival was poorer in plants grown on low sucrose/high CO2 compared to those grown on high sucrose/high CO2.

Effects on sugar beet (Beta vulgaris L.) of current and elevated CO2 and temperature alone and in combination and their interactions with abundant and deficient nitrogen supply (HN and LN, respectively) have been studied in three experiments in 1993, 1994 and 1995. Averaged over all experiments, elevated CO2 (600 μmol mol–1 in 1993 and 700 μmol mol–1 in 1994 and 1995) increased total dry mass at final harvest by 21% (95% confidence interval (CI) = 21, 22) and 11% (CI = 6, 15) and root dry mass by 26% (CI = 19, 32) and 12% (CI = 6, 18) for HN and LN plants, respectively. Warmer temperature decreased total dry mass by 11% (CI = – 15, – 7) and 9% (CI = – 15, – 5) and root dry mass by 7% (CI = – 12, – 2) and 7% (CI = – 10, 0) for HN and LN plants, respectively. There was no significant interaction between temperature and CO2 on total or root dry mass. Neither elevated CO2 nor temperature significantly affected sucrose concentration per unit root dry mass. Concentrations of glycinebetaine and of amino acids, measured as α‐amino‐N, decreased in elevated CO2 in both N applications; glycinebetaine by 13% (CI = – 21, – 5) and 16% (CI = – 24, – 8) and α‐amino‐N by 24% (CI = – 36, – 11) and 16% (CI = – 26, – 5) for HN and LN, respectively. Warmer temperature increased α‐amino‐N, by 76% (CI = 50, 107) for HN and 21% (CI = 7, 36) for LN plants, but not glycinebetaine.

Water stress greatly affects plant processes, including photosynthesis, decreasing dry matter production over much of the earth’s surface so that harvested crop yields are below the genetic potential. Need to provide food and fibre for a large and rapidly growing human population (particularly in drought prone areas) makes the long-sought goal of selecting less sensitive crops ever more urgent. However, the mechanisms of response and ways of avoiding or over-coming stress are still not well understood. Selection breeding has not substantially improved production under stress, but would benefit — in the long run — from better understanding of the basic mechanisms. Application of molecular biological methods to improve responses to stress depends crucially upon knowledge of mechanisms: here photosynthesis is considered, to clarify the basic metabolic limitations and impacts on other stress responses.

ABSTRACTWinter wheat (Triticum aestivuin cv. Mercia) was grown in a controlled‐environment facility under simulated Held conditions at ambient (360μmol mol−1) and elevated (690 μmol mol−1) CO2 concentrations. Some of the plants were shaded to mimic cloudy conditions during three periods of about 20d duration between terminal spikelet and start of grain‐fill, giving 16 treatments in all. Elevated CO2, increased grain yield by about 20%, while shading in any period decreased yield, with the greatest effect in the last period, encompassing anthesis. No interactions between these effects were significant for grain yield, but there were complex interactions for mean grain size. Observed effects of shading and elevated CO2 on biomass production were well predicted by a simulation model. Observed effects of treatments on yield could be related to effects on biomass using a simple model which assumes that yield is proportional to biomass production, with coefficients of 0.42 (g grain yield g−1 biomass) for the first two periods and 0.74 for the last period. Wheat models should therefore include developmental changes in sensitivity of yield to biomass production, but biomass changes induced by different CO2 concentrations or light environments can be treated as having equivalent effects on grain yield.

ABSTRACTWinter wheat (Triticum aestivum L., ev. Mercia) was grown in a controlled‐environment facility at two CO2 concentrations (targets 350 and 700 μmol mol−1), and two temperature regimes (tracking ambient and ambient + 4°C). Observations of phenology, canopy growth, dry matter production and grain yield were used to test the ARCWHEAT1 simulation model. Dry‐matter production and grain yield were increased at elevated CO2 concentration (27 and 39%, respectively) and reduced at increased temperature (−16 and −35%, respectively). ARCWHEAT1 substantially underestimated canopy growth for all treatments. However, differences in the facility environment from field conditions over the winter, indicated by the unusually rapid canopy growth observed in this period, meant that empirical model relationships were being used outside the conditions for which they were developed. The ARCWHEAT productivity submodel, given observed green area indices as inputs, overestimated the effect of CO2 on productivity. An alternative, more mechanistic submodel of productivity, based on the SUCROS87 and Farquhar & von Caemmerer models, simulated observed crop biomass very closely. When these productivity simulations were inputed into the ARCWHEAT1 partitioning and grain‐fill submodels, grain yield was predicted poorly, mainly as a result of the assumption that the number of grains is proportional to total growth during a short pre‐anthesis phase. While yield was not correlated with growth in this phase, it was correlated with growth in longer pre‐anthesis phases, indicating that ARCWHEAT1 could be improved by taking into account the contribution of earlier growth in determining yield.

Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO, concentrations (350 and 700 mu mol mol(-1)) two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4 degrees C] and two rates of nitrogen supply (equivalent to 489 kg ha(-1) and 87 kg ha(-1)). Leaves grown at 700 mu mol mol(-1) CO2 had slightly greater photosynthetic capacity (10% mean increase over the experiment) than those grown at ambient CO2 concentration, but there were no differences in carboxylation efficiency or apparent quantum yield. The amounts of chlorophyll, soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area did not change with long-term exposure to elevated CO2 concentration. Thus winter wheat, grown under simulated field conditions, for which total biomass was large compared to normal field production, did not experience loss of components of the photosynthetic system or loss of photosynthetic competence with elevated CO2 concentration. However, nitrogen supply and temperature had large effects on photosynthetic characteristics but did not interact with elevated CO2 concentration. Nitrogen deficiency resulted in decreases in the contents of protein, including Rubisco, and chlorophyll, and decreased photosynthetic capacity and carboxylation efficiency. An increase in temperature also reduced these components and shortened the effective life of the leaves, reducing the duration of high photosynthetic capacity.

ABSTRACTWinter wheat (Triticum aestivum L., cv. Mercia) was grown in chambers under light and temperature conditions similar to the UK field environment for the 1990/1991 growing season at two levels each of atmospheric CO2 concentration (seasonal means: 361 and 692 μmol mol−1), temperature (tracking ambient and ambient +4°C) and nitrogen application (equivalent to 87 and 489 kg ha−1 total N applied). Total dry matter productivity through the season, the maximum number of shoots and final ear number were stimulated by CO2 enrichment at both levels of the temperature and N treatments. At high N, there was a CO2‐induced stimulation of grain yield (+15%) similar to that for total crop dry mass (+12%), and there was no significant interaction with temperature. This contrasts with other studies, where positive interactions between the effects of increases in temperature and CO2 have been found. Temperature had a direct, negative effect on yield at both levels of the N and CO2 treatments. This could be explained by the temperature‐dependent shortening of the phenological stages, and therefore, the time available for accumulating resources for grain formation. At high N, there was also a reduction in grain set at ambient +4°C temperature, but the overall negative effect of warmer temperature was greater on the number of grains (‐37%) than on yield (‐18%), due to a compensating increase in average grain mass. At low N, despite increasing total crop dry mass and the number of ears, elevated CO2 did not increase grain yield and caused a significant decrease under ambient temperature conditions. This can be explained in terms of a stimulation of early vegetative growth by CO2 enrichment leading to a reduction in the amount of N available later for the formation and filling of grain.

ABSTRACTThe requirements for the experimental study of the effects of global climate change conditions on plants are outlined. A semi‐controlled plant growth facility is described which allows the study of elevated CO2 and temperature, and their interaction on the growth of plants under radiation and temperature conditions similar to the field. During an experiment on winter wheat (cv. Mercia), which ran from December 1990 through to August 1991, the facility maintained mean daytime CO2 concentrations of 363 and 692 cm3 m−3 for targets of 350 and 700 cm3 m−3 respectively. Temperatures were set to follow outside ambient or outside ambient +4°C, and hourly means were within 0.5°C of the target for 92% of the time for target temperatures greater than 6°C. Total photosynthetically active radiation incident on the crop (solar radiation supplemented by artifieal light with natural photoperiod) was 2% greater than the total measured outside over the same period.

The effect of short-term water stress on photosynthesis of two sunflower hybrids (Helianthus annuus L. cv Sungro-380 and cv SH-3622), differing in productivity under field conditions, was measured. The rate of CO(2) assimilation of young, mature leaves of SH-3622 under well-watered conditions was approximately 30% greater than that of Sungro-380 in bright light and elevated CO(2); the carboxylation efficiency was also larger. Growth at large photon flux increased assimilation rates of both hybrids. The changes in leaf composition, including cell numbers and sizes, chlorophyll content, and amounts of total soluble and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein, and in Rubisco activity and amount of ribulose-1,5-bisphosphate (RuBP) were determined to assess the factors regulating the differences in assimilation of the hybrids at high and low water potentials. The amounts of chlorophyll, soluble protein, Rubisco protein and the initial activity of Rubisco and its activation state did not differ significantly between hybrids. However, unstressed leaves of SH-3622 had more, smaller cells per unit area and 60% more RuBP per unit leaf area than that of Sungro-380. Water stress developing over 4 days decreased the assimilation of both hybrids similarly. Changes in the amounts of chlorophyll, soluble and Rubisco protein, and Rubisco activity and activation state were small and were not sufficient to explain the decrease in photosynthesis; neither was decreased stomatal conductance (or stomatal "patchiness"). Reduction of photosynthesis per unit leaf area from 25 to 5 micromoles CO(2) per square meter per second in both hybrids was caused by a decrease in the amount of RuBP from approximately 130 to 40 micromoles per square meter in SH-3622 and from 80 to 40 micromoles per square meter in Sungro. Differences between hybrids and their response to water stress is discussed in relation to control of RuBP regeneration.