No abstract

Photosynthetic responses of sunflower plants grown for 52 d in ambient and elevated CO(2) (A=350 or E=700 micromol mol(-1), respectively) and subjected to no (control), mild or severe water deficits after 45 d were analysed to determine if E modifies responses to water deficiency. Relative water content, leaf water potential (Psi(w)) and osmotic potential decreased with water deficiency, but there were no effects of E. Growth in E decreased stomatal conductance (g(s)) and thereby transpiration, but increased net CO(2) assimilation rate (P(n), short-term measurements); therefore, water-use efficiency increased by 230% (control plants) and 380% (severe stress). Growth in E did not affect the response of P(n) to intercellular CO(2) concentration, despite a reduction of 25% in Rubisco content, because this was compensated by a 32% increase in Rubisco activity. Analysis of chlorophyll a fluorescence showed that changes in energy metabolism associated with E were small, despite the decreased Rubisco content. Water deficits decreased g(s) and P(n): metabolic limitation was greater than stomatal at mild and severe deficit and was not overcome by elevated CO(2). The decrease in P(n) with water deficiency was related to lower Rubisco activity rather than to ATP and RuBP contents. Thus, there were no important interactions between CO(2) during growth and water deficit with respect to photosynthetic metabolism. Elevated CO(2 )will benefit sunflower growing under water deficit by marginally increasing P(n), and by slowing transpiration, which will decrease the rate and severity of water deficits, with limited effects on metabolism.

Winter 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 |imol mol"^), temperature (tracking ambient and ambient +4 °C) and nitrogen application (equivalent to 87 and 489 kg ha* 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.

The effect of short-term water stress on photosynthesis of two sunflower hybrids (Helianthus annuus L. cv Sungro-380 and cv , differing in productivity under field conditions, was measured. The rate of CO2 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 C02; 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. Photosynthetic CO2 assimilation per unit area of leaf surface depends on the capacity of the plant's photosynthetic mechanism and on those environmental factors, such as CO2 supply and radiation, which are the substrates for the process, and on those conditions (e.g. temperature and water supply) that affect the mechanism. The rate of photosynthesis, in combination with the leaf area, determines plant productivity (2,10,11,18 CO2 fixation than C3 plants in bright light and high temperatures, and the biochemical and physiological origins of this are well established (19). However, although there are differences in assimilation rate between species within the C3 and C4 groups, differences between closely related plants are less well established. Also, the causes of differences and how they relate to the characteristics of the photosynthetic mechanism between species and varieties are poorly understood (19,21,28).Genetic variation in rates of photosynthesis per unit leaf area (Pn2) has been reported for cultivars of some crop species (2, 9, 12). Possible causes of the differences in Pn between genotypes are variations in amounts or activities of specific proteins, pigments, etc. and differences in leaf structure, cell size, or stomatal frequency. Small but consistent differences in assimilation were detected between hexaploid wheats under well-watered field conditions (9), but such differences between other wheat genotypes were attributed to ploidy (2). Substantial differences (up to 35%) have been reported between cultivars of field bean (13) and in pea (21), the latter attributed to variation in Chl content. Assimilation rates were best correlated with the ratio of cell surface area to leaf surface area (19), in analyses of 112 C3 and 6 C4 species. The correlations among cell size, ploidy, and assimilation rates (22) and also the effects of nutrition on the relationship (18) makes the analysis complex. Variation in Pn has also been attributed to the amounts and activities of Rubisco. Both the amount and the activity of Rubisco can limit photosynthesis in C3 plants (11,18,22,26,28,29). Genetic variability in Rubisco content related to ploidy ...

Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO2 concentrations (350 and 700 μmol mol−1), two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4°C] and two rates of nitrogen supply (equivalent to 489 kg ha−1 and 87 kg ha−1). Leaves grown at 700 μmol 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.