The photosynthetic responses of wheat (Triticum aestivum L.) leaves to different levels of drought stress were analyzed in potted plants cultivated in growth chamber under moderate light. Low-to-medium drought stress was induced by limiting irrigation, maintaining 20 % of soil water holding capacity for 14 days followed by 3 days without water supply to induce severe stress. Measurements of CO2 exchange and photosystem II (PSII) yield (by chlorophyll fluorescence) were followed by simultaneous measurements of yield of PSI (by P700 absorbance changes) and that of PSII. Drought stress gradually decreased PSII electron transport, but the capacity for nonphotochemical quenching increased more slowly until there was a large decrease in leaf relative water content (where the photosynthetic rate had decreased by half or more). We identified a substantial part of PSII electron transport, which was not used by carbon assimilation or by photorespiration, which clearly indicates activities of alternative electron sinks. Decreasing the fraction of light absorbed by PSII and increasing the fraction absorbed by PSI with increasing drought stress (rather than assuming equal absorption by the two photosystems) support a proposed function of PSI cyclic electron flow to generate a proton-motive force to activate nonphotochemical dissipation of energy, and it is consistent with the observed accumulation of oxidized P700 which causes a decrease in PSI electron acceptors. Our results support the roles of alternative electron sinks (either from PSII or PSI) and cyclic electron flow in photoprotection of PSII and PSI in drought stress conditions. In future studies on plant stress, analyses of the partitioning of absorbed energy between photosystems are needed for interpreting flux through linear electron flow, PSI cyclic electron flow, along with alternative electron sinks.

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Genetically engineered tobacco (Nicotiana tabacum) with the ability to synthesis glycinebetaine was established by introducing the BADH gene for betaine aldehyde dehydrogenase from spinach (Spinacia oleracea). The genetic engineering enabled the plants to accumulate glycinebetaine mainly in chloroplasts and resulted in enhanced tolerance to high temperature stress during growth of young seedlings. Moreover, CO 2 assimilation of transgenic plants was significantly more tolerant to high temperatures than that of wild-type plants. The analyses of chlorophyll fluorescence and the activation of Rubisco indicated that the enhancement of photosynthesis to high temperatures was not related to the function of photosystem II but to the Rubisco activase-mediated activation of Rubisco. Western-blotting analyses showed that high temperature stress led to the association of Rubisco activase with the thylakoid membranes from the stroma fractions. However, such an association was much more pronounced in wild-type plants than in transgenic plants. The results in this study suggest that under high temperature stress, glycinebetaine maintains the activation of Rubisco by preventing the sequestration of Rubisco activase to the thylakoid membranes from the soluble stroma fractions and thus enhances the tolerance of CO 2 assimilation to high temperature stress. The results seem to suggest that engineering of the biosynthesis of glycinebetaine by transformation with the BADH gene might be an effective method for enhancing high temperature tolerance of plants.High temperature stress is one of the environmental factors that limit plant growth (Levitt, 1980;Boyer, 1982;Frova, 1997). Photosynthesis is one of the physiological processes that are most sensitive to high temperature stress (Berry and Bjö rkman, 1980). Inhibition of photosynthesis by high temperature stress is a common occurrence for plants in tropical and subtropical regions and the temperate zones where plants are exposed periodically to high temperatures (Larcher, 1995). It is well documented that high temperature stress causes damage to photosynthetic electron transport (Berry and Björkman, 1980). For many years, PSII has long been considered the most heatsensitive component of the photosynthetic apparatus (Berry and Bjö rkman, 1980). However, it appears that PSII is unaffected at temperatures that inhibit CO 2 fixation (Weis, 1981a(Weis, , 1981b, and that PSII is only inhibited by severe heat stress when the temperature is higher than 45°C (Havaux, 1993(Havaux, , 1996. Recent studies seem to support that PSII activity is not limiting at temperatures that inhibit CO 2 fixation and that CO 2 fixation is most sensitive to heat stress due to the inhibition of activation of Rubisco via a direct effect on Rubisco activase (Feller et al., 1998; Crafts-Brandner, 2004a, 2004b;Haldimann and Feller, 2004).Glycinebetaine (GB) is one of the organic compatible solutes that can accumulate rapidly in many plants under salinity stress, drought, and low temperature (McCue and Hanson, 1990...

The effects of exogenous application of glycinebetaine (GB) (10 mM) on growth, leaf water content, water use efficiency, photosynthetic gas exchange, and photosystem II photochemistry were investigated in maize plants subjected to salt stress (50 and 100 mM NaCl). Salt stress resulted in the decrease in growth and leaf relative water content as well as net photosynthesis and the apparent quantum yield of photosynthesis. Stomatal conductance, evaporation rate, and water use efficiency were decreased in salt‐stressed plants. Salt stress also caused a decrease in the actual efficiency of PSII (ΦPSII), the efficiency of excitation energy capture by open PSII reaction centers (Fv′/Fm′), and the coefficients of photochemical quenching (qP) but caused an increase in non‐photochemical quenching (NPQ). Salt stress showed no effects on the maximal efficiency of PSII photochemistry (Fv/Fm). On the other hand, in salt‐stressed plants, GB application improved growth, leaf water content, net photosynthesis, and the apparent quantum yield of photosynthesis. GB application also increased stomatal conductance, leaf evaporation rate, and water use efficiency. In addition, GB application increased ΦPSII, Fv′/Fm′, and qP but decreased NPQ. However, GB application showed no effects on Fv/Fm. These results suggest that photosynthesis was improved by GB application in salt‐stressed plants and such an improvement was associated with an improvement in stomatal conductance and the actual PSII efficiency.