PHOTOSYNTHETICA 44 (2): 312-315, 2006 312 BRIEF COMMUNICATION Effects of water stress and high temperature on gas exchange and chlorophyll fluorescence in Triticum aestivum L. I.A. HASSAN Department of Botany, Faculty of Science, Alexandria University, 21526 El Shatby, Alexandria, Egypt Abstract Wheat plants grown in controlled growth chambers were exposed to drought stress (DS) and high temperature (HT) singly and in combination (DS+HT). The effects of these two stresses on net photosynthetic rate (P N ), stomatal conductance (g s ), intercellular CO 2 concentration (C i ), quantum efficiency of photosystem 2 (Ф PS2 ), variable to maximum chlorophyll (Chl) fluorescence (F v /F m ), photochemical (q p ) and non-photochemical (NPQ) Chl fluorescence, and yield were investigated. Grain yield was decreased by 21 % due to DS, while it was increased by 26 % due to HT. P N , g s , C i , and Chl fluorescence were dramatically reduced to DS, HT, and their interaction, except NPQ which showed an increase due to HT. Additional key words: grain yield; intercellular CO 2 concentration; quantum efficiency of photosystem 2; spikelet; stomatal conductance; yield. ——— Drought stress (DS) and high temperature (HT) are among the most important environmental factors that limit net photosynthetic rate (P N ) (Hassan et al. 1998, Yordanov et al. 1999, 2000). Water stress leads to a pro- gressive suppression of photosynthetic carbon assimi- lation in desiccation-tolerant and intolerant wheat plants (Deltoro et al. 1998). Reduction in P N is attributed to both stomatal and non-stomatal control (Ort et al. 1994, Shangguan et al. 1999). Increases in temperatures may cause expansion of production into higher elevations. The grain filling period may be reduced as HT may detrimentally affect sensitive developmental stages such as flowering, thereby reducing grain yield and altering crop quality. Increases in temperature by 2–4 °C caused a reduction in yield of many crops depending on species and cultivar used (Yates and Strzpek 1998). Exposure of plants to water stress can alter response to increased temperature. Havaux (1992) stated that exposure of potato leaves to HT caused an increase in activity of photosystem 2 (PS2) in water-stressed plants, as indicated by a slight increase in variable to maximum chlorophyll (Chl) fluorescence (F v /F m ). PS2 plays an important role in the response of plants to environmental stresses and it is a very sensitive component of the photosynthetic systems (Berry and Björkman 1980, Chaves 1991, Yordanov et al. 1999, 2000). The change in photosynthesis would be reflected ultimately in growth and yield. The aim of the present investigation was to study the effects of DS and HT singly and in combination on Chl fluorescence, P N , stomatal conductance (g s ), and yield of wheat plants. Seeds of wheat (Triticum aestivum L.) were obtained from the Department of Agronomy, Alexandria University, Egypt. They were sown in 20 cm 2 pots, 5 seeds in each pot with multipurpose compost in a glasshouse [temperature 24–25 o C day/night, air humidity 60–65 %, 16-h photoperiod with additional lighting at 450 µmol m –2 s –1 (photosynthetic photon flux density, PPFD)], total irradiance was 675 µmol m –2 s –1 . After appearance of expanded top leaf (flag leaf) (12 d after sowing), plants were thinned to one seedling per pot. Pots then were transferred to eight closed climate controlled growth chambers. Two chambers were assigned to DS treatment (withholding irrigation till soil moisture of 13 %). Two other chambers were assigned to HT treatment (40–42 o C), two others were assigned to interaction between DS and HT, while the last two ——— Received 25 July 2005, accepted 24 October 2005. Fax: +002 03 39 11 794, e-mail: i_hassan82@hotmail.com Acknowledgments: I am indebted to the British Council for their continuous financial support. I thank Prof. Nigel Bell (Imperial College, London) and Dr. I. Twefik (Westminster University, UK) for their help in analyses.