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Terminal drought stress during grain filling period has recently become more common in the semiarid Mediterranean regions, where wheat (Triticum aestivum) is grown as an important winter cereal crop. The objective of this experiment was to study the effect of terminal drought stress on grain yield, gas exchange variables, and some physiological traits of nine bread wheat cultivars. An experiment was carried out in a split-plot arrangement using randomized complete blocks design with three replications during the 2010-2011 season at the research farm of Razi University, Iran. Based on the results obtained, post anthesis water deficit significantly decreased grain yield, biomass, 1,000 grain weight, and harvest index of wheat cultivars. Under terminal drought stress and control treatments, there were significant differences between cultivars in terms of all traits studied. Also, terminal drought stress decreased leaf net photosynthesis rate (P_n), stomatal conductance (g_s), transpiration rate, Chlorophyll a, b, and a/b, and increased leaf temperature and sub-stomatal CO₂ concentration. Cultivars differed in their response to water stress. In general, tolerant cultivars showed a higher P_n and g_s and leaf water content under both moisture conditions compared with susceptible ones. A greater reduction in g_s and transpiration rate and smaller reduction in P_n under stress condition led to a remarkably higher photosynthetic water use efficiency of the tolerant cultivars. Finally, it can be concluded that planting wheat variety DN-11 in areas with post-anthesis water stress was recommendable for maximizing grain yield.

Keywords: Photosynthesis, Relative water content, Stress Susceptibility Index, Water use efficiency

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