Water stress is one of the major abiotic stresses in agriculture worldwide. In order to assess photosynthesis response and grain yield of 25 wheat genotypes under water deficit (post-anthesis stress) conditions, a 2-year study (2010-12) was carried out as a split-plot arrangement using randomized complete block design with three replications. The most sensitive gas exchange variable to water deficit was found to be mesophyll conductance (gm) (62% reduction), followed by photosynthesis rate (Pn) (42% reduction). Water deficit also reduced grain yield by an average of 45%. Pn and gm were significantly correlated with grain yield under both conditions. Higher chlorophyll content was associated with higher Pn under water deficit conditions. Maintenance of greater green leaf area during grain filling period was related to greater grain yield. Genotypes with higher Pn and gm were those with optimum grain yield (i.e. cvs. Zarrin and Darya), hence, Pn and gm were found to be the appropriate indices for screening wheat genotypes under the terminal water deficit conditions.

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