Effect of heat stress during grain filling on photosynthesis and grain yield of bread wheat (Triticum aestivum L.) genotypes

To investigate the effect of heat stress during grain filling period on photosynthesis, gas exchanges and chlorophyll fluorescence parameters of bread wheat genotypes, two different experiments in the field and phytotron of Chamran University in Ahwaz, Iran, was implemented during 2010-2011 growing season. In the first experiment, 10 medium maturity spring bread wheat genotypes in two; optimum (1 Nov.) and late (21 Dec.) sowing dates as a factorial arrangements in randomized complete block design with three replications were planted. In the second experiment the same genotypes were planted in the pots in randomized complete block design with three replications. Ten days after anthesis, all pots of each genotype were divided in two groups randomly, and then were moved to two separate phytotrons with different temperature treatments i.e.; normal (25/16 °C (max/min)) and heat stress (37/25 °C (max/min)). Pots remained in this condition until maturity. Results showed that under heat stress condition, due to late sowing, flag leaf net photosynthetic rate decreased by at least 11% and at the most 29.5% depending on wheat genotypes. Mesophyll conductance reduction and increasing stomatal conductance were associated with photosynthetic rate reduction. But in cv. Falat and line S- 78-11, stomatal conductance was reduced under heat stress conditions due to late sowing, and stomatal closure was a determining factor in photosynthetic loss. Non-stomatal factors contributed to photosynthetic loss in other genotypes. In late sowing date (mild heat stress conditions in the field) the quantum yield of PSII (ΦPSII) and maximum quantum efficiency of PSII under given light conditions (F′v/F′m) decreased, however, nonphotochemical quenching (NPQ) increased and despite a significant decrease in maximum quanthum yield of PSII under given dark conditions (Fv/Fm), it remained in normal range, due to increase in photoprotective. In severe heat stress conditions (in phytoteron) ΦPSII, F′v/F′m and Fv/Fm parameters significantly reduced, and NPQ values decreased. In fact, under severe heat stress conditions, sensitivity electron transport chain and Fv/Fm parameter increased in sensitive genotypes. Considering the lack of correlation between grain yield and chlorophyll fluorescence parameters in both experiments and also the different rankings of genotypes in different environments, chlorophyll fluorescence parameters solely can not be used as a criterion for the selection of heat tolerant wheat cultivars.