Field evaluation of short-term heat stress pre- and post-flowering on physiological parameters in spring bread wheat (Triticum aestivum L. ) under warm climate of Ahvaz

Heat stress during reproductive development is the main limitation in the production of wheat in most of the wheat fields in the world. It is important to recognize the physiological and molecular mechanisms associated with heat tolerance and the detection of screening methods in improving plants to tolerate heat. In Iran data from the effect of short-term heat wave on yield and physiological mechanisms do not exist under field conditions. The objective of this study was to determine the effects of short-term heat stress near flowering and early grain filling on grain yield and physiological parameters.

This research was conducted at research farm of Khuzestan Agricultural Sciences and Natural Resources University, located 35 km northeast of Ahwaz, in 2014. Four wheat genotypes (Chamran. Maroon, Arvand and Atrak) were exposed to heat stress (maximum 35 °C) for a three-day in the field with a portable heat chamber at two different stages, near flowering (H1) and early grain set (H2). Chlorophyll content was measured using manual chlorophyll. Stomatal conductance was performed from the top three leaves of selected plants. Chlorophyll concentration was calculated using the Arnon method (1949). Cell membrane thermostability (CMTS) were calculated using the following equation: CMTS (%) = [1- (EC1 / EC2)] × 100where EC1 and EC2 are the primary electrical conductivity (before the autoclave) and the secondary (after the autoclave), respectively. Proline was maesured using Bates method (1973). Ascorbate peroxidase (APX) activity was measured using Nakano and Asada method (1987). Catalase (CAT) activity was measured using Aebi method (1983), peroxidase activity (POX) was measured using Chance and Maehly method (1955), Superoxide dismutase was measured using Biochamp and Fridovich method (1971). Malon de aldehyde (MDA) was measured using Hess and Packer (1969). All calculations were performed using the SAS-9.4 statistical software. Factor analysis was done by using principal component analysis and Varimax rotation on the temporary factor.

The results of analysis of variance showed that there were significant differences between heat stress levels and the traits of genotypes. Short-term heat stress (H1 or H2) decreased the average grain yield of Chamran, Maroon, Arvand and Atrak genotypes by 19.6, 18.6, 17.8 and 11.2 percent. Heat stress significantly caused to reduce water potential, chlorophyll fluorescence, leaf relative water content and cell membrane thermostability whereas stomatal conductance, chlorophyll destruction rate and proline content increased. Although there was no significant effect of short term heat stress on total chlorophyll concentration in this experiment, there was a negative correlation between grain yield and total chlorophyll concentration (r = -0.67 in H1 and r = -0.77 in H2). The activity of catalase, peroxidase and superoxide dismutase increased significantly in response to H1 or H2. The results of the factor analysis showed that four factors explained 86.7% and three factors explained 86.4% the variance among varieties in H1 and H2, respectively. Atrek, Chamran and Arvand varieties tolerated heat stress than Maroon variety by more photosynthesis persistency, higher metabolite content and more enzyme defense mechanism. It could be argued that cultivars with slower rate of leaf senescence after heat exposure and more enzymatic protection could be more tolerant to heat stress.