Screening for heat tolerant genotypes in bread wheat (T. aestivum L.) using stress tolerance indices

Heat is a major stress that severely affects  wheat productivity. The objective of the current study was estimating the effect of heat stress during the grain-filling stage for screening heat tolerant wheat genotypes by measuring six stress tolerance indices for grain yield and also measured the % of decline on other important traits. A set of 190 wheat genotypes along with four local checks were evaluated in two separate experiments, one under normal condition (timely sown) and the other under heat stress condition (late sown) in augmented block design at the research farm of Department of Genetics and Plant Breeding, C.C.S. University, Meerut (U.P) during rabi season of 2014-2015. The calculation of % declines revealed that there was  a significant decline in the performance of traits under heat stress environment (late sown) in comparison to a normal environment (timely sown). Three traits, namely grain yield/plant, biological yield/plant and grain weight/ spike showed 57.15 %, 53.66 % and 47.73 % decline, respectively under heat stress environment indicating that above mentioned three traits are highly affected by heat stress. To evaluate the susceptible and tolerant genotypes for  grain yield under heat stress environment, six stress tolerance indices viz. heat susceptibility index (HSI), mean productivity (MP), tolerance (TOL), stress tolerance index (STI), trait stability index (TSI) and trait index (TI) were calculated based on the grain yield under normal (timely sown) and stress (late sown) environments. The results of correlation between stress tolerance indices and grain yield in normal environment showed a significant positive relationship with MP (0.822**), STI (0.568**), TSI (0.316**), HSI (0.314**), TI (0.188*) and TOL (0.084). However, in stress environment, grain yield showed a significant positive correlation with TI (1.000**), STI (0.907**), MP (0.713**) and TOL (0.129) and negative correlation with HSI (-0.860**) and TSI (-0.808**).   Based on the correlation analysis, we have selected four stress indices viz.  HSI (heat susceptibility index), MP (mean productivity), STI (stress tolerance index) and TI (trait index) for their use in the selection of heat tolerant wheat genotypes. The results indicated that 9 genotypes (G51, G64, G71, G114, G119, G134, G139, G148 and G150) were heat tolerant and 12 genotypes (G1, G3, G7, G27, G38, G40, G77, G107, G136, G160, G171 and G187) were highly heat susceptible. The genotype G77 was the most heat sensitive genotype while genotype G119 and G139 showed a high yield under heat stress environment. Therefore, genotypes G119 and G139 were identified as a suitable genotype under late sowing environment and can be recommended for heat stress environment. Further, these two genotypes may be also employed in breeding programs aimed for developing heat tolerant varieties of wheat.