Combining Ability of Physiological and Yield Traits of Bread Wheat Diallel Crosses under Timely and Late Sowing Dates

WHEAT (Triticum aestivum L.) is a vulnerable crop to heat stress. A set of 12 wheat genotypes were evaluated under timely and late sowing dates using physiological and yield traits during the two years of 2015/2016 and 2016/2017. Consequently, seven genotypes were used in a half-diallel mating system. The parents and their 21F1 hybrids were evaluated under timely and late sowing dates during 2018/2019. The results indicated that both chlorophyll absorbance at anthesis and flag leaf temperature depression at milk stage were strongly associated (r= 0.6; P<0.001) with grain yield under heat stress induced by late sowing date. Therefore, using these traits is highly recommended screening tools for heat stress tolerance in wheat breeding programs. Significant differences were observed between the parental genotypes for their GCA and between their crosses for SCA for all traits under heat stress. The additive constituent of gene action was predominant. Among the seven parents, 1, 4, and 6 were the best general combiners for grain yield, 1000 grain weight, chlorophyll absorbance at anthesis, flag leaf temperature depression and canopy temperature depression at milk stage under heat stress. The best specific crosses for grain yield were 6×9, 6×11 and 1×4, for flag leaf temperature depression at milk stage were 6×11, 6×9 and 10×11 and for canopy temperature depression at milk stage were 4×6, 1×4 and 6×11 under heat stress. Taking advantage of both additive and non-additive components requires using bi-paternal and multi-parental mating approaches in wheat breeding programs targeted to develop heat-tolerant varieties.