Genetic Control of Thermo-Tolerance in Spring Wheat as Measured by Canopy Temperature Depression and Cell Membrane Thermostability

Terminal heat stress is an important production constraint for spring wheat and affects many plant biological activities. This comprehensive three years study was conducted to determine the genetic mechanism of heat tolerance through diallel analysis biometric technique in three environmental conditions i. e. stress free (Timely planting) and two heat stress regimes (Late planting and polyhouse-timely planting environments). Initially two hundred wheat strains of diverse origin were screened and identified four thermo-tolerant and three thermo-labile strains during year 2009-2010. Forty two F1 hybrids were developed by hybridizing seven genotypes according to 77 full diallel fashion during 2010-2011. Response of forty nine genotypes to high temperature stress was measured by canopy temperature depression (CTD), cell membrane thermo stability (CMT) and grain yield per plant during next crop year 2011-2012. Data were subjected to diallel analysis of variance and estimation of variation of genetic parameters. Additive gene effects were highly significant for three traits in all test regimes. The overall dominance components were smaller but highly significant in stress free and heat stress indicating important role of dominance. Likewise highly significant b1 (Directional dominance) item indicated the directional dominance deviations of the genes. Symmetrical gene distribution and unimportant role of specific genes for canopy temperature depression and grain yield per plant were represented by non-significant b2 (symmetry of gene distribution) and b3 (specific gene effects) items respectively. Regression and array variances analyses suggested the adequacy of model for canopy temperature depression and grain yield per plant. Estimation of genetic components of variation indicated the importance of additive gene effects in acquired thermotolerance for all the traits in three test regimes. Results indicated that heat tolerance based on CMT and CDT measurements can be enhanced by utilizing the genetic variability existing within genetic resources. In conclusion, diallel biometrical technique and integrated use of modified pedigree method of selection by involving parents like Ch-86, Bhakkar-2002, SH-02 and V00183 and use of specific crosses like Bhakkar-02 SH-02, V00183 Chakwal-86 and Chakwal-86 SH-02 would be more effective for the evolution of post anthesis thermotolerant spring wheat varieties for hot irrigated dry climates.