Assessment of Diversity for Terminal Heat Tolerance under Different Sowing Conditions in Bread Wheat (Triticum aestivum L. em.Thell.)

Wheat crop is facing various abiotic stresses due to changing environmental conditions as a result of global warming and heat stress is one of them which affects wheat crop during different growth stages and ultimately reduces grain yield very significantly. The present investigation was carried out with thirty two diverse genotypes of bread wheat in completely randomized block design with three replications at Norman E. Borlaug Crop Research Centre, G.B. Pant University of agriculture & Technology Pantnagar for the screening of wheat genotypes for terminal heat tolerance under three environments viz., timely sown (E1), late sown (E2) and very late sown (E3) conditions and to estimate the effect of high temperature on grain yield. Based on prevailing temperature, timely sown condition was considered as heat stress free environment while late sown and very late sown conditions were considered as heat stress environments. Genotypic performance varied substantially over the non-stress and heat stress environments. The observations were recorded on sixteen agronomic traits and three physiological traits. The statistical analysis forgenetic divergence study was done using Mahalanobis D2 statistics and clustering of genotypes was done using Tocher method. Terminal heat tolerance was measured by calculating heat susceptibility index (HSI) for the stress environments. On the basis of genetic diversity analysis, the maximum percent contribution towards genetic divergence was contributed by plant height while the lowest contribution was from harvest index. The 32 genotypes were grouped into six clusters. Cluster-II had maximum number of genotypes (11) while cluster-VI had only single genotype. Cluster-V exhibited the highest intra-cluster while the lowest intra-cluster distance was exhibited by cluster-VI. The highest inter-cluster distance was observed between cluster-III and cluster-VI whereas the lowest inter-cluster distance was observed between cluster-IV and cluster-I. Cluster-VI exhibited highest cluster means for yield contributing traits like grain weigh per spike, grain yield per plant, harvest index while cluster-V exhibited the highest cluster means for physiological traits like relative water content and canopy temperature depression. On the basis of heat susceptibility index, most of the genotypes were found tolerant and moderately tolerant for heat stress conditions. The genotypes bearing the desired values from different clusters can be exploited in future breeding programme for the improving wheat genotypes for heat stress conditions. These genotypes can be used as donor parents in heat tolerance breeding programme.