GENETIC BASIS OF VARIATION FOR PHYSIOLOGICAL AND YIELD CONTRIBUTING TRAITS UNDER NORMAL AND HIGH TEMPERATURE STRESS IN Gossypium hirsutum L.

Cotton is severely affected by heat stress in most parts of Pakistan which results in poor quality and low cotton yield. Sixty cotton strains were screened against heat stress on the basis of cellular membrane thermostability, chlorophyll contents, canopy temperature, node number of first fruiting branch, boll weight and seed cotton yield to explore the genetics of variation among genotypes for heat tolerance. Five tolerant and five sensitive parents were sorted out and crossed in Line x Tester mating design during 2018.The F-1 crosses along with parents were grown in field under normal and heat stress conditions by using two sowing dates to create heat stress in the field. Sowing during 1st week of April was considered as heat stressed because its peak flowering period was supposed to be coincided with the maximum temperature of May -June; while, sowing during 1st week of June was taken as normal because its peak flowering period coincided with normal temperature of August. Data regarding cell membrane themostability (CMT%), chlorophyll contents, canopy temperature, node number to first fruiting branch, bolls/plant, boll weight, ginning out turn and seed cotton yield under normal as well as heat stressed conditions were collected which showed significant differences among genotypes for all the traits under investigation. Most of the traits showed greater mean square for female x male interaction than female and male individual mean squares and higher dominance variance than additive variance. Broad sense heritability for investigated traits was observed in the order of boll weight (94.56), seed cotton yield (94.51), CMT% (91.88), canopy temperature (90.48), node number to first fruiting branch (75.68) and chlorophyll contents (51.93). FH-458 and PB-76 indicated high general combining ability for most of the traits whereas the cross combination FH-458x FH-326 was described as the best cross showing 56.85% better parent heterosis for seed cotton yield under heat stress conditions. The present study was planned to investigate the genetics of some important physiological and agronomic traits of cotton which is a pre-requisite for the development of high yielding cotton varieties under high temperature stress.