Deciphering thermotolerance in cotton genotypes using integrated morpho-physiological and biochemical markers at early stage

Heat stress critically affects plant growth, development, and physiological balance, resulting in substantial yield losses. Understanding early responses to high temperature is crucial to identifying resilient genotypes for breeding activities. In this study, heat tolerance in 25 cotton genotypes was evaluated at the seedling stage using morphological and physio-biochemical indicators. The experiment was carried out under controlled conditions using a completely randomized experimental design with three replications; the plants were exposed to three temperature regimes: normal (T 1 : 32/28°C), moderate heat stress (T 2 : 45/30°C, 2 h exposure), and severe heat stress (T 3 : 48/32°C, 4 h exposure). Increasing temperature stress caused significant reductions in shoot and root length as well as fresh and dry biomass. In contrast, biochemical attributes, including total soluble sugars, proline, superoxide dismutase, catalase, peroxidase, ascorbate peroxidase (APX), sodium, potassium, and electrolyte leakage increased significantly ( P ≤ 0.05), whereas total soluble protein content declined. The multi-trait genotype–ideotype distance index (MGIDI) identified genotypes G9 (National Institute for Biotechnology and Genetic Engineering (NIBGE)-IR-824), G11 (Nuclear Institute for Agriculture and Biology (NIAB)-846), G18 (NIAB-512), and G21 (NIAB-896) as the most heat-tolerant under both moderate and severe stress. These findings provide valuable insights for identifying and utilizing heat-resilient cotton genotypes, supporting future breeding programs aimed at improving thermal tolerance in cotton.