Abstract
BACKGROUND: Escalating global temperatures pose an ongoing threat to cotton production by disrupting essential morphological, physiological, and metabolic processes during early plant development. These early stages are critical for crop establishment, yet the genetic basis of heat tolerance at this phase remains insufficiently characterized. Therefore, advancing our understanding of early-stage responses is essential for the development of heat-tolerant genotypes. METHODS: A total of 79 cotton genotypes were assessed under controlled conditions and two elevated temperature treatments, HST-1 (45 °C) and HST-2 (48 °C), with emphasis on their morphological and physiological responses to heat stress under glasshouse conditions. Box plot was used to visualise trait distributions. Correlation analysis elucidated interrelationships among various traits. Principal component analysis (PCA) was conducted to reduce dimensionality and identify major axes of variation. The multi-trait genotype-ideotype distance index (MGIDI) was employed to rank genotypes based on overall performance. RESULTS: Heat stress significantly reduced shoot and root elongation, biomass accumulation, and relative water content, while increasing excised leaf water loss. Photosynthetic rate, stomatal conductance, and instantaneous water use efficiency declined markedly, although transpiration rate remained relatively stable. PCA effectively distinguished tolerant from susceptible genotypes and revealed key traits associated with tolerance, while MGIDI identified eight top performing lines: G1 (NIAB-377), G4 (NIAB-868), G8 (NIBGE-IR-15), G9 (NIAB-512 33/4), G39 (NIA-Noori), G67 (Cyto-535), G69 (NIAB-512), and G78 (SLH-01). CONCLUSION: Our results show substantial genotypic variability in early-stage heat stress responses in cotton and establish a rigorous multivariate framework for trait-based selection. This integrative multivariate approach facilitates the targeted selection of thermo-tolerant genotypes with enhanced morphological and physiological traits, advancing the breeding of climate-resilient cotton.