Differential regulation of heat stress during the seedling and squaring stage on the reproductive development and yield formation of cotton

Global warming has resulted in an increasing frequency of high-temperature (HT) stress during the critical growth stages of cotton. However, systematic investigations into its effects during the seedling and squaring stages remain limited. In this study, the cotton cultivar ‘CCRI425’ was subjected to HT treatments at both stages to evaluate the impacts on floral bud differentiation, young square development, and associated metabolic regulation. HT stress during the seedling stage delayed floral bud differentiation, reduced bud number, and induced morphological abnormalities. These changes were linked to hormonal imbalances, specifically increased levels of auxin (IAA) and cytokinin (CTKs) and decreased levels of abscisic acid (ABA) and jasmonic acid (JAs) in the apical meristems. Concurrently, disrupted sugar and antioxidant metabolism further impaired reproductive development, ultimately leading to a 56.7 % reduction in seed cotton yield. In contrast, HT during the squaring stage exerted more severe inhibitory effects on young square development, disturbing sugar metabolism and ROS scavenging in both stamens and pistils. This disturbance resulted in decreased pollen viability and stigma receptivity, culminating in a 78.1 % yield loss. By revealing stage-specific metabolic disruptions in cotton reproductive development under HT stress, this study provides critical insights into the mechanisms limiting yield and offers a theoretical basis for breeding heat-tolerant varieties and mitigating high-temperature damage.