Boll-leaf system photosynthesis reveals systemic chilling responses in cotton

1. A multidimensional evaluation system was established to assess chilling tolerance in cotton, based on key physiological and biochemical traits. 2. Functional divergence between main stem and sympodial leaves revealed distinct stress response strategies under chilling conditions. 3. The net photosynthetic rate of the boll-leaf system [ P n (BLS)] was proposed as a novel and efficient indicator for rapid screening of chilling-tolerant cultivars. Chilling stress is a major abiotic factor that limits the yield and quality of cotton ( Gossypium hirsutum L.), particularly during the flowering and boll-setting stages. Developing a scientific and precise evaluation system for chilling tolerance is important for the screening and breeding of tolerant cultivars. In this study, 14 early maturing upland cotton cultivars were subjected to a three-day chilling stress treatment (15°C/10°C, day/night) during the flowering and boll-setting stages. Physiological and biochemical parameters, including photosynthetic pigments, gas exchange, chlorophyll fluorescence, antioxidant enzyme activities, and membrane lipid peroxidation, were measured in both the main and sympodial leaves. A comprehensive evaluation index (D-value) was constructed based on principal component analysis (PCA) and membership function and validated by the resistance coefficient of whole-plant biomass (RCBM). Systematic cluster analysis was used to classify chilling tolerance levels, and partial least squares regression (PLSR) was applied to identify key physiological indicators. The results showed that chilling stress significantly suppressed photosynthesis and induced oxidative damage, with sympodial leaves being more sensitive than main stem leaves. PCA revealed that main-stem leaves primarily relied on a “photosynthesis-antioxidant coordinated defense” mechanism, while sympodial leaves exhibited a “damage signal-driven” response pattern. The D-value based on main-stem leaf traits was highly correlated with the whole-plant biomass resistance coefficient ( R 2 =0.92, P <0.001), indicating that it is the most effective indicator for assessing plant chilling tolerance. PLSR analysis identified the net photosynthetic rate ( P n ) and peroxidase (POD) activity as the core indicators shared by both leaf types. Furthermore, the resistance coefficient of boll-leaf system photosynthesis [ P n (BLS)] was also significantly correlated with RCBM ( R 2 =0.84, P <0.001), suggesting its great potential as a simple and efficient indicator for rapid screening. This study developed a reliable and systematic evaluation strategy for chilling tolerance in cotton, highlighted the predictive value of main stem leaf traits, and proposed P n (BLS) as a mechanistic indicator reflecting systemic photosynthetic responses under chilling stress.