High Temperature Inhibits Fertilization Efficiency, Carbohydrate Metabolism, and Energy Levels in Cotton Pistils.

Successful pollen tube growth and fertilization of the ovule is essential for the development of seeds and yield in cotton (Gossypium hirsutum L.). Pollen tube growth has a high energy requirement relative to vegetative tissues, and any abiotic stress negatively affecting the availability of energy reserves in the pistil should negatively impact fertilization. Cotton plants exposed to optimal (30/20°C) or high day temperature (38/20°C) conditions during flowering were analyzed for in vivo fertilization efficiency, soluble carbohydrates , and ATP content in the pistil, and gas exchange, chlorophyll content, quantum efficiency, and ATP content of the subtending leaf on the day of anthesis. Pollen tubes were visualized in ovules 24 h after anthesis via UV microscopy, and fertilization efficiency was expressed as the ratio of fertilized ovules to total ovules per ovary. In the pistil, fertilization efficiency, soluble carbohydrates, and ATP content declined under heat stress. Subtending leaf photosynthesis, photochemical efficiency, and chlorophyll content decreased under heat stress, whereas stomatal conductance increased and ATP levels remained unchanged. We propose that the major limitations to subtending leaf photosynthesis under heat stress are reduced quantum efficiency of photosystem II and enhanced cyclic electron flow, which maintains ATP content but decreases CO₂ fixation in source leaves. We conclude that heat stress primarily limits fertilization efficiency either by decreasing pollen tube growth or guidance to the ovules, and that the energy demands of growing pollen tubes or developing ovules cannot be met under heat stress due to decreased source leaf activity.