Physiological causes of cotton fruit abscission under conditions of high temperature and enhanced ultraviolet-B radiation

An experiment was conducted in sunlit controlled environment growth chambers to determine the physiological mechanisms of fruit abscission of cotton (Gossypium hirsutum L. cv. NuCOTN 33B) grown in high temperature and enhanced ultraviolet (UV)-B radiation. Six treatments included two levels of optimum (30/22°C) and high (36/28°C) day/night temperatures and three levels of biologically effective UV-B radiation (0, 7, and 14 kJ m^-2 per day). Both the temperature and UV-B treatments were imposed from seedling emergence through 79 days after emergence (DAE). High temperature did not negatively affect either leaf net photosynthetic rates (Pn) or abscission of cotton squares (floral buds with bracts) but significantly decreased boll retention. Plants exposed to 7 kJ UV-B radiation retained 56% less bolls than the 0 kJ UV-B control plants at 79 DAE, despite no significant differences in leaf Pn measured at squaring and flowering. At 53 DAE, leaf Pn of plants grown in high UV-B radiation (14 kJ m^-2 per day) decreased by 11%, whereas total non-structural carbohydrate (TNC) concentrations in the leaves, floral buds, and young bolls decreased by 34, 32, and 20%, respectively, compared with the control plants. The high UV-B radiation significantly increased square abscission. Square abscission was not related to leaf TNC concentration but closely correlated with TNC in floral buds (r = -0.68, P < 0.001). Young boll abscission was highly correlated with TNC concentrations in both the leaves (r = -0.40, P < 0.01) and the bolls (r = -0.80, P < 0.001). Our results indicate that non-structural carbohydrate limitation in reproductive parts was a major factor associated with fruit abscission of cotton grown under high temperature and enhanced UV-B radiation conditions.