To Study the genotypic differences of physiological characteristics in leaves subtending bolls and its relationship with the boll weight forming, field experiments were conducted in Nanjing, Jiangsu Province （cotton belts in lower basin of the Yangtze River） in 2004-2005 by using four cotton genotypes. The study focused on the time course of C/N ratio, soluble protein content and internal source protective enzyme activity in the leaves subtending bolls. Flowers were labeled at anthesis and leaves subtending boll were sampled every 7 days from 10 DPA （day post-anthesis） in 2004 and every 7 days from 5 DPA in 2005. The results of 2-year experiments showed that the leaves subtending bolls of SC-15 and KC-1 had more active nitrogen metabolism and higher soluble protein content after anthesis, and had higher physiological activities and longer duration of function, including higher activity of internal source protective enzymes, lower MDA content during the middle and later development stages of bolls. These phenomena led longer time for rapid increasing of boll weight, higher fiber weight in single boll and resulted in higher boll weight finally. On the contrary, the leaves subtending bolls of DSC-1 had shorter duration of function, and lower physiological activities including higher C/N ratio, sharp drop of internal source protective enzyme activities and higher extent of lipid peroxidation.These items caused the shorter time for rapid increasing of boll weights, lower speed for fiber weight increasing and resulted in lower boll weight finally. The items in AC-33B were between the above two types. From above results, we suggested that under our experiment condition, the genotypic difference of the physiological characteristic in leaves subtending bolls might be one of important reasons causing, the difference in boll weights forming.
leaf subtending boll