Abstract: 【Objective】Effects of physiological age of cotton plant on the physiological mechanism of fiber thickening development and fiber strength were studied.【Method】 By setting different seeding times, puting the fiber thickening development process of boll which situated in the different positions of cotton plant under the same temperature conditions, mean daily temperature for summer boll, early autumn boll and late autumn boll within boll age of 25 to 50 days were 22.0℃,19.4℃ and 15.3℃ accordingly.【Result】The results of 2-year experiment showed that: there were double effect of temperature and physiological age of cotton plant on fiber thickening development and fiber strength formation, the physiological age of cotton plant was the principal factor and there were evidently effects on fiber thickening development of summer boll for the top fruiting branches and early autumn boll for the lower fruiting branches, boll weight and lint percent and fiber strength in these portion were lower than the others. Low temperature in fiber thickening development process was the principal factor that affected fiber thickening development and fiber strength of late autumn boll. Under this testing condition of which the mean daily temperature for late autumn boll was 15.3℃, the fiber sucrose synthetase activity was evidently fall, and the β-1, 3-glucanase activity moved up, the cellulose accumulation quantity and accumulation rate reduced obviously, boll weight was smaller than 2 g and fiber strength was less than 20 CN·tex-1. 【Conclusion】Effect of the cotton physiological age on the fiber strength enhanced more and more among summer, early autumn and late autumn bolls.

Key words: Cotton physiological age, Cotton fiber, Thickening development, Fiber strength