Abstract: Fiber strength is one of the most important quality indexes of cotton (Gossypium) fiber and contributes highly to the yarn strength. Normally, fiber strength is determined by fiber’s secondary wall thickening stage in which many complex physiological and biochemical processes are involved. To investigate the physiological characteristic of cotton fiber thickening in seasonal bolls and its relationship with fiber strength, the experiments were carried out in Nanjing, Jiangsu Province in 2004–2005 by choosing four cultivars with significant difference in fiber strength as materials. Boll samples were labeled on anthesis day (July 15, July 25, Aug. 25, and Sept. 10) and expressed as PSB (pre-summer boll), SB (summer boll), EAB (early autumn boll) and LAB (late autumn boll), respectively. The results of two-year experiments showed that there were great differences in matter transformation and related enzyme activities during cotton fiber thickening in seasonal bolls, and which resulted in different fiber strengths. The diversity of physiological indexes between bolls with different anthesis dates was similar in every cultivar (genotype). Under suitable temperature condition (mean daily temperature 26.0–28.5℃ during boll age from 10 to 50 days) and plant physiological age (about the third-ninth fruit branch), the cotton seasonal bolls (PSB and SB), which had more soluble sugar and sucrose transformed, higher β-1,3-glucan content at the onset of the secondary wall thickening, and higher activities of sucrose synthetase and β-1,3-glucanase, would get longer and tempered cellulose accumulation and higher strength fiber. EAB had worse temperature environment for fiber development than PSB during the later period, but it had higher matter transformation rate and better characteristic of cellulose deposition than PSB. As the result, fiber strength of EAB was higher than that of PSB. With the worse temperature condition (mean daily temperature lower than 20℃ during boll age from 10 to 50 days) and cotton plant se-nescence (above the sixteenth fruit branch), LAB had the longest period for cellulose rapid deposition, the lowest matter transfor-mation rate and cellulose accumulation speed. And all these resulted in the lowest fiber strength.

Key words: Cotton, Cotton seasonal boll, Cotton fiber thickening development, Physiological characters, Fiber strength