Effects of different defoliants on the defoliation，ripening effect， yield and quality of cotton in summer-sowing short-season cultivation in Yangtze River basin

【Objective】 To study the response of cotton to different defoliation and ripening agents under the shortseason cultivation mode of summer sowing，in order to provide theoretical basis for the screening of defoliation and ripening agents suitable for cotton planted in Yangtze River basin. 【Method】 The finished defoliant Xinsaili （50% thifensulfuron-ethylene）was used as the control（CK），and ethylene glycol and additives were used as the common components，and 4 defoliant formulations（corresponding to treatments labelled S，D，L and T）were made with 98% thifen-sulfuron，98% thifensulfuron+50% diuron，98% potassium dihydrogen phosphate and 10% abscisic acid respectively， and the defoliation rate，flocculation rate，yield related traits and cotton fiber quality indexes were determined after different defoliant treatments. The effects of different defoliant treatments on cotton defoliation and ripening as well as yield and quality were compared by correlation analysis and principal component analysis. 【Result】 The defoliation rate and flocculating rate of cotton showed an increasing trend as the time after defoliant spraying passed. The defoliation rate of CK was the highest（96.57%）on day 10，followed by D treatment（95.76%），and defoliation rate of L treatment was significantly lower than others（P<0.05，the same below）；the flocculating rate also showed a similar trend，with CK being the highest，D treatment being the second highest，and L and T treatments being significantly lower than that of other treatments. In terms of the yield of cotton，it was the highest in D treatment，followed by S，CK and T treatments， and L treatment was the lowest. In the late（August 30）flowering cotton bolls，D treatment was the best in terms of yield traits；in fibre quality，there was no significant effect of the treatments on the uniformity index，Micronaire value and elongation rate（P>0.05）. The average length of upper half was the optimal on CK，followed by D treatment；the fiber strength was the highest in D treatment. The best fibre quality was also observed in the later（August 30）flowering bolls under the treatments of CK and D. Combined with correlation analysis and principal component analysis，D and S treatments had better defoliation and ripening effects，and could increase cotton yield and improve cotton fiber quality to different degrees. 【Conclusion】 Thifensulfuron+diuron mixtures，thifensulfuron and the finished product of Xinsaili defoliant have fine effects，and can be applied in cotton production under the short-season cultivation mode of summer sowing in Hunan area of Yangtze River basin.