Potassium (K) application alleviates the negative effect of drought on cotton fiber strength by sustaining higher sucrose content and carbohydrates conversion rate

Potassium (K) application can alleviate the negative effect of drought on fiber strength of cotton, but the involved physiological mechanism is still unclear. To explore this, two cotton varieties, Siza 3 and Simian 3, were planted under three K levels of 0, 150 and 300 kg K2O ha(-1). These were exposed to two water regimes consisting of a well-watered level (WW) of 75 +/- 5% soil relative water content (RSWC) and a short-term drought stress (DS) of 40 +/- 5% SRWC at flowering and boll development stage of cotton. Results revealed that cotton fiber strength of the 7th main-stem fruiting branch (FB7, middle branch) and 3rd main-stem fruiting branch (FB3, lower branch) significantly decreased under drought conditions. The K application significantly increased cotton fiber strength on all FBs under both WW and DS conditions and alleviated the fiber strength decline on FB3 and FB7 under drought. Correspondingly, K application alleviated the DS-caused decrease of sucrose content, cellulose content, sucrose and callose conversion rate, and SuSy and beta-1,3-glucanase activities. Correlation analysis also revealed that sucrose content was the most associated to final cotton fiber strength, followed by callose and sucrose conversion rate. Greater increase in enzymes activity, carbohydrate content and conversion rate, and final fiber strength by K application was observed in Siza 3 than in Simian 3 under DS. Summarily, since the carbohydrate content and enzymes activity under DS increased with the increase of K application, the negative effect of DS on fiber strength on FB3 and FB7 gradually reduced. Sustaining higher sucrose content and carbohydrate conversion rate (CCR) would be the strategy for K alleviating the DS-induced decline in fiber strength in cotton.