Potassium (K) supply affects K accumulation and photosynthetic physiology in two cotton ( Gossypium hirsutum L.) cultivars with different K sensitivities

A two-year field experiment was established to investigate the differences in the process of potassium (K) accumulation and photosynthetic physiology of functional leaves of two cotton (Gossypium hirsutum L.) cultivars with contrasting K sensitivity (Simian 3, low-K-tolerant cultivar and Siza 3, low-K-sensitive cultivar) under three K levels (0,150 and 300 kg K2O ha−1). Results in 2012 and 2013 indicated that the maximum theoretical accumulation of K, the maximum and average accumulation rate of K increased in the K-supply treatments (150 and 300 kg K2O ha−1), and the K accumulation in reproductive and reproductive organs was increased by K application. Decreased net photosynthetic rate (Pn) in the 0 kg K2O ha−1 treatment was attributed to stomatal limitation at the boll setting stage for Simian 3 and at the peak flowering stage for Siza 3. After which, non-stomatal factors (low chlorophyll content, unbalanced chlorophyll a to chlorophyll b ratio, negative chlorophyll fluorescence parameters and decreased carboxylation efficiency) dominated the reduction in Pn under K deficiency. High total and initial Rubisco, sucrose phosphate synthase and sucrose synthase (SuSy) (except SuSy in Simian 3) activities did not result in high hexose, sucrose and starch contents in the K-supply treatments, because of a higher rate of sucrose export. Compared with Simian 3, the sensitivity of Siza 3 to low-K was showed in the following aspects: (1) the duration of K rapid-accumulation was increased by K application; (2) Pn, chl a + b and chlorophyll fluorescence parameters, especially qN, under K deficiency were more negative; (3) the occurring time of non-stomatal limitation under K deficiency was earlier; (4) the net photosynthetic rate of unit chlorophyll was significantly promoted by K application; (5) Siza 3 needed higher leaf K content to maintain Rubisco activation state; and (6) SuSy activity was increased by K application.