Potassium application affects carbohydrate metabolism in the leaf subtending the cotton (Gossypium hirsutum L.) boll and its relationship with boll biomass

Field experiments were conducted in 2012 and 2013 with two cotton (Gossypium hirsutum L.) cultivars (Simian 3, low-K tolerant; Siza 3, low-K sensitive) under three levels of potassium (K) fertilization (0, 150 and 300 kg K2O ha−1). Results showed that K application increased leaf K concentration, net photosynthetic rate (Pn), stomatal conductance (Gs), plant biomass and stimulated boll biomass (capsule wall, seed and lint biomass). K application increased the proportion of lint biomass, decreased the proportion of seed biomass, and did not affect the proportion of capsule wall biomass. Specific leaf weight (SLW), maximum/minimum sucrose contents and nonstructural carbohydrate (hexose, sucrose, starch) decreased, but sucrose transformation rate in LSCB increased in both cultivars after K application, the leaf critical K levels for hexose content, sucrose content and starch content were 1.1%, 1.3%, 1.2-1.4% in Simian 3 and were 1.6%, 1.7% and 1.7-1.8% in Siza 3, respectively. The activities of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), cytosolic fructose-1,6-bisphosphatase (cy-FBPase), sucrose phosphate synthase (SPS), sucrose synthase (SuSy) and amylase activities increased by K application, whereas soluble acid invertase (SAI) activity decreased. SPS and SuSy activities in Siza 3 were more sensitive than that in Simian 3. Correlation analysis revealed that higher Pn, sucrose transformation rate and SPS activity in LSCB were necessary to improve boll biomass, but the accumulation of sucrose in LSCB was not beneficial to boll biomass.