Cultivar sensitivity of cotton seed yield to potassium availability is associated with differences in carbohydrate metabolism in the developing embryo

Storage lipids and proteins in cottonseed have important industrial values and carbohydrate metabolism is the basis for the biosynthesis of oil and protein in cottonseed. In order to investigate the effects of potassium (K) fertilizer on carbohydrate metabolism of cottonseeds in two cotton (Gossypium hirsutum L.) cultivars with different K sensitivities, a two-year field experiment was conducted with a low K-tolerant cultivar (Simian 3) and a low K-sensitive cultivar (Siza 3) under three K levels (0, 150 and 300 kg K2O ha−1). Results showed that boll number and seed index were higher in the K application treatments (150 and 300 kg K2O ha−1) than in the 0 kg K2O ha−1, resulting in high cottonseed yield. Embryo weight was increased by K application, but seed coat weight was not influenced. K application did not change protein content, but markedly increased oil and non-structural saccharide contents, and K concentration was positively correlated with oil and non-structural carbohydrate contents. In addition, higher non-structural carbohydrate content in the K application treatments than in the 0 kg K2O ha−1 was attributed to higher contents of starch, sucrose and fructose, and sucrose increased to a greater extent than other carbohydrates with K application. Higher fructose content in the K application treatments was closely related to higher sucrose synthase (SuSy) and acid invertase activities. Compared with Simian 3, the sensitivity of Siza 3 to K was evidenced in the following ways: (1) boll number, cottonseed yield, embryo biomass, and the seed coat to embryo ratio were increased more by K application for Siza 3 than Simian 3; (2) oil and carbohydrate accumulations in embryo were more responsive to K concentration in Siza 3 than Simian 3; (3) the increases in the contents of sucrose, starch and fructose and the activities of enzymes (SuSy and acid invertase) caused by K application were larger in Siza 3 than Simian 3; (4) the increases in sucrose phosphate synthase and alkaline invertase activities resulting from K application was only detected in Siza 3.