Potassium application alleviates the negative effects of salt stress on cotton (Gossypium hirsutum L.) yield by improving the ionic homeostasis, photosynthetic capacity and carbohydrate metabolism of the leaf subtending the cotton boll

Field experiments were conducted in 2017 and 2018, in order to investigate the effects of potassium (K) application on photosynthetic capacity and carbohydrate metabolism of the leaf subtending the cotton boll (LSCB) under salt stress. Three salinity treatments (EC = 1.68∼1.78, 6.21∼6.42 and 10.59∼11.08 dS m−1) with three K levels (0, 150, 300 kg K2O ha-1) applied in each salinity treatment, were imposed on two cotton (Gossypium hirsutum L.) cultivars differing in salt sensitivity (CCRI-79, salt tolerant and Simian 3, salt sensitive). The results indicated that under salt stress: (1) K application increased LSCB K+/Na+ ratio since Na+ concentration decreased and K+ concentration increased. Cotton boll (capsule wall, seed and lint) biomass, total boll number, boll weight and seed cotton yield were also increased; (2) K application improved net photosynthetic rates (Pn) of LSCB by increasing stomatal conductance rates (Gs), chlorophyll a and b concentrations and chlorophyll a/b ratio; (3) K application decreased soluble sugar, sucrose and starch concentrations of LSCB through increases in sucrose phosphate synthase (SPS) and sucrose synthase (SuSy) activities and decreases in sucrose acid invertase (SAI) activity. The results of this study indicated that K fertilizer application rates have a regulating effect on LSCB’s photosynthetic properties and carbohydrate metabolism under salt stress, thus providing a theoretical foundation for increasing cotton yield under salt stress.