Sucrose metabolism in cotton subtending leaves influenced by potassium-to-nitrogen ratios

Nutrient management is important for enhancing crop productivity economically, as well as for environmental protection. The present study explored the best relative ratio of potassium (K) to nitrogen (N) under a new short-season, high-density planting model, based on the sucrose metabolism in the subtending leaves of cotton in relation to yield. A 2-year (2016–2017) field experiment was conducted in a randomized complete block design along with four replicates. The treatments were three K relative ratios to N [K08 {K (K2O): N = 0.8:1 (168:210 kg ha−1)}, K10 {K (K2O): N = 1:1 (210:210 kg ha−1)}, and K12 {K (K2O): N = 1.2:1 (252:210 kg ha−1)}]. The plot size was 36.48 m2 comprised of four rows. Plants were sown on raised beds. Row spacing was managed to 0.76 m with a plant-to-plant distance of 0.14 m to maintain a planting density of 9 plants m−2. The results indicated that increased K application ratios substantially favored sucrose metabolism in the subtending leaves to attain higher cotton yields. K10 and K12 upregulated the photosynthetic rate (Pn) of subtending leaves by 17–32% and 33–59%, sucrose phosphate synthase activities by 4–17% and 14–28%, and sucrose synthase activities by 12–25% and 21–36%, respectively. Glucose-6-phosphate dehydrogenase and soluble acid invertase activities were significantly downregulated. K10 improved seed and lint yield by 10–24% with no differences between K10 and K12, suggesting K10 (K2O: N; 1:1) to be the best ratio for cost-effective cotton production under the new planting model in the Yangtze River Valley, China.