Potassium Application Affects Key Enzyme Activities of Sucrose Metabolism during Seed Filling in Vegetable Soybean

Pot experiments were conducted in 2014 and 2015 with two vegetable soybean [Glycine max (L.) Merr.] cultivars (‘Zhongkemaodou 1’ and ‘121’) under normal rates of nitrogen and phosphorus application with three potassium (K) fertilization treatments: no K application (K0), 120 kg K2SO4 ha−1 at seeding (K1), and 120 kg K2SO4 ha−1 at seeding + 1% K2SO4 foliar application at flowering (K2). Potassium application significantly increased seed sucrose concentration, and the highest seed sucrose concentrations were uniformly observed in the treatment of foliar K application after flowering. At 7 wk after flowering, the highest stage of seed sucrose concentration, K1 and K2 treatments increased sucrose concentration by 19.8 to 44% and 29.1 to 86.4% for Zhongkemaodou 1, and 7.2 to 42.1% and 26.2 to 63.8% for 121, respectively. Changes of net activities of sucrose‐metabolizing enzymes in seed paralleled the trend of sucrose accumulation. Potassium application increased the activities of sucrose phosphate synthase and sucrose synthase with increased sucrose synthesis but reduced the activity of sucrose synthase with increased sucrose decomposition. Potassium application also reduced the activities of acid invertase and neutral invertase, especially around seed ripening. Potassium fertilization enhanced seed net activities of key enzymes in sucrose metabolism and subsequently induced the increase in sucrose concentration of vegetable soybean.