Objective  To optimize the fertilization for improving fig quality and yield, the internal regulation mechanism of potassium (K) on sugar metabolism of the fruit was studied.

  Method  Two-year-old Bojihong fig (Ficus carica) plants were cultivated under common practice, except that 4 levels of K₂SO₄ were applied on the soil for the experimentation. The K applications included CK at 0g.plant⁻¹, K1 at 125 g.plant⁻¹, K2 at 250 g.plant⁻¹, and K3 at 375 g.plant⁻¹. Contents of soluble sugars and starch as well as activities of enzymes related to the sugar metabolism of the fruits were determined for a correlation analysis.

  Result  (1) Fructose and glucose were the main soluble sugars contained in the fruit with the contents increased as the fruit was developing. The combined starch and soluble sugars content decreased with fruit maturation, and the starch declined while the sugars on the rise. (2) With the K applications, both the contents and the compositions of the of soluble sugars in the figs significantly increased, while starch decreased during the mid and late stages of the fruit development. K2 produced the greatest soluble sugars increase among all treatments. (3) The K applications significantly rose the activities of AI and SS (in the starch degradation direction), but not on NI, in the early and late stages of the fruit development. However, they extremely significantly enhanced the NI activity as the fruits matured resulting in fructose and glucose accumulations in the figs. They also boosted, and maintained at high level, on the activities of α- and β-amylases facilitating the conversion of starch to sugars. In addition, the K applications promoted SPS activity in all developmental stages but exerted little effect on SS (in the synthesis direction) that encouraged the sucrose formation n the fruit.

  Conclusion  The K applications increased activities of the enzymes related to sugar metabolism and promoted starch degradation that benefitted the accumulation of desirable soluble sugars in figs.