Effects of potassium on fruit soluble sugar and citrate accumulations in Cara Cara navel orange (Citrus sinensis L. Osbeck)

Sugars and organic acid are key factors for sensory properties of fruit quality. Potassium (K) is an essential macroelement and is also known as quality element. This study aims to investigate how K affects quality improvement particularly sucrose, fructose, glucose and citrate accumulations in Cara Cara navel orange. K fertilizer increased fruit sucrose, fructose, glucose and citrate concentrations and the ratios of total soluble sugars and citrate, particularly the ratios increased more in the treatment of 0.50 kg/plant K fertilizer. These results showed that suitable amount of K fertilizer could improve fruit quality of Cara Cara navel orange. Further study showed that, at early stage of fruit development, K fertilizer played more important roles in degrading sucrose due to the increased activities of sucrose-degrading enzymes including invertase and sucrose synthase-cleavage activity. Conversely, at late stage of fruit development, K fertilizer played more important roles in re-synthesizing sucrose due to the higher activities of sucrose-synthesizing enzymes including sucrose synthase-synthetic activity and sucrose phosphate synthase. The above results suggested that the upregulations of sucrose-degrading and sucrose-synthesizing enzyme activities caused by K fertilizer were conducive to improving sucrose metabolism and sink strength, and then increased the concentrations of sucrose, fructose and glucose in fruit. K fertilizer increased the activities of citrate-synthesizing enzymes including citrate synthetase and phosphoenolpyruvate carboxylase, but had no significant impacts on the activities of citrate-degrading enzyme, indicating that K fertilizer increased citrate accumulations at least partially by promoting its synthesis. This study finally suggested that K fertilizer improved the accumulations of soluble sugar and citrate in Cara Cara navel orange.