Abstract: 【Objective】 Potassium overdose is found to be one of the most important causes for poor coloring of apple fruit. The influence of the combination of different fertilizers (nitrogen, phosphorus, calcium and magnesium) on apple fruit coloring, nutrient content and skin pigments, was studied for the purpose of investigating the mechanism of skin yellowing of ‘Fuji’ apple fruit resulted from over-application of potassium. Hopefully, this study will provide theoretical references for effective prevention of yellowing of apple fruit skin. 【Method】 The nine-year old ‘Fuji’ apple tree grafted on Malus robuste rootstock with an average yield of about 60 kilograms per plant was used as experimental material. Neutral wet soil with medium fertility was used in the experiment. Based on the principles of uptake and interaction of different nutrient elements, fertilization of this study took reference to the fertilizing experience and results of former studies. Each apple plant was fertilized with 0, 125, 250, 500, 1 000 and 2 000 g K2O combined with constant amount of either N-P, N-P-Mg or N-P-Ca. Apple coloring degree, content of anthocyanin, chlorophyll, carotenoids, and content of major mineral elements in both fruit and soil were measured. 【Result】 In all three fertilization combination sets (N-P, N-P-Mg, and N-P-Ca), fruit coloring was firstly improved and then decreased as K2O amount increased, with best fruit coloring in N-P-Mg treatment and worst coloring in N-P-Ca treatment. In N-P and N-P-Ca treatments, 250 g K2O per plant was optimal for fruit coloring, and 1 000 g K2O per plant was optimal for N-P-Mg treatments. In N-P combinations, K2O application was significantly positively correlated with fruit K content and soil K residual while significantly negatively correlated with fruit Mg content. In N-P-Mg, there was no significant correlation between K application and content of mineral nutrients in apple fruit. In N-P-Ca combinations, fruit N content was significantly negatively correlated with K application, and fruit Fe content was significantly positively correlated with K application, with average N content 37% higher than that in N-P treatments. With the increase of K application, the content of chlorophyll in fruit skin increased first and then decreased in N-P treatments, decreased constantly in N-P-Mg treatments, and decreased first then increased in N-P-Ca treatments. With the increase of application K fertilizer, the content of carotenoid decreased first and then increased in N-P-Mg treatments, basically remained constant in N-P treatments, and constantly increased in N-P-Ca treatments. In all three combinations, the content of carotenoid reached peak with highest application of K fertilizer, but anthocyanin content was the lowest at this point, which probably caused the yellowing of fruit skin and poor coloring. 【Conclusion】 The color percent of ‘Fuji’ apple increased at first, and then decreased as the dose of potassium fertilizer added. Over application of potassium promoted absorption potassium and inhibited absorption magnesium of the trees with the concentration of anthocyanin decreasing and that of carotenoid increasing, which resulted in the fruit poor coloring. Fertilization with magnesium made optimal dose for fruit coloring from 250 g to 1 000 g per plant that alleviated the negative effects of K over-use on fruit coloring. Fertilization with calcium enhanced the concentration of N and Fe in fruit to decrease the concentration of anthocyanin and increase that of carotenoids. This study suggests that fruit coloring depends on the interaction of N, P, K, Ca, Mg, Fe, and Mn and it is especially important to keep the balance between different elements while fertilization.

Key words: potassium fertilizer , Fuji apple , coloration , impact factor , mechanism