Fruit sugar and organic acid were significantly related to fruit Mg of six citrus cultivars

The fruit quality of 6 citrus cultivars growing in the same orchard was determined at ripening stage in both 2014 and 2015. We further measured the components of sugar (sucrose, fructose and glucose), organic acid (citric, malate and quinic acid), enzymes related to Glycolysis and Krebs cycle and mineral elements at 5 stages of fruit development in the second year. The results showed that at ripening stage of both years, ‘Newhall’ cultivar had higher TSS concentration and the TSS/TA ratio but lower TA concentration, while ‘Flame’ cultivar was exactly opposite. Sucrose and citric acid were the most accumulated compounds in fruit during the fruit development of 6 citrus cultivars. Fruit sucrose concentration increased from 9.26 mg·kg−1 at 60 DAFB to 50.92 mg·kg−1 at 180 DAFB, and the citric acid concentration increased from 1.41 mg·kg−1 at 60 DAFB to 29.87 mg·kg−1 at 90 DAFB or 29.02 mg·kg−1 at 120 DAFB then decreased till ripening (5.47 mg·kg−1). We found ACO was the key enzyme resulting in the difference of citric acid accumulation, but not quite clear in sucrose metabolism. The fruit mineral nutrient concentrations of 6 cultivars during the fruit development were 0.94–1.92% of N, 0.11–0.23% of P, 1.03–1.37% of K, 0.31–1.15% of Ca, 0.11–0.29% of Mg, 3.97–72.34 mg·Fe kg−1, 1.93–10.64 mg·Mn kg−1, 1.56–10.73 mg·Cu kg−1, and 0.90–16.80 mg·Zn kg−1. We also analyzed the relationship among each sugar, organic acid component and mineral nutrient in this study by curve estimation and PCA analysis. The results indicated that only Mg was significantly correlated with both sugar and organic acid component, negative and positive respectively. It suggested that the accumulation of sugar and organic acid might be related to the dynamic changes of fruit Mg concentrations of 6 citrus cultivars.