Comparative analysis of leaf photosynthetic characteristics and fruit sugar content in trees of Citrus cultivar "Huangguogan" of different age

Citrus trees of cv. Huangguogan, 8-, 30-, and 280-year-old (8s, 30s, and 280s, respectively), were used to explore the photosynthesis and related enzyme activity. In spring shoot leaves, net photosynthetic rate (PN) of the 280s was significantly lower than that of the 30s and 8s, but it was opposite in summer shoot leaves. PN of the summer shoots was higher than that of the spring shoots. PN was significantly positively correlated with stomatal conductance, intercellular CO2 concentration, and transpiration rate, but was negatively correlated with maximum photochemical quantum yield of PSII photochemistry. The effect of age on variable fluorescence/minimal fluorescence was similar to that of minimal fluorescence, variable fluorescence, maximal fluorescence yield of the dark-adapted state. Fruits of the 280s had significantly lower sugar components than those of the 8s and 30s. Results showed that fruits of the 8s and 30s actively accumulated photosynthesis products, which was verified by the higher PN in the spring shoots, lower acidic invertase and neutral invertase activities, and higher sucrose phosphate synthase than that of the 280s.