Abstract: To explore the effects of zinc levels on the synthesis and translocation of photosynthetic products from leaves to fruits, and to lay a theoretical foundation for improving fruit quality through zinc supplementation during the critical period of apple fruit development, a field experiment was carried out with a eight-year old ‘Hanfu'/GM256/Malus baccata Borkh apple. We used the ¹³C tracer method to examine the effects of different zinc levels (ZnSO₄·H₂O 0, 0.1%, 0.2%, 0.3%, 0.4%, expressed by CK, Zn₁, Zn₂, Zn₃, Zn₄, respectively) on translocation of photosynthate to fruit during the stage of fruit expanding. The results showed that, with increasing zinc concentration, Rubisco enzyme activity, net photosynthetic rate, sorbitol and sucrose content, sorbitol 6-phosphate dehydrogenase, and sucrose phosphate synthase enzyme activities of leaves first increased and then decreased, with the highest values being observed in Zn₃ treatment. Zn₃ treatment significantly increased the ¹³C assimilation capability of leaves. Compared with other treatments, the ¹³C of self-retention (labeled leaves and labeled branches) was lowest in Zn₃ treatment (61.2%) and the output of ¹³C photoassimilates was highest in Zn₃ treatment (38.8%). ¹³C absorption of apple fruit showed a trend of Zn₃ ＞ Zn₂ ＞ Zn₄ ＞ Zn₁ ＞ CK. In summary, foliar zinc application under appropriate concentration (0.3% ZnSO₄·H₂O) enhanced photosynthesis, increased the assimilation capability of leaves, and promoted the directional transportation of photosynthate to fruit.

Key words: apple, zinc, ¹³C-photoassimilates, translocation, fruit