Mechanism by Which High Foliar Calcium Contents Inhibit Sugar Accumulation in Feizixiao Lychee Pulp

The problem of Feizixiao lychee fruit cracking is typically solved by the application of calcium to the leaves. However, lychee trees are sensitive to excessive amounts of calcium, and in practice, it is easy to spray excessive amounts that result in fertilizer burns. This paper intends to explore the effects of excessive calcium fertilizer application on lychee fruit pulp quality and the underlying molecular physiological mechanism. Adult Feizixiao lychee trees were used as test materials; concerning treatment, a 54 μM anhydrous CaCl2 aqueous solution was sprayed onto the leaves, and water was used as a control (CK). The levels of pulp sugar and the activities of key enzymes involved in glucose metabolism were observed, and transcriptome analysis and genetic screening were performed on the pulp. Spraying excessive amounts of calcium onto the leaf surfaces caused the downregulation of trehalase-encoding genes and SUS-encoding genes, thus inhibiting the activities of trehalase, SS-I and SS-II, and further inhibiting the accumulation of glucose, fructose, and sucrose. Moreover, upregulation of VIN gene expression enhanced AI activity and inhibited sucrose accumulation, thus inhibiting upregulation of NI gene expression during fruit growth and expansion; in turn, this inhibited the increase in NI activity in the fruit pulp, which then decreased the glucose and fructose accumulation in the pulp in the high-calcium treatment group compared with the CK group. The downregulation and expression of CHS family genes may lead to a decrease in chalcone accumulation, which may lead to damage caused by active oxygen production in the fruit pulp, thus inhibiting the accumulation of soluble sugars in that tissue.