The Effects of Mixed Foliar Nutrients of Calcium and Magnesium on the Major Bypass Respiratory Pathways in the Pulp of ‘Feizixiao’ Litchi

Abstract

During the period of ‘Feizixiao’ litchi fruit pericarp’s full coloring, there is a phenomenon of “sugar withdrawal” in the pulp, and the mixed foliar nutrients of calcium and magnesium (Ca+Mg) can effectively overcome this phenomenon. One of the reasons for this may be that it is related to the influence of the mixed nutrients of Ca+Mg on the bypass respiratory pathways of the pulp. The major fruit quality indicators, the rates of cytochrome and cyanide-resistant respiratory pathways (CP and AP) in the pulp and the activities of their key enzymes, were observed continuously in 2021 and 2022, and the deferentially expressed genes (DEGs) related to the two bypass respiratory pathways in the pulp were screened by RNA-seq analysis, with a qPCR of the random genes performed to verify the results. Ca+Mg treatment kept the content of the total soluble sugar in the pulp stable and higher than that the control in the ripening stage; Ca+Mg treatment increased the activities of electron-transferring enzymes in the electron transport chain, such as NADH dehydrogenase (ND), succinate dehydrogenase (SDH), cytochrome bc1 complex, and cytochrome c (Cyt c) through up-regulating their gene expression. In terms of the rate-limiting enzymes in the pulp, Ca+Mg treatment increased the activity of cytochrome oxidase (COX) in the CP pathway by up-regulating the expression of COX genes, then increased the CP respiratory rate and inhibited the CP respiratory rate decrease; meanwhile, it also inhibited the activity of AOX (alternate oxidase) in the pulp in the AP pathway by down-regulating the expression of AOX genes, then inhibited the increase in the AP respiration rate. The qPCR validation of randomly selected DEGs showed a significant unitary linear correlation between their expression levels and the results of the RNA-seq analysis. Therefore, one of the physiological mechanisms on the mixed foliar nutrients of Ca and Mg overcoming the phenomenon of “sugar withdrawal” in the ‘Feizixiao’ litchi pulp could be to promote CP and to inhibit AP, and then to delay the ripening and senescence of the pulp.

Keywords

• sugar withdrawal phenomenon
• bypass breathing pathway