The Physiological and Molecular Mechanisms of Fruit Cracking Alleviation by Exogenous Calcium and GA3 in the Lane Late Navel Orange

Fruit cracking is very common in the production and cultivation of citrus, and can lead to decreases in its yield and quality. Bacteria can easily invade cracked fruit and cause mildew, accelerate the spread of diseases and pests, affect the appearance of the fruit, and reduce its economic benefits. In order to explore a method for alleviating citrus cracking, the Lane Late navel orange, which is a citrus that easily cracks, was studied via treatment with 1.0 g·L−1 chelated calcium (Ca) or 50 mg·L−1 gibberellin (GA3). The fruit cracking rate, external and internal quality, active oxygen metabolism and expression levels of related genes, cell wall structure components, and metabolism-related enzyme activity and the expression levels of related genes were determined. The results showed that Ca and GA3 treatment significantly reduced the fruit cracking rate and increased the longitudinal and transverse diameter, single-fruit weight, pulp quality, and peel quality, but had no significant effect on the internal quality of the fruit. Ca and GA3 treatment also improved the activities of antioxidant enzymes (SOD and CAT), enhanced the scavenging ability for active oxygen species, and thus reduced the contents of H2O2 and MDA and decreased the superoxide anion production rate. At the same time, Ca and GA3 treatment decreased the activities of protopectin-degrading enzymes (PME, PL, and PG) and cellulase (CX), prevented the degradation of pectin and cellulose in the cell wall, and increased their contents in the peel, thus improving the ductility and toughness of the peel and reducing the occurrence of cracked fruit. Ca and GA3 treatment significantly increased the relative expression levels of antioxidase-related genes (CsSOD and CsCAT) in fruit peel and decreased the relative expression levels of CsPPO and cell wall metabolism-related genes (CsPME, CsPL, CsPG, and CsCX). In summary, this study confirmed that exogenous calcium and gibberellin can reduce the fruit cracking rate by regulating the scavenging ability for active oxygen species and the cell wall metabolism of the Lane Late navel orange at the physiological and molecular level, laying a theoretical foundation for further analysis of citrus fruit cracking mechanisms and clarifying that spraying exogenous calcium and gibberellin on the citrus fruit surface is an effective production measure for preventing and alleviating fruit cracking. In particular, gibberellin is better than exogenous calcium.