Effects of spraying calcium on fruit cracking and related physiological characteristics and microstructure of Junzao jujube

Abstract：【Objective】Junzao jujube is one of the main varieties of jujube cultivated in Xinjiang. Due to the frequent occurrence of calcium imbalance, physiological diseases such as fruit cracking causes great economic losses. Calcium plays an important role in the growth and development of fruit trees, and exogenous calcium application can significantly reduce splitting rate of fruit. The cell wall materials, cell wall degrading enzymes, and peel structure are closely related to the occurrence of fruit cracking. The purpose of this study is to explore the effects of treatments of calcium preparations on fruit splitting rate, cell wall substances, cell wall degrading enzymes and micro structure of jujube fruit peel. 【Methods】In this experiment, 12-year-old Junzao jujube trees were used as the test material. Different calcium solutions (0.5%, 0.25% and 0.125% CaCl2 and 0.1% organic calcium) were foliar sprayed atthe young fruit stage. Fruit cracking rate, peel cell wall components (cellulose, water-soluble pectin, ionbound pectin, and water-soluble pectin), and activities of related degrading enzymes (cellulase and pectinase) were observed or analyzed. Tissue structure was analyzed at the green ripening, white ripening, color change and brittle ripening stages. Excel 2021 software was used to process and plot the data, and Duncan’s new complex range test was carried out with DPS 9.01 software.【Results】Different calcium spray treatments significantly reduced the cracking rate, and the effect of 0.5% CaCl2 treatment was the best, in which the cracking rate was reduced by 48% compared with control. Calcium treatments slowed the decomposition rate of cellulose and prospection in the cell wall. During the growth and development of the fruit, the cellulose content in the peel first increased and then decreased, and the highest cellulose content occurred in the white ripening period. With the increase of fruit ripeness, cellulase activity increased, which intensified the decomposition of cellulose, resulting in significant decrease in the cellulose content. The calcium treatments could reduce cellulase activity and increase fruit cellulose content. The cellulose content was the highest under 0.25% and 0.125% CaCl2 treatments, and the cellulose content at the high incidence period of fruit cracking increased by 113% and 119%, respectively. The calcium treatments significantly reduced the activities of pectinase and cellulase in the cell wall, and the cellulase activity of fruit under 0.5% CaCl2 treatment was the lowest in the crisp ripening period, the most cracking-sensitive period, which was 32.2% lower compared with control. Pectinase activity under 0.25% CaCl2 treatment decreased by 41.1% compared with control. The content of water-soluble pectin increased with fruit ripeness and reached a maximum during the brittle ripening period. The calcium treatments significantly reduced the content of water- soluble pectin, and the 400- fold liquid 0.25% CaCl2 treatment had the best inhibitory effect on the increase in water-soluble pectin content in fruit, which was reduced by 19% compared with control. In contrast, the calcium treatments significantly increased the content of ion- bound pectin and covalently bound pectin, which were highest under 0.5% CaCl2 treatment and 32.9% and 38.4% higher than control, respectively. With the growth and development of the fruit, the cuticle layer gradually thinned, while the epidermis and subepidermal layer gradually thickened. The number of cell cavities also increased, and the tightly arranged pulp cells became loose, and the crack resistance of the peel was weakened. Different calcium treatments increased the thickness of the cuticle layer, epidermal cells, and subepidermal cells of the peel so that the cuticle remained intact; the occurrence of cracks and the number of cavities in the pulp cells were reduced. 【Conclusion】In the process of ripening of Junzao jujube fruit, the increases in cellulase and pectinase activities and water-soluble pectin content accelerates the rapid degradation of cellulose and protopectin, and intensifies the occurrence of fruit cracking. Calcium application at the young fruit stage can increase the content of cellulose, ion-bound pectin and covalently bound pectin, reduce the activities of cellulase and pectinase, and increase the thickness of the cuticle layer of the peel, and thus increase the hardness and elasticity of the peel and reduce the occurrence of fruit cracking. Calcium treatments in the young fruit stage can be used as an important measure to reduce the fruit cracking in Junzao jujube.