Effects of postharvest calcium treatmenttreatment on the firmness of persimmon (Diospyros kaki) fruit based on a decline in WSP

Persimmon fruit is a typical respiratory climacteric fruit. Previous studies have found that water-soluble pectin (WSP) was closely related to fruit firmness, and calcium was involved in pectin bridging, which also affected fruit firmness. In this research, the effect of calcium treatment (2% CaCl2, 30 min) on persimmon fruit firmness was studied from the perspective of WSP. Firmness of persimmon fruit treated with calcium increased by 40% and the WSP content decreased by 14% after storage for 15 d Meanwhile, the activity of pectin degrading enzymes polygalacturonase (PG), pectin methylesterase (PME) and pectin lyase (PL) decreased by 39%, 24%, and 11%, respectively. Fruit firmness was negatively correlated with the WSP content and activity of pectin degrading enzymes. Therefore, the lower pectin degrading enzyme activity delayed the decrease of the WSP molecular weight and branching rate, and inhibited the increase of WSP content, thus maintaining the integrity of cell wall structure and higher firmness of persimmon fruit. Additionally, calcium treatment could delay the water migration in the cell wall and vacuole, further maintaining persimmon fruit firmness. The results of this research not only help reveal the mechanism of persimmon fruit softening, but also provide reference for the application of calcium in persimmon fruit preservation.