Effects of different calcium agents on fruit firmness and related cell wall metabolites in 'Hanfu' apple

Abstract:【Objectives】After different calcium treatments, fruit firmness, cell wall components and enzyme activities of apple fruit were measured to explore the physiological mechanism of the effects of treatments with different types of calcium on the fruit firmness and to provide a reference to extending shelf life and maintaining fruit quality.【Methods】Firm ripe, medium and uniform sized'Hanfu'apple were hand-harvested with stalk in the morning from an orchard in Xingcheng, Liaoning, China, and were transported to the laboratory in the day. A total of 400 fruit were randomly divided into five groups and 10 fruit were randomly selected to determine the initial values of various parameters, and the remaining fruit were used for calcium treatments. Fruit of the five groups were immersed in calcium nitrate, calcium chloride, calcium amino acid, calcium sorbitol, or in clear water as control group. Theconcentration of calcium ion in each treatment was 2% and immersion time was 15 min. The fruit were then stored at room temperature. For each group, 10 fruit were randomly sampled every 7 d during storage. The fruit firmness, soluble pectin, protopectin, cellulose, and the activities of pectin methylesterase (PME) , polygalacturonase (PG) and cellulase (CX) were monitored during storage to explore the relationship between fruit firmness and the metabolites of cell walls.【Results】Fruit firmness is the basis of fruit storage. With the extension of storage, fruit firmness decreased rapidly for the first 7 days and decreased slowly from day 7 to day 28. The four calcium treatments all increased the fruit firmness. At 35 days of storage, the firmness of fruit treated with calcium sorbitol was significantly higher than that of the control, while the other Ca treatments were not significantly different from the control. Compared to the control, the firmness of fruit treated by calcium nitrate, calcium chloride, calcium amino acid and calcium sorbitol was increased 4%, 6%, 6.6% and 7.9%, respectively. During storage, the content of soluble pectin increased rapidly for the first 7 days and increased steadily from day 7 to day 35. The content of the soluble pectin of all the calcium treatments was lower than that of the control, indicating that calcium treatments could significantly inhibit the increase of soluble pectin content in fruit. At 35 days of storage, the content of soluble pectin treated with calcium amino acid, calcium sorbitol or calcium nitrate was significantly lower than that of the control, while there was no significant difference between calcium chloride treatment and the control. The content of soluble pectin in the treatments of calcium nitrate, calcium chloride, calcium amino acid and calcium sorbitol was 13.7%, 5.0%, 19.4% and 26.6%lower than in the control, respectively. The contents of the protopectin and cellulose decreased during storage and were higher in all calcium treatments than in the control, indicating that calcium treatments could effectively inhibit the decline of protopectin and cellulose in fruits. At 35 days of storage, the content of protopectin was 17.2%, 21.0%, 25.3% and 29.6% higher and the content of cellulose 28.9%, 36.5%, 53.3% and 68.5% higher than that of the control in the treatments of calcium nitrate, calcium chloride, calcium amino acid and calcium sorbitol, respectively. Cell wall degrading enzymes play an important role in fruit softening. During the storage, the activities of PME, PG and CX all increased and calcium treatments inhibited the increase in the enzyme activities, as they were all significantly lower compared with the control. At 35 days of storage, the activity of PME was 19.8%, 21.9%, 27.9% and31.8% lower; the activity of PG 5.4%, 8.7%, 15.3%, 19.6% lower, and the activity of CX 22.1%, 22.6%, 43.7%, 50.1% lower than the control in the treatments with calcium nitrate, calcium chloride, calcium amino acid and calcium sorbitol, respectively. Correlation analysis showed that fruit firmness was significantly negatively correlated with soluble pectin, and PG, CX and PME activities, but significantly positively correlated with protopectin and cellulose.【Conclusion】Postharvest calcium treatments could significantly reduce the activities of PME, PG and CX enzymes, inhibit the degradation of protopectin and cellulose, and suppress the rise of soluble pectin, and thus maintain the fruit firmness. Based on our results, application of calcium sorbitol gave the best effect among all the treatments, and therefore it is recommended for commercial use.