A study on the variation of cell wall components and activities of their degradation enzymes in sweet cherry during fruit development

Abstract：【Objective】The study was conducted to understand the relationship between the texture changes of sweet cherry and changes in cell wall components and their degrading enzyme activities during fruit ripening.【Methods】Sweet cherry trees of three cultivars, including the hard meat- type ‘Tieton’, the conventional type‘Hongdeng’and the soft meat-type‘Jiahong’, cultivated in a 12-yearold open orchard were used as the experiment materials. Samples were collected during core hardening, color- break (whitening), coloring, and full maturity to determine the fruit firmness, cell wall components, and the activities of cell wall degrading enzyme.【Results】‘Hongdeng’fruit was harder than the other two varieties in the early stage of fruit development, but the range of decline was large during the later stages.‘Jiahong’had the lowest hardness after color-break. With the development of the fruit, the hardness difference between the three varieties gradually decreased, and there was no significant difference between the them at full maturity, but the fruit hardness of‘Tieton’was higher than the other two varieties. The WSP content of the three varieties gradually increased from core hardening to colorbreak, and decreased or remained stable after the coloring period. The WSP content in‘Hongdeng’increased from the core hardening stage to the coloring stage (189.49%), while‘Tieton’WSP content in-creased by 43.60% . The WSP content of the three varieties was negatively correlated with hardness. The ISP content in the three varieties decreased first and then increased, rapid decreases in both ISP content and hardness were found during color-break. The ISP content peaked during core-hardening, after which the content decreased significantly in the three cultivars. During the fruit development, the difference gradually decreased. The CSP content in‘Tieton’and‘Hongdeng’decreased after color- break, while that in‘Jiahong’declined after the coloring period. Among them,‘Hongdeng’had the highest rate of decline (45.08%) with an increase towards full maturity. The change pattern was opposite to that of WSP. As the fruit matured, cellulose content in‘Hongdeng’and‘Jiahong’was significantly reduced, which was significantly lower than that in‘Tieton’in all periods. For‘Tieton’, there was no significant change in cellulose content from core hardening to fruit coloring. Although its cellulose content dropped significantly during maturation, it was still significantly higher than that in the other two varieties. The hemicellulose content of the three varieties showed a gradual decline during fruit development, and‘Tieton’and‘Hongdeng’had a higher content than that in‘Jiahong’. From color-break to the coloring period, hemicellulose content gradually decreased till full maturity when it had no significant difference. The activity of PG of the three varieties increased at the beginning, then remained stable or slightly decreased. The increase rate of PG activity in‘Tieton’was the highest (110.56% ), and it peaked during coloration in‘Tieton’and‘Hongdeng’. During the coloration and maturation periods, the activity of PG in‘Tieton’,‘Hongdeng’and‘Jiahong’decreased in turn and differed significantly. PME activity peaked during core Ihardening.‘Hongdeng’had a significantly higher PME activity than ‘Tieton’and‘Jiahong’.‘Jiahong’showed no significant difference in PME activity during the four stages, while in the‘Hongdeng’, a significant decrease occurred from core hardening to color break. In ‘Tieton’, the activity of PME decreased significantly from core hardening stage to coloration stage. The PL activity in the three varieties fluctuated with time and had no correlation with fruit firmness. The α-L-Af activity in the three varieties was the lowest during core hardening, and‘Tieton’had a significantly higher α-L-Af activity than the other two varieties. During fruit development, the activity of α-L-Af in‘Tieton’was the smallest, however, the rate of rise in‘Jiahong’was the largest. The activity of the enzyme in‘Hongdeng’rose to 392 nmol·g-1 ·min-1 during the maturation period, which was significantly higher than those in the other two varieties. The β-Gal activity of the three varieties was very low in the core hardening stage, and then, there was an upward trend. Among them,‘Tieton’had a lowest rate of increase,‘Hongdeng’had a highest rate of increase. In‘Jiahong’, a significant increase in enzyme activity was found and then the activity maintained high. The Cx activity of the three varieties showed an upward trend, but the growth rate in‘Tieton’and‘Jiahong’was higher than that in‘Hongdeng’. The difference in fruit development gradually increased. During the maturation period, the activity of this enzyme in‘Jiahong’continued to rise and became significantly higher than that in‘Tieton’ and‘Hongdeng’. During the fruit development of‘Tieton’, the change in hardness was significantly positively correlated with Cx. For‘Hongdeng’, the change in hardness was positively correlated with the change in hemicellulose and cellulose contents, but the activities of α-L-Af, β-Gal and PME were significantly negatively correlated. During the development of‘Jiahong’fruit, fruit hardness was significantly negatively correlated with WSP content and the activities of Cx, α-L-Af, and PME.【Conclusion】During the ripening of sweet cherry fruit, the degradation of protopectin and the hydrolysis of cellulose are the key factors for fruit softening. The degradation of fruit cell wall components is the result of a synergistic effect of multiple enzymes. The lower activities of PME and α-L-Af in‘Tieton’than that of the other two varieties might be the main reason for its higher fruit hardness. The low degrada-tion rates of cellulose and protopectin, high PG activity and low β- Gal activity might be secondary causes for its high hardness. The Cx enzyme seems to have different effects in‘Jiahong’and‘Tieton’ and seems not significantly affect the fruit hardness in‘Hongdeng’, which might be the difference between these varieties.