Phenotypic changes of ‘Kyoho’ berries following NaL treatment
Different concentrations of NaL (0.1%, 0.5% and 1% NaL) were used to treat postharvest ‘Kyoho’ berries, and the phenotypic changes of grape clusters within 10 days were observed. After 2 d of storage, abscission symptoms were visible in the control and treatment group, and as the storage time extended, the abscission of CK became more pronounced. The abscission rate of ‘Kyoho’ fruit after treatment with different concentrations of NaL was significantly decreased compared with the control in different storage periods (Figure 1). The results indicated that NaL treatment had an inhibitory effect on grape berry abscission, and the 1% NaL treatment had the strongest inhibitory effect.
Effect of NaL treatment on the physiological characteristics of ‘Kyoho’ berries
The abscission rate of grapes gradually increased during storage, and NaL treatment inhibited the abscission of grape berries (Figure 2A). The grapes began to drop after 2 d of storage, and the abscission rate of ‘Kyoho’ berries after NaL treatment was always lower compared with the control (Figure 2A). The abscission rate of CK reached 63.71% after 10 d of storage. However, the abscission rate of grapes treated with 0.1%, 0.5% and 1% NaL were only 25.8%, 21.4% and 12.4% on the same day, respectively.
The rate of weight loss increased rapidly as the storage time extended. After 10 d of storage, the weight loss rate of the control reached 6.83%. Compared with the control group, the weight loss rate of ‘Kyoho’ berries was reduced after treatment with different NaL concentrations (Figure 2B). The 1% NaL concentration had the strongest inhibitory effect, and the weight loss rate was only 2.92% after 10 d of storage. Similarly, the browning degree of grape rachis increased as the storage time extended, and the browning index of rachis was consistently lower in the NaL treatment groups than in CK (Figure 2D). In addition, the fruit firmness of the CK and NaL treatments consistently decreased during storage; however, there were no significant differences in the fruit firmness between any of the treatments (Figure 2C). Thus, NaL treatment reduced the weight loss and rachis browning level of ‘Kyoho’ berries, and 1% NaL had the strongest effect (Figure 2B-D).
Effect of NaL treatment on the fruit quality of ‘Kyoho’ berries
TSS and TA are important indexes for evaluating the taste and flavor of grapes. The TSS and TA of ‘Kyoho’ berries in all groups did not change significantly throughout the 10 d of storage at 20°C (Figure 3A, C). The VC content of grapes decreased after 4 d of storage, and the VC content was lower in the NaL treatments than in the control (Figure 3B). Similarly, NaL treatment inhibited the reduction in the reducing sugar content in grapes during storage (Figure 3D), and the treatment with the strongest effect was 1% NaL, which reduced the loss of postharvest quality. Thus, NaL treatment reduced the loss of VC and reducing sugar, improved the postharvest quality of ‘Kyoho’ berries and did not affect the taste and flavor of the fruit.
Effect of NaL treatment on the MDA content and membrane permeability of ‘Kyoho’ berries
MDA accumulation can damage the cell membrane and accelerate fruit senescence. During storage, the content of MDA continuously increases. Compared with the control group, the accumulation of MDA in grapes was inhibited by NaL treatment (Figure 4A). After 2 d of storage, the membrane permeability of grape began to increase as the storage time extended. The membrane permeability of CK peaked 6 d after storage and then began to decrease. After NaL treatment, the membrane permeability of grape increased slowly and was consistently lower than that of CK (Figure 4B). This indicated that NaL treatment inhibited MDA accumulation, decreased membrane permeability and delayed grape senescence. The effect of the 1% NaL treatment was the strongest.
Effect of NaL treatment on the activities of cell wall-degrading enzymes of ‘Kyoho’ berries
To explore why NaL treatment can inhibit the abscission of table grapes, changes in the activity of four cell wall-degrading enzymes in ‘Kyoho’ berries were measured at 20℃. During storage, the activities of PG, POD, Cx and LOX increased continuously, which was positively correlated with the abscission rate of ‘Kyoho’ berries. Compared with the control, NaL treatment inhibited the increase in PG, POD, Cx and LOX activities (Figure 5A-D), and the effect of the 1% NaL treatment was the strongest. These results indicated that NaL treatment could effectively inhibit the increase in cell wall-degrading enzyme activity.
Effect of NaL treatment on the activity of energy metabolism-related enzymes of ‘Kyoho’ berries
Based on the physiological and phenotypic results, it was found that the 1% NaL treatment was the most effective for inhibiting berry abscission and maintaining fruit quality of berries. To explore whether NaL treatment affect the energy state of postharvest grape fruit, 1% NaL treated samples of ‘Kyoho’ berries were selected for the determination of the activity of enzymes involved in energy metabolism-related indexes, including mitochondrial ATPase (H+-ATPase and Ca2+-ATPase), mitochondrial respiratory metabolic enzymes (SDH and CCO). The activity of H+-ATPase, Ca2+-ATPase, SDH and CCO was higher in the 1% NaL treatment group than in the control group during the storage period, and the difference was significant during the late part of the storage period (Figure 6A-C). After harvest, the activity of CCO increased before 6 d of storage and then decreased. The activity of CCO was higher in the NaL treatment group than in the control (Figure 6D). This indicated that NaL treatment increased the activity of H+-ATPase, Ca2+-ATPase, SDH and CCO.
Effect of NaL treatment on ATP, ADP, and AMP content and energy charge of ‘Kyoho’ berries
ATP is the main energy source for biological tissues and cell metabolism and thus indicates the physiological state of plants. The ATP content decreased during storage. The ATP content of the 1% NaL treatment group decreased slowly and was higher compared with the control group (Figure 7A). During storage, the content of ADP peaked at 4 d of storage and then began to decline (Figure 7B). In contrast to the trend in ATP, the content of AMP increased continuously. NaL treatment inhibited the increase in AMP content, which was always lower compared with the control (Figure 7C). The pattern of energy charge was consistent with the ATP results, NaL treatment increased the energy charge (Figure 7D). Thus, NaL treatment maintained the ATP content and energy charge of ‘Kyoho’ berries during storage.