Study On Mechanisms Of Mealiness Or Leaheriness Development In Chilling Injured Peach Fruit

Peach fruit (Prunus persica) are very popular for their attractive color, appealing flavor, soft and juicy texture, and rich in nutrient. But peach fruit are very perishable after harvest due to their rapid ripening and softening for high temearature in mid summer and climacteric characteristics. Low temperature can effectively delay ripening and senescence processes, but fruit are susceptible to chilling injury with prolonged storage time. It menifested itself as mealiness, leatheriness, dry or fresh browning. At present, although many technologies such as controlled atmosphere, heat treatment、low temperature conditioning or natural volatile compounds were used to alleviate chilling injury. But for the characteristics and mechanisms of mealiness or leatheriness are unclear, the effects of these technologies are controversial. It is therefore crucial to study the mechanisms and control technologies of chilling injury. Basing on the kinetic of firmness of peach fruit in normal ripening and the effect of chilling temperatures and maturities on chilling injury. The present study was to investigate the differences between mealiness and leatheriness of cell wall metabolisms, subcellular water distribution. And effects of hot air treatment, 1-MCP, exogenous ethylene treatments on mealiness or leatheriness were also studied. The results were as follows:(1) Effects of various storage temperatures from 10 to 30℃(283.15 to 303.15 K) on firmness were investigated, and a model for predicting the effects of storage temperature on fruit firmness was developed. The model contained two kinetics, a zero-order kinetic in pre-climacteric stage with the energy activation of 68.8 kJ/mol, and a first-order kinetic in climacteric stage with the energy activation of 76.9 kJ/mol. In validate, the coefficient of determination (R2) and root mean squared error (RMSE) between the predicted and measured firmness were 0.995 and 0.567 N, respectively. The model gave satisfactory predictions of fruit firmness in postharvest peach fruit. (2) The chilling injury intensity and symptoms in peach fruit at 0,3,5 and 7℃and effect of maturity on chilling injury at 0,5℃were investigated. Leatheriness and mealiness were two different types of disorders. The lower the temperature, the lower susceptibility. At 0℃, leathereness was the main CI symptom, while at 5℃, mealiness was the main CI symptom. The higher maturity, the earlier mealiness, but the later leatheriness. Juice rate, TSS and TA signinficantly decreased after mealiness or letheriness, and the effect of leathiness was higher than that of mealiness.(3) The changes in cell wall pectins and lignin in normally ripening (juicy) and in mealy or leathery peach fruit was investigated. Fruit at 5℃15d plus 3d at 20℃, a higher level of PME, exo-PG and a lower level of endo-PG, and a higher level of CSF and NSF1 and a lower proportion of WSF and NSF2, inducing mealiness symptom. Fruit at 0℃35d plus 3d at 20℃, low exo-PG and endo-PG and higher level of PAL and POD, losing fruit ripening and inducing leatheriness symptoms. These results suggested that mealiness was due to the imbalance of pectin degradation, while leatheriness was due to lack of ripening ability and lignin synthesis.(4) Proton NMR relaxation was used to evaluate the changes of water compartments in normal ripening, chilling-injured peach fruit. Four different Proton NMR relaxation times can identify four populations of water with different relaxation characteristics. The observed relaxation times originated from particular water compartments:the vacuole, the extra cellular space, the cytoplasm and the cell wall. The relaxation time distributions during ripening showed that relative area in extra cellular space increase and in vacuole decrease during ripening. Both mealiness and leatheriness decreased water content in extracelluar space. These results suggested that the low water content in extracelluar space cause dry fresh of peach frui, NMR could be used to detect the CI.(5) Peach fruit were treated with 5 uL/L 1-MCP or 5 ul/L ethylene at 20℃for 12h, and then stored at 5℃or 0℃to study the effects of ethylene metabolisms on the development of mealiness or leatheriness, respectively. In the later stage at 0℃, ACC content, ACO activity and ethylene production decreased, fruit lost ripening ability and manifested as leatheriness. Exogenous ethylene alleviated leatheriness, while 1-MCP advanced it. Fruit at 0℃, ripen with increase of ethylene production, and manifested as mealiness.1-MCP alleviated mealiness, while exogenous ethylene advance it. These results suggested that endogenous was necessary to fruit ripening, but excess ethylene accelerate the development of mealiness. (6) Effect of hot air threatment (HAT) on leateriness of peach fruit stored at 0℃was investigated. The higher PME but low level of exo-PG and endo-PG in the later stage at 0℃, fruit lost ripening ability and manifested as leatheriness. A higher ethylene production, PME, exo-PG and endo-PG activities increased in heat-treated fruit, restored the ripening ability. Exo-PG activities increased but endo-PG decreased, inducing the developmnet of mealiness. These results suggested it may be related with the increase of ethylene after HAT, which have different effects on the development of mealiness or leatheriness.