Study On Mechanism Of Aril Breakdown In Postharvest Longan Fruit Inoculated With Phomopsis Longanae Chi

Longan （Dimocarpus longan Lour.） is a famous subtropical fruit species in the South of China. It’s rich in nutritional, medicinal value and health value. Since longan fruit mature during the hot and humid August/September in China, It is hard to storage and transport. Because the postharvest physiological metabolism is vigorous, the postharvest longan pulp very easily causes aril breakdown. Long-distance transportation, marketing and consumption was limited by its short storage time. Aril breakdown was one of the most important factors affected the edible quality and commodity value seriously. Study on the relationships of aril breakdown in postharvest longan fruit inoculated with pathogenic bacteria is rarely reported.The Phomopsis longanae Chi. is one of the main pathogenic bacteria that cause aril breakdown after inoculated postharvest longan fruit. This paper chose longan cv. Fuyan as the experiment materials, which were pre-harvest bagging to control pathogen latent infection, the postharvest longan fruits were divided into inoculated with Phomopsis longanae Chi. group and control group, and packaged by polyethylene film bags （0.015 mm）and storaged at （28±1）℃; then we studied the efffects of cell wall metabolism, active oxygen metabolism and membrane lipid peroxidation, lignin（resistance-related material）and its enzyme activity in aril breakdown of postharvest longan fruit after inoculated with Phomopsis longanae Chi., the mechanism of aril breakdown was concluded. The rusults were as follows:1) Compared to the controlled group, the aril breakdown of postharvest longan fruit inoculated with Phomopsis longanae Chi. was extremely serious. Aril breakdown rate accelerated obviously and aril breakdown index was higher than the controlled group（P<0.05）.2) Compared to the controlled group, the contents of cell wall substances, soluble pectin, protopectin, cellulose, semicellulose of pulp inoculated with Phomopsis longanae Chi. were decreased progressively, the contents of soluble pectin, cellulose, semicellulose of pulp were lower than the controlled group（P<0.05）,but the protopectin was higher （P<0.05） during the storage time; The activities of PE, PG, Cx were higher than controlled group, they were increased at first and then decreased; the activity of PG was higher than the controlled group（P<0.05）. But, the activity of Cx was significantly higher （P<0.01）. They accelerated the degradation of soluble pectin, cellulose, semicellulose; The activity ofβ-galactosidase was lower than the controlled group during 0-2 days, the peak of enzyme activity appeared in the 3th day; The result showed that PE played the key role at the prophase of aril breakdown; PG, Cx andβ-galactosidase played the key role at the middle phase of aril breakdown after the infection of Phomopsis longanae Chi.; it showed that the increase of cell wall degrading enzymes activities at the different time accelerated the degradation of cell wall components , lead to the development of aril breakdown. The protection system of longan fruit was damaged quickly that accelerated aril breakdown.3) Compared to the controlled group, the productive rate of O₂^-., the activities of SOD, APX, the content of MDA were higher significantly （P<0.05）, the activity of CAT, the contents of GSH, AsA were lower（P<0.05）, and the activity of LOX was significantly higher （P<0.01）; The results showed that the rate of O₂^-. was increasing rapidly, and the decrease of CAT,GSH,AsA in the longan defense system after the Phomopsis longanae Chi. infection lead to the accumulation of O₂^-., which induced the increase of SOD, APX; The O₂^-. can’t be cleaned normally, that promoted the activity of LOX, started the membrane lipid peroxidation and increased the toxic content of MDA, they caused damage of cell membrane system and accelerated the progress of aril breakdown seriously.4) Compared to the controlled group, the content of lignin, the activities of PAL, POD were higher; The results showed that the synthesis of resistance substances such as lignin was accelerating with the increase of PAL and POD, which can strengthen the thickness of cell wall, added the lignification of cell organization and improved the resistance of longan; However, the results also showed that there were not significant difference with lignin content and POD activity between the two groups. In another word, the increase content of resistance substances improved the resistance of longan shortly, but it couldn’t prevent the infection from Phomopsis longanae Chi.; So the aril breakdown of longan fruits aggravated during the storage time.