Pre-harvested Spraying DA-6 For Delaying Aril Breakdown Of Harvested Longan Fruits And Its Action Mechanism

Longan(Dimocarpus longan Lour.)is a famous subtropical fruit with high nutritional and medicinal value.Because longan fruits mature in the hot season during August to September,the fruits deteriorate rapidly after harvest due to pericarp browning,aril breakdown and fruits decay,which effects the quality and shelf life of longan fruit.'Fuyan' longan(Dimocarpus longan Lour.cv.'Fuyan'),the chief longan cultivar in Fujian Province of China,were treated with DA-6 before harvest,the objectives of this research were to clarify the action mechanism of pre-harvested spraying DA-6 for delaying aril breakdown of harvested longan fruits by investigating active oxygen metabolism,membrane lipid metabolism,tissue energy metabolism and cell wall metabolism in relation to aril breakdown,and the changes of activities of disease defense-related enzymes in relation to disease-resistance of fruit.The results were showed as follows:The relationship between the active oxygen metabolism and delaying aril breakdown of harvested longan fruits by pre-harvested spraying DA-6:Pre-harvested spraying DA-6 kept hinger activities of active oxygen enzymes of longan fruits such as dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX),and contents of endogenous antioxident substances such as ascorbic acid(AsA)and glutathione(GSH),which result in strengtheing the capacity of scavenging active oxygen,inhibiting peroxidation of membrane lipid and accumulation of malondialdehyde(MDA)content,and then delaying aril breakdown of longan fruits.The relationship between the membrane lipid metabolism and delaying aril breakdown of harvested longan fruits by pre-harvested spraying DA-6:Pre-harvested spraying DA-6 decreased activities membrane lipid-degrading enzymes such as phospolipase D,lipase and lipoxygenase,which decreased the degradation of unsaturated fatty acids,such as linoleic acid(C18.2)and linolenic acid(C18:3),and inhibited the increase of the relative content of the saturated fatty acid palmitic acid(C16:0)and stearic acid(C18:0)than control fruits during storage,maintained high level of the index of unsaturated fatty acids(IUFA)and the ratio of unsaturated fatty acids to saturated fatty acids,resulted in maintaining the integrity of cell membrane.The relationship between the tissue energy metabolism and delaying aril breakdown of harvested longan fruits by pre-harvested spraying DA-6:Pre-harvested spraying DA-6 could inhibite the decrease of ATP and ADP,maintain relatively higher energy charge level,increase Ca2+-ATPase?Mg2+-ATPase?H+-ATPase and NAD kinase activities,higher content of NADP(H),decreased NADH content and NAD content,and then delaying aril breakdown of longan fruits.The relationship between the cell wall metabolism and delaying aril breakdown of harvested longan fruits by pre-harvested spraying DA-6:Pre-harvested spraying DA-6 decreased activities of cell wall-degrading enzymes such as petinesterase(PE),polygalacturonase(PG),cellulase and ?-galactosidase,retarded the decrease of contents of cell wall material,protopectin,soluble pectin,cellulose and semicellulose,and delayed an increase of content of soluble pectin,maintained the relative integrity of cell wall and delayed the aril breakdown of longan fruits.The relationship between the changes of activities of disease defense-related enzymes and delaying the disease-resistance of harvested longan fruits by pre-harvested spraying DA-6:Pre-harvested spraying DA-6 maintained activities of disease defense-related enzymes,such as chitinase,?-1,3-gulcanase,phenylalanine ammonialyase and peroxidase activity and increased the content of lignin,which effectively delayed the longan fruits decay because of pathogens.