Study On Physiological And Biochemical Mechanism Of Inhibiting Browning In Pericarp Of Harvested Longan Fruits By Exogenous Ascorbic Acid Treatment

Longan(Dimocarpus longan Lour.)is one of the most economically important fruits with high commercial value and nutritional value in the south of China.But most of longan fruits mature in August or Spetember under high temperature conditions in China,the fruit are extremely easy to dehydrate,brown and infected by pathogen after harvest,which has a serious impact on fruit quality and commercial value,bringing great difficulties to its long-term storage and long-distance transportation.The pericarp browning has been considered a main factor affecting its storage life and quality of Postharvest longan fruit.Our previous studies have shown that,accumulation of reactive oxygen species,which leads to membrane lipid peroxidation,may be a key factor caused the browning of Longan Fruit,but its physillogical mechanism is not clear,So it is necessary to sdudy the physillogical mechanism,how does AsA control longan browning.Exogenous reactive oxygen scavenger(ascorbic acid,AsA)was used in the postharvest longan fruits in an attempt to further understand the physiological and biochemical mechanism of browning development induced by reactive oxygen species in pericarp of harvested longan fruit.'Fuyan' longan(Dimocarpus longan Lour.'Fuyan')fruits,the major longan cultivar in Fujian Province of China were used as the experiment material.The objectives of this research were to investigate the effects of exogenous ascorbic acid on postharvest physiology and quality of harvested 'Fuyan']ongan fruits.The another objectives of this research were to clarify the physiological and biochemical mechanism of inhibiting browning in pericarp of harvested longan fruits by exogenous ascorbic acid treatmen by investigating active oxygen metabolism,tissue energy metabolism,membrane lipid metabolism and phenolics metabolism.The rusults were showed as follows:1.AsA treatment retarded the process of longan pericarp browning.2.AsA treatment reduced the respiration rate,it also delayed the increase of cell membrane relative leakage rate,slowed aril breakdown index and nutrient substance degradation of longan fruits,retarded the change of apparent color of fruit and the process of longan pericarp browning,reduced weight loss and decay,maintained a higher rate of healthy fruit percent.3.AsA treatment could increase the activities of superoxide dismutase(SOD),catalase(CAT),ascorbate peroxidase(APX)and contents of glutathione(GSH),ascorbic acid(AsA)and flavonoids in pericarp of harvested longan firuits.It inhibited the increase of the production rate of O2-and content of malondialdehyde(MDA).From the results it can be concluded that AsA treatment enhanced the capacity of scavenging reactive oxygen,which reduced the accumulation of reactive oxygen species,inhibited the membrance lipid peroxidation and maintained the stability and integrity of cellular membrane structure.4.AsA treatment decreased activities of membrane hydrolysis enzyme(lipoxygenase,esterase,phospholipase D)in pericarp of harvested longan fruits,delayed the degradation of unsaturated fatty acids of membrane lipids,such as oleic acid(C18:1),linolenic acid(C18:3),retarded the increase of saturated fatty acids such as palmitic acid(C16:0),stearic acid(C18:0),Lignoceric acid(C24:0),maintained relatively higher index of unsaturated fatty acids(IUFA)and unsaturated degree of fatty acids,inhibited the membrance lipid peroxidation,maintained the stability and integrity of cellular membrane structure,along with the integrity of cellular compartmentation in the membrane structure,retarded the process of longan pericarp browning.5.AsA treatment maintained relatively higher ATP content and energy charge level,increased activities of Ca2+-ATPase,Mg2+-ATPase,H+-ATPase,maintained the ion balance inside and outside of cells and the function of cell membrane on choosing and compartmentalization.6.AsA treatment maintained higher content of total phenolic,decreased PAL activity,inhibited the change BPPO,BPOD to FPPO,FPOD respectively,decreased activities of FPPO and FPOD,maintained integrity and compartmentation of enzyme and browning substrates in the membrane structure,restrained pericarp browning of longan fruits.