Physiological And Biochemical Mechanism Of Browning Development Caused By Active Oxygen In Pericarp Of Harvested Longan Fruits

Longan(Dimocarpus longan Lour.)is a famous subtropical fruit in the world.Longan is grown commercially in many countries,including China,Thailand,Vietnam,India,Australia and some tropical and subtropical regions in the U.S.,which account for most of the world production.Longan is one of the most economically important fruits in the south of China.The longan industry has undergone rapid expansion since 1990 in China,the largest cultivation and production nation in the world,with 73.6%cultivated area and 59.7%output.Because longan fruits mature under high temperature and humidity,the fruits deteriorate rapidly after harvest due to pericarp browning,aril breakdown and fruit decay,amongst which pericarp browning is the most important factor affecting the shelf life of longan fruit.Previous studies showed that senescence and browning in plant tissues may be attributed to the decrease of active oxygen scavenging capacity and the increase accumulation of active oxygen.Therefore,increasing the capacity of active oxygen scavenging or decreasing the accumulation of active oxygen should be a vital way to delay pericarp browning and extend storage life of harvested longan fruits.Exogenous supply of active oxygen,hydrogen peroxide,was used to postharvest longan fruits in an attempt to understand postharvest physiology and quality,active oxygen metabolism,respiratory metabolism,energy metabolism,membrane lipid metabolism and phenolics metabolism and its relationship to browning induced by the accumulation of reactive oxygen species in pericarp of harvested longan fruits.Exogenous active oxygen scavenger,propyl gallate,was also used to longan fruits in an attempt to further understand the crucial role of active oxygen scavenger on the membrane integrity and the maintain of cellular compartmentation,discover the possible physiological and biochemical mechanism of inhibiting browning in pericarp of harvested longan fruits by exogenous propyl gallate..The results were as follow.1.The relationship between the active oxygen-induced browning and cellular decompartmentation in pericarp of longan fruits(1)Effects of active oxygen(hydrogen peroxide)treatment on quality and physiology of harvested longan fruitsActive oxygen(hydrogen peroxide)treatment could increase respiration rate,accelerate the increase of cell membrane relative leakage rate,accelerate the degradation of pulp nutrient ingredients such as total sugar,sucrose and vitamin C,and accelerate the degradation of pericarp pigment substance,such as chlorophyll,carotenoids,flavonoids and total phenols,increase pericarp browning index and aril breakdown index,and keep lower healthy fruit percentage.(2)The relationship between the hydrogen peroxide-induced browning and the metabolism of active oxygen in pericarp of harvested longan fruitsActive oxygen(hydrogen peroxide)treatment could accelerate the production rate of O2-.increase MDA content,reduce activities of active oxygen scavenging enzymes such as superoxide dismutase(SOD),catalase(CAT)and ascorbate peroxidase(APX),and contents of endogenous antioxidant substances such as ascorbic acid(AsA),glutathione(GSH)and carotenoid,which result in weakening the capacity of scavenging active oxygen and accelerating the accumulation of active oxygen,and then accelerating pericarp browing of longan fruits.(3)The relationship between the hydrogen peroxide-induced browning and the metabolism of respiratory in pericarp of harvested longan fruitsActive oxygen(hydrogen peroxide)treatment could significantly enhance activities of respiratory terminal oxidase(CCO and AAO)and respiration rate,decrease energy charge,inhibit activity of NAD kinase(NADK),accelerate the decrease in NADP(H)contents,maintain high NADH content during the whole storage time and NAD content in the early stage,which lead to accelerating electron transfer,and then accelerate pericarp browing of longan fruits.(4)The relationship between the hydrogen peroxide-induced browning and the metabolism of energy in pericarp of harvested longan fruitsActive oxygen(hydrogen peroxide)treatment could reduce contents of ATP and ADP,and energy charge(E.C),inhibit activities of Ca2+-ATPase,Mg2+-ATPase and H+-ATPase,breaking the ion balance inside and outside of cells and the function of cell membrane on choosing and compartmentalization,and accelerate pericarp browing of longan fruits.(5)The relationship between the hydrogen peroxide-induced browning and the metabolism of membrane lipids in pericarp of harvested longan fruitsActive oxygen(hydrogen peroxide)treatment could significantly promote activities of lipoxygenase(LOX)and lipase,reduce relative contents of the unsaturated fatty acids such as linoleic acid and linolenic acid,decrease the index of unsaturated fatty acids(IUFA)and the ratio of unsaturated fatty acids to saturated fatty acids(U/S),break integrity of cell membrane structure,and then accelerate pericarp browning of longan fruits.(6)The relationship between the hydrogen peroxide-induced browning and the metabolism of phenolic in pericarp of harvested longan fruitsActive oxygen(hydrogen peroxide)treatment could enhance activities of phenolase such as PPO and POD,which accelerated enzymatic oxidation of polyphenols and pericarp browning of longan fruits.2.Effects of active oxygen scavenger on postponing cellular decompartmentation and controlling pericarp browning of longan fruits(1)Effects of active oxygen scavenger(propyl gallate)treatment on quality and physiology of harvested longan fruitsActive oxygen scavenger(propyl gallate)treatment could effectively reduce fruit respiration rate,delay the increase of cell membrane relative leakage rate in pericarp,keep higher contents of pulp nutritional ingredients such as total sugars,sucrose and vitamin C,and higher contents of chlorophyll,carotenoid,flavonoid and total phenolics in pericarp,decrease pericarp browning index and aril breakdown index,and keep higher healthy fruit percentage.(2)Effects of active oxygen scavenger(propyl gallate)treatment on active oxygen metabolism in pericarp of harvested longan fruits and its relation to pericarp browningActive oxygen scavenger(propyl gallate)treatment could decelerate the production rate of O2-,decrease MDA content,enhance activities of active oxygen scavenging enzymes such as SOD,CAT and APX,and contents of endogenous antioxidant substances,such as AsA,GSH and carotenoid,which result in strengthening the capacity of scavenging active oxygen,inhibiting peroxidation of membrane lipid and accumulation of MDA content,and then delaying pericarp browing of longan fruits.(3)Effects of active oxygen scavenger(propyl gallate)treatment on respiratory metabolism in pericarp of harvested longan fruits and its relation to pericarp browningActive oxygen scavenger(propyl gallate)treatment could inhibite activity of AAO throughtout storage time and activity of CCO in the early stage,inhibite respiratory rate,delay the decrease of energy charge.Meanwhile,longan fruits treated with propyl gallate kept higher activity of NAD kinase(NADK),higher content of NADP(H),decreased NADH content and NAD content in the early stage,which lead to decelerating electron transfer,and then delay pericarp browning of harvested longan fruits(4)Effects of active oxygen scavenger(propyl gallate)treatment on energy metabolism in pericarp of harvested longan fruits and its relation to pericarp browningActive oxygen scavenger(propyl gallate)treatment could inhibite the decrease of ATP and AMP,maintain relatively higher energy charge level,increase Ca2+-ATPase,Mg2+-ATPase,H+-ATPase activities,which inhibit the losing of the function of cell membrane on choosing and compartmentalization,kept the ion balance inside and outside of cells and the function of cell membrane on choosing and compartmentalization,and then delaying pericarp browing of longan fruits.(5)Effects of active oxygen scavenger(propyl gallate)treatment on membrane lipids metabolism in pericarp of harvested longan fruits and its relation to pericarp browningActive oxygen scavenger(propyl gallate)treatment could inhibit activities membrane lipid-degrading enzymes such as LOX and lipase,decrease the degradation of unsaturated fatty acids,maintain high level of the index of unsaturated fatty acids(IUFA)and the ratio of unsaturated fatty acids to saturated fatty acids(U/S),maintain the integrity of cell membrane structure,and then inhibit pericarp browning of longan fruits.(6)Effects of active oxygen scavenger(propyl gallate)treatment on phenolic metabolism in pericarp of harvested longan fruits and its relation to pericarp browningActive oxygen scavenger(propyl gallate)treatment could inhibit activities of phenolase such as PPO and POD,which delay enzymatic oxidation of polyphenols and pericarp browning of longan fruits.