[The relationship between the desiccation-induced browning and the metabolism of active oxygen and phenolics in pericarp of postharvest longan fruit].

Longan (Dimocarpus longan Lour.)fruits are very susceptible to water loss and peri-carp browning, and postharvest pericarp brown-ing is the most important factors degrading thequality of longan fruit and shorting storage life.Pericarp browning has been attributed todesiccation, chilling, heat stress, senescence andpest or pathogen attack. Desiccation is the mostmain factor of induced-pericarp browning inlongan. The relationship between water loss frompericarp and pericarp browning in longan cv.Fuyan fruits using open plastic punnets andsealed polyethylene bags at 10°C±1°C and 50%relative humidity, and the effect of pericarp wa-ter loss of the fruit on active oxygen metabolismand phenolics metabolism were investigated.Water loss resulted in rapid pericarp browning.Development of pericarp browning was higherwith higher rate of water loss from pericarp andstorage time (from 0 to 6 days) (Fig.1A and B).Water loss from pericarp was positively corre-lated with pericarp browning index significantly(P0.01). Water loss from pericarp resulted inreduced activities of reactive-oxygen-scaveng-ing enzymes (SOD, CAT, APX and GR) (Fig.3),decreased amounts of endogenous antioxidantsubstances (AsA and GSH) (Fig.4), and increasedrates of O2·ˉ production (Fig.2A), MDA content(Fig.2B) and relative leakage rate (Fig.1C),which showed that membrane structure wasbroken. Water loss from pericarp resulted in anincrease in activity of PPO (Fig.6A), and obvi-ous reductions in total phenolic and flavonoid contents (Fig.5B and C), whereas there was notobvious change in anthocyanin content (Fig.5A).These results show that phenolics and flavonoidsare the main substrates for PPO during desicca-tion-induced browning. Water loss from pericarpcaused a significant increase in activity of POD(Fig.6B), which also plays an important role indesiccation-induced browning in pericarp of lon-gan fruit. Water loss from pericarp caused anincrease in pH value (Fig.1D), which resulted inchanges in anthocyanin structure and color, thedegradation of anthocyanin became easier. Theresults suggest that desiccation-induced brown-ing of longan pericarp may be due to a decreasein activities of reactive-oxygen-scavenging en-zymes and amounts of endogenous antioxidantsubstances, an accumulation of active oxygen,an increase in membrane lipid peroxidation, aninjury of the integrity of cellular membranestructure, which, in turn, may cause cellular de-compartmentation, resulted in PPO and POD,located in plastid and other organelle, to comeinto contact with phenolic and flavonoidsubstrates, located in vacuole, to form brownpolymers (Fig.7).