Responses Of Phospholipase D And Antioxidant System To Mechanical Wounding In Postharvest Cucumber Fruits

Mechanical damage is considered as a type of stress, which is also the main cause of losses in the postharvest horticultural products. It can produce several signals that migrate through injury tissure into uninjured tissue and induce some physiological responses. In rescent years, the research on responses of mechanical wounding in postharvest horticultural products is highly attentioned.This study was to investigate the response of phospholipase D (PLD), lipoxygenase (LOX) and antioxidant system to mechanical wounding in postharvest cucumber (Cucumis sativus L. cv. Biyu-2) fruits. "Wound respiration" and "wound-induced ethylene", membrane-associated Ca2+ content, activities of PLD and LOX, contents of phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidic acid (PA) and electrolyte leakage were determined in cucumber fruits following mechanical wounding. Results show that "wound respiration" and "wound-induced ethylene" were induced by mechanical wounding. PLD and LOX activities increased upon wounding, while membrane-associated Ca2+ content decreased. Accompanying with the increase of PLD and LOX activities, accumulation of PA and loss of PC and PI were observed in all fruits, at the same time, higher electrolyte leakage was observed with activities of PLD and LOX increased. Results suggest that PLD and LOX might be the main hydrolytic enzymes of phospholipids in postharvest cucumber fruits participating in the mechanical wounding injury.Individual cucumber fruit were wounded via puncture in the navel, middle and neck tissues. Changes of "wound respiration" and "wound-induced ethylene", lectrolyte leakage, activities of PLD, LOX, contents of PC, PI, PA and electrolyte leakage, membrane-associated Ca2+ content were found in navel, middle and neck tissues of cucumber fruits at 4 h after wounding. Both "wound respiration" and "wound-induced ethylene" was significantly higher in navel end than other parts. Activities of PLD and LOX were induced by wounding, which higher activities of PLD and LOX was observed in navel end while membrane-associated Ca2+ content decreased. This result indicated that more mature tissue (navel) exhibited more sensitive in responses to wounding than younger tissues (middle and neck).We studied the variation of cucumber fruits antioxidant system to mechanical wounding. The rusults indicated that mechanical wounding could induced O2·- in cucumber fruits. During the storage of cucumber after wounding, the accumulation of H2O2 was provided to undergo a increasing-decreasing-increasing change, while the activity of SOD, POD, CAT, APX, and MDHAR could reach a maximu and then descent in the following hours. The activity variation of DHAR showed similar tendency to anti-oxidant enzymes with stable descend in the last storage period. Mechanical wounding could active the anti-oxidant enzymes in cucumber. Though the amplitude was much smaller, variation of GR activity in wounded cucumber was similar to H2O2 content. However, the wounded cucumber was always showing lower activity to control fruits. The contents of ASA and GSH were lower in wounded fruits than control, while the contents of DHA and GSSH were higher in wounded cucumber fruits than control fruits.We found that the order for contents of O2·- and H2O2 was navel end> middle> neck end. There was higher levels of O2·- and H2O2 in wounded cucumber fruits than control fruits. The activities of SOD, POD, CAT, MDHAR, APX, DHAR in navel end were higher than other parts of cucumber fruits. But activity of GR was different from other anti-oxidant enzymes, GR activity in navel end was the lowest in three parts of cucumber. Contents of ASA and GSH were as similar as GR in navel end, which was lower than control fruits. There were the highest contents of DHA and GSSG in navel than in middle and neck end, which induced lower GSH/GSSG in navel end. Therefore, the results suggest that response to mechanical wounding stress was maturity-dependent.Mechanical wounding could upregulate the gene expression of PLD, LOX, and some oxidation relative enzymes with time-effect. The expression levels of PLD and LOX in wounded fruits were significantly higher than control fruits. The expression of anti-oxidant enzymes such as SOD, POD, CAT, APX, MDHAR, and DHAR was higher in wounded cucumber fruits than control fruits in early hours of storge. However, the expression of these enzymes reduced sharply, and even lower than the control fruits during the later period of storge (8-10 h). The expression regulation of GR was accordance with other anti-oxidant enzymes though its expression was always lower than control fruits.The expressions of PLD, LOX, SOD, POD, CAT, APX, MDHAR, and DHAR in different cucumber parts were all higher than their control fruits following an order of fruit navel end> middle> neck end. The expression abundance of GR gene showed an opposite tendence to the above enzymes with an order of neck end> middle> fruit navel end. We finally concluded that the expression of anti-oxidant enzymes increased with the maturity of cucumber fruits.