| Abstract: | Gamma irradiation (1.0 kGy) of intact, newly harvested, mature muskmelon fruit (Cucumis melo L. var. reticulatus Naud.) appears to have an immediate deleterious effect, but also a long-term beneficial effect, on the integrity and function of the plasma membrane (PM) of hypodermal mesocarp tissue. The initial consequences of gamma irradiation included an increase in the free sterol:phospholipid ratio, resulting at least in part from deglycosylation of steryl glycosides, a decrease in the spinasterol:7-stigmastenol ratio in each of the PM steryl lipids (free sterols, steryl glycosides, and acylated steryl glycosides), and a decrease in H+-ATPase activity. Irradiation did not increase protein loss, suggesting that the decrease in H+-ATPase activity resulted from either direct inactivation of the enzyme or altered PM ordering caused by the steryl lipid modifications. The long-term beneficial effects of irradiation, observed following 10 days of commercial storage, included greater retention of total PM protein, a diminished decline in total PM phospholipids (PL) and in the PL:protein ratio, and maintenance of greater overall H+-ATPase activity (activity was the same as in controls on a per mg protein basis, but there was > 30% more protein in the PM of stored irradiated fruit). These results indicate that 1 kGy gamma irradiation administered prior to storage slowes the progression of two key parameters of senescence, PM protein loss and PL catabolism. |
