Quality Maintenance of Postharvest Horticultural Crops by Stress Treatments and Approach for the Elucidation of Its Mechanism

This review deals with the effectiveness and mechanism of stress treatments, such as heat and UV treatments (hot air, hot water, and UV-B), on the quality maintenance of postharvest green horticultural crops such as broccoli florets and green sour citrus fruits (lime, Nagato-yuzukichi, and yuzu), in which chlorophyll (Chl) degradation is a main factor in quality deterioration. Postharvest stress treatments effectively suppressed Chl degradation with senescence during storage. In broccoli florets and lime fruit, hydrogen peroxide, which is produced by postharvest stress treatments, could induce the activation of the ascorbate-glutathione (AsA-GSH) cycle, and enhancement of the AsA-GSH cycle might be involved in the suppression of senescence in such produce. The mechanism of peroxidase-mediated Chl degradation and the control of Chl-degrading enzymes by stress treatments were determined using broccoli florets and Nagato-yuzukichi fruit. Peroxidase oxidized phenolic compounds such as apigenin, naringin and p-coumaric acid, which have a hydroxyl group at the p-position, to form phenoxy radicals, and Chl a could be degraded by the formed phenoxy radicals to colorless low-molecularweight compounds. It was found that the cationic isoperoxidase related to Chl degradation, which is localized in the chloroplast, increased with senescence during storage and enhancement of the activity was effectually inhibited by stress treatments. In addition to peroxidase, the activities and gene expression of other Chl-degrading enzymes were controlled by stress treatments. Thus, stress treatments seem to be a useful method for controlling Chl degradation in stored green fruits and vegetables.