Abstract:

The 12-year-old of‘Red Globe’grapes were treated with DL-4-chlorophenylalanine（CPA，200 μmol · L^-1，melatonin synthesis inhibitor），50–200 μmol · L^-1 melatonin（MT）and distilled water（control），respectively，by foliar spraying at early June（22 d of leaf age）. The sprays were applied five times with an interval of 30 d. The leaf chlorophyll content，reactive oxygen species levels，superoxide dismutase（SOD），peroxidase（POD），catalase（CAT），and ascorbic acid–glutathione（AsA–GSH）cycle enzyme activities in chloroplast，mitochondrial，and cytosolic fractions were measured periodically. The results showed that the content of chlorophyll rapidly decreased，reactive oxygen species levels in each subcellular fraction gradually increased，and the activity of antioxidant enzymes and AsA–GSH cycle enzyme gradually decreased at 115 d of leaf development. Among subcellular fractions，the $\mathrm{O}_{2}^{\overline{·}}$ level was highest in the chloroplasts and the H₂O₂ level was highest in the cytosol during leaf senescence. 150 μmol · L^-1 MT treatment significantly decreased the $\mathrm{O}_{2}^{\overline{·}}$ and H₂O₂ levels，while the activities of SOD，POD，and CAT were significantly increased in each subcellular fraction. Meanwhile，the MT treatment at 150 μmol · L^-1 significantly increased the activities of ascorbate peroxidase（APX），ascorbate oxidase （AAO），dehydroascorbate reductase（DHAR），monodehydroascorbate reductase（MDHAR），and glutathione reductase（GR）in each subcellular fraction，leading to a significant increase in AsA，DHA and GSH contents，as well as the contents of chlorophyll a，chlorophyll b，and carotenoid in grape leaves. As a result，the senescence process of leaves was delayed. On the contrary，the CPA treatment inhibited the activities of antioxidant enzymes and AsA–GSH cycle enzyme in each subcellular fraction，and the reactive oxygen species levels in each subcellular fraction were increased and the chlorophyll content decreased. In summary，exogenous MT increases the AsA and GSH contents in leaf subcellular fractions by increasing SOD，POD，CAT and AsA–GSH cycle enzyme activities，and thus enhances the antioxidant capacity of leaves，which in turn effectively scavenges ROS，resulting in delaying the leaf senescence in grape.

Key words: grape, melatonin, leaf senescence, reactive oxygen, antioxidant, ascorbic acid-glutathione cycle