Melatonin, a multiple signal molecule, plays important roles in delaying senescence during the development of plants. Because few species have been studied for the effect of exogenous melatonin on anti-aging, the plausible mechanism of melatonin of anti-aging effects on other plant species has remained largely unknown. In the present study, the effects of exogenous melatonin on leaf senescence in kiwifruit were examined during natural aging after melatonin (200 μM) or water (Control) pretreatment. The decreased membrane damage and lower hydrogen peroxide (H2O2) content due to the enhanced scavenging activity of antioxidant enzymes peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) demonstrated that melatonin effectively delayed the aging of kiwifruit leaves. Likewise, owing to up-regulated expression of chlorophyll a/b-binding protein (CAB) gene in the sampled leaves pretreated with melatonin, chlorophyll degradation decreased. Therefore, osmoregulatory substances in sampled leaves accumulated (e.g., soluble sugar and soluble protein) and seedling cell environment stability was maintained. Simultaneously, melatonin decreased H2O2 concentration owing to increased glutathione (GSH) and ascorbate (AsA) content, and the expression levels of glutathione reductase (GR), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) were up-regulated by melatonin application, indicating that the increase of GSH and AsA was attributed to the expression of these genes. In addition, a large amount of flavonoids accumulated in seedlings pretreated with melatonin, and transcript levels of eight genes involved in flavonoid synthesis, including phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxymate (C4H), chalcone synthase (CHS), flavanone 3-hydroxylase (F3H), flavonol synthase (FNS), leucoanthocyanin reductase (LAR), anthocyanin reductase (ANR), flavonoid 3-O-glucosyltransferase (UFGT) were enhanced in response to melatonin application. These results indicated that melatonin delayed aging of kiwifruit leaves by activating the antioxidant capacity and enhancing flavonoid biosynthesis. All of these results can provide clear proof that melatonin plays a key roles in delaying leaf senescence.

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In this study, kiwifruit seedlings were used as the materials, were applied 100μmol/L melatonin (MT) solution by root irrigation, to investigated the effects of MT on kiwifruit' antioxidant activity under low light. The results showed the content of TPC, TFC and TFAC of kiwi leaves under MT treatment were significantly higher than that under water and weak light treatments. The antioxidant capacity was measured by three different methods (FRAP, DPPH and ABTS), showed MT treatment significantly increased free radical scavenging ability of kiwi leaves. The indexes of MT treatment showed a trend of first increased and then decreased and last increased. The results provided a theoretical basis for improving the yield and quality of kiwifruit. Introduction Kiwifruit is the vine deciduous plant belonged to Actinidiaceae Actinidia, which also called 'longevity fruit' due to its high content of VC [1]. China is the origin of kiwifruit, with rich wild kiwi resource [2] , and also the biggest producer of kiwi. However, kiwifruit production is often susceptible to a variety of stresses, such as low light, drought, and other factor, often resulting reduction of yield. Low light environment abound in main growing area of kiwifruit in China, e.g., the southwestern of Sichuan. MT is an indole small molecules, the chemical name is N-acetyl-5-methoxytryptamine [3]. It has a variety of functions on plant, including the regulation of plant growth and development, such as changing the florescence [4] , anti-aging [5]. All the functions are beneficial to improve the biological production of plants. Therefore, it is of great theoretical and practical significance to study the influence of applying exogenous MT on kiwi antioxidant activity Materials and methods Plant materials and treatments Seeds of kiwifruit were collected in September, 2015. After stored at 4℃ for two monthes, seeds were treated with variable temperature at 4℃ 16h, 24℃ 8h for two weeks, and then, placed at 25℃ for 10-15 days to germinate. Sowed the sprouting seeds, and placed them in the artificial climate chamber (light-dark cycle: 12h/12h, diurnal temperature: 25℃/ 20℃). When had 2-3 true leaves, seedlings were moved into pots.

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Melatonin is a ubiquitous indol molecule and has powerful functions in relieving abiotic stress responses. In our study, we used the kiwifruit seedlings as the materials and investigated the efficiency of different concentration melatonin (50 μM, 100 μM and 200 μM) on protect photosynthesis compared with control under low light. The results indicated that each concentration of melatonin significantly alleviated the degradation of chlorophyll and carotenoid and had no significant difference. Attributing to the preservation of chlorophyll by melatonin, the melatonin application groups accumulated more soluble sugar, soluble protein and proline than control group. In general, combining the results of photosynthetic pigment, photosynthate and proline, 200 μM of melatonin had the mostly significant effect on protecting photosynthesis.