| ’Nai’ (Prunus salicina Lindl.) is a kind of special and famous fruit in south China.Since ’Nai’ fruit mature in the hot season, the harvested fruits are not easy to storage and become ripening, softening and decay after about 10 days storage at ambient temperature, which limit its storage, transportation, marketing and consumption.1-methylcyclopropene (1-MCP), a new type of inhibitor of ethylene action, can effectively delay ripening and senescence and extend storage life of harvested fruits and vegetables. ’Younai’ (Prunus salicina Lindl. cv. Younai) fruits, the major ’Nai’cultivar in Fujian Province of China were used as the experiment material. The objectives of this research were to investigate the optimum conditions of 1-MCP treatment for maintaining fruit quality and prolonging storage life of ’Younai’ fruits.The another objectives of this research were to clarify the action mechanism of 1-MCP treatment for delaying ripening and senescence of harvested ’Younai’ fruits by investigating cell wall metabolism in relation to softening during fruit ripening, active oxygen metabolism, tissue energy metabolism and membrane lipid metabolism in relation to fruit senescence, phenolics metabolism in relation to senescence-induced browning, and the changes of activities of disease defense-related enzymes in relation to disease-resistance of fruit. The rusults were showed as follows:1. Effects of different concentrations of 1 -methylcyclopropene (1 -MCP)treatments on physiology and fresh-keeping of harvested Younai fruits. Compared with the control fruits, the treatments with 1-MCP at 0.3 μL/L, 0.6 μL/L, 0.9 μL/L or 1.2 μL/L lowered respiration rate and respiratory peak of Younai fruits, delayed the increase of cell membrane relative leakage rate of fruit, inhibited the decrease of hue angle (h°) value of fruit surface, kept higher fruit firmness, and kept higher contents of titratable acid in fruit and chlorophyll in pericarp, retarded the change of apparent color of fruit, and decreased fruit weight loss and fruit decay. Among the above 1-MCP treated fruits, the fruits pre-treated with 1.2 μL/L 1-MCP for 12 hours could keep best fruit quality. After 20 days storage at (25±1)℃,the fruits still maintained fresh green with 87% healthy fruit, the texture of flesh was crisp and hard, and there were proper sour-sweet of taste and rich aroma of the fruit. From the results, it could be concluded that pre-treatment with 1.2 μL/L 1-MCP for 12 hours could be used as the optimum conditions for maintaining fruit quality and prolonging storage life of Younai fruits during storage at (25±1)℃.2. Effects of 1.2 μL/L 1-MCP treatments on physiology and fresh-keeping of harvested Younai fruits. 1.2 μL/L 1-MCP treatment obviously decreased respiration rate, the value of respiratory peak and ethylene production of ’Younai’ fruits, delayed an increase of cell membrane relative leakage rate of fruit, inhibited a decrease of hue angle (h°) value of fruit surface, kept higher firmness and contents of titratable acid,soluble sugar, and reducing sugar in fruit, higher chlorophyll content in pericarp,retarded the change of apparent color of fruit, and decreased fruit weight loss and fruit decay.3. Effects of 1.2 μ/L 1-MCP treatments on fruit softening and cell wall metabolism of harvested Younai fruits during ripening. 1-MCP treatment kept higher fruit firmness,obviously decreased activities of cell wall-degrading enzymes such as pectinesterase (PE), polygalacturonase (PG), cellulase and β-galactosidase, retarded the decrease of contents of cell wall material (CWM), covalent-bound pectin (CBP),cellulose and hemicellulose, meanwhile delayed an increase of content of water-soluble pectin (WSP). From the results, it could be concluded that 1-MCP could obviously decrease cell wall metabolism and fruit softening of harvested ’Younai’fruits, and prolong fruit storage life.4. The relationship between the active oxygen metabolism and delaying fruit senescence of harvested ’Younai’ fruits by 1.2 μL/L 1-MCP treatment. 1-MCP treatment kept higher activities of active oxygen scavenging enzymes of ’Younai’fruits such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX), and contents of endogenous antioxidant substances such as ascorbic acid (AsA) and glutathione (GSH), obviously decreased O2-· production rate and malondialdehyde (MDA) content. 1-MCP treatment increased active-oxygen-scavenging capacity and delayed membrane lipid peroxidation of’Younai’ fruits, which retarded their ripening and senescence.5. The relationship between the tissue energy metabolism and delaying fruit senescence of harvested ’Younai’ fruits by 1.2 pL/L 1-MCP treatment. 1-MCP treatment could promote the accumulation of ATP content at the late stage of fruit storage, maintain a higher level of energy charge during the whole storage, which resulted in maintaining normal metabolism of’Younai’ fruits. ’Younai’ fruits treated with 1-MCP could keep higher activity of NAD kinase (NADK), lower NAD(H)contents and higher NADP(H) contents.6. The relationship between the membrane lipid metabolism and delaying fruit senescence of harvested ’Younai’ fruits by 1.2 μL/L 1-MCP treatment. 1-MCP treatment decreased activities membrane lipid-degrading enzymes such as phospholipase D, lipase and lipoxygenase (LOX). 1-MCP treated fruits inhibited higher relative contents of the unsaturated fatty acids,such as linoleic acid (C18:2) and linolenic acid (C18:3),and a lower relative contents of the saturated fatty acids, such as palmitic acid (C16:0) and stearic acid (C18:0) than control fruits during storage. As a result, the index of unsaturated fatty acids (IUFA) and unsaturated degree of fatty acids were decreased. From the results, it could be concluded that 1-MCP treatment delaying ripening and senescence of ’Younai’ fruits may be due to decreases of activities of phospholipase D, lipase and LOX, which may reduce the degradation of unsaturated fatty acids of membrane lipids and maintain higher unsaturated degree of fatty acids, in turn, may reward a transition of membrane lipid from predominantly liquid crystalline to gel-phase lipid, and the formation of nonbilayer lipid structures,resulted in maintaining integrity of cell membrane structure and compartmentalization within the cell.7. Effects of 1.2 μL/L 1-MCP treatments on senescence-induced browning and phenolics metabolism of harvested Younai fruits. 1-MCP treatment decreased browning degree of ’Younai’ fruits, kept higher contents of polyphenol and flavonoid during storage, but increased activity of PPO at the early stage of fruit storage. The inhibition of ’Younai’ fruit decay by 1-MCP may be related to maintaining the integrity of cellular membrane structure and the normal compartmentation of phenolic compounds and phenolase, which inhibit their contact and the enzymatic oxidation of phenolics by phenolase.8. The relationship between the changes of activities of disease defense-related enzymes and delaying the decrease of disease-resistance of harvested ’Younai’ fruits by 1.2 μL/L 1-MCP treatment. 1-MCP treatment increased activities of disease defense-related enzymes, such as chitinase, β-1,3-glucanase, polyphenoloxidase (PPO)and peroxidase (POD), significantly inhibited decay incidence of ’Younai’ fruits. But 1-MCP treatment decreased phenylalanine ammonialyase (PAL) acticity, which may be due to PAL production related to ethylene action. The inhibition of ’Younai’ fruit decay by 1-MCP is related to the increasing of activities of disease defense-related enzymes. The induced disease resistance may be involved in the mechanism by which 1-MCP treatment can inhibit ’Younai’ fruit decay. |
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