Effect of postharvest polyamines treatment on black spot disease control and storage quality of‘Zaosu'pear fruit

Astract:【Objective】‘Zaosu'pear (Pyrus bretchneideri) is one of the important early cultivars grown in the north of China. The fruit is popular due to its green peel, good taste and crispy fleshy texture. However, it is often harvested in the summer and susceptible to postharvest losses due to fungal diseases and quality deterioration characterized by peel yellowing and the decrease in soluble solids content. Alternaria rot, caused by Alternaria alternata, is one of the most important diseases of pear fruit and, in general, synthetic chemical fungicides are the primary means to control postharvest diseases. However, public concern over food safety has prompted the investigation of methods of disease control that are less harmful to human health and the environment. It is of the utmost importance to develop alternatives to the use of chemical fungicides. Polyamines are low molecular weight organic polycations, positively charged with physiological pH, that are present in a wide range of living organisms. In plants, polyamines are involved in different physiological processes, such as growth, development and response to abiotic and biotic stresses. Therefore, the effects of different concentrations of spermidine on black spot control and storage quality of‘Zaosu'pears were evaluated.【Methods】A uniform size of‘Zaosu'pear fruits, free of physical injuries or apparent decay, were commercially harvested and chosen as the experimental materials. The fruitswere washed in a 2% (φ) sodium hypochlorite solution for 2 min, rinsed with tap water and air-dried prior to use. The pear fruit was dipped with 0, 0.5, 1, 2 mmol·L^-1 spermidine for 15 min, and then dried in air for 2 h at room temperature. The control effect of spermidine treatment on the black spots of pear fruit were evaluated and the changes in the content of the reactive oxygen species (ROS) , activities of ROS producing and scavenging related enzymes and physiological and quality indexes of pear fruits during storage were also measured. All the test data were calculated using a Microsoft Excel 2010 standard deviation and plotted. SPSS 19.0 software was used to analyze the variance of the data, using Duncan's multiple difference significant analysis.【Results】Postharvest application of spermidine significantly reduced the lesion diameter of the black spots in pear fruit caused by A. alternata compared to the control treatment (P<0.05) , except for the group treated with spermidine at 2 mmol·L^-1, the inhibitory rate of the fruit treated with spermidine at 1 mmol·L^-1 increased to 55.1% on the fifth day. The rate of O^·-2 production in pear fruit tissue gradually decreased during storage, while the content of H2 O2 gradually increased with the spermidine treatment promoted by the production of O·-2 and H2 O2, and delayed by the decrease of NADPH oxidase (NOX) acitivities during the later storage. Spermidine treatment also enhanced the activities of the antioxidant enzymes such as superoxide dismutase (SOD) , peroxidase (POD) , catalase (CAT) and ascorbate peroxidase (APX) of pear fruit. Furthermore, spermidine treatment reduced the respiration rate and ethylene release of fruit and significantly delayed the respiration and ethylene peak, decreased the weight loss and effectively reduced the decrease of and firmness and soluble solids content of pear fruit during storage.【Conclusion】The exogenous spermidine at 1 mmol·L^-1 could effectively inhibit the development of black spots and improve disease resistance and storage quality of the‘Zaosu'pear fruit. It is suggested that exgenous spermidine treatment might enhance the fruit resistance through regulating the reactive oxygen metabolism in pear fruit tissue, and thus effectively maintain fruit storage quality.