| Ponkan (Citrus reticulata Blanco) is one of the most widely-cultivated loose skin mandarins in China. Generally, commercially harvested ponkan fruits should be treated with a post maturation process by storage. However, physiological disorders such as puffiness and granulation were usually observed in long-term stored ponkan. Therefore, improvement and optimization parameters for ponkan storage remain to be one of the most important issues encountered in citrus industry. However, exploration of the molecular characters during ponkan storage had been rarely carried out, which hampered the innovation of citrus postharvest technology. In present study, mature ponkan fruits were harvested, and storage experiments were performed under ambient (about 20℃) and cold storage (about 4℃) conditions. The experiments were repeated in two harvest years of 2006 to 2007, and 2007 to 2008. To monitor the quality changes during fruit storage, contents of soluble sugars and acids were measured by gas chromatography (GC); moreover, expression of citric acid and ethylene related genes were assayed by realtime RT-PCR. The main results were as follows:1. Analyses of organic acid contents in pulp and peel revealed that the main organic acid in ponkan was citric acid. Under cold storage conditions, the change patterns of citric acid were different in two harvest years, increasing tendency was observed in 2006 to 2007, while undulately decreasing tendency was detected in 2007 to 2008.2. Resemblance in contents of fructose, glucose and sucroses in pulp and peel was observed in cold stored fruits in 2 harvest years. In all cases, the soluble sugars showed a slight accumulation in pulp; while, fructose and glucose contents decreased in peel, and sucrose increased throughout the storage treatments.3. Combination the results of organic acids and sugars contents and gene expression patterns in pulp and peel, significante changes were found. High variability of acid, sugar and gene expression was monitored in peel than that in pulp through the storage, which illuminated that the peel was more sensitive to storage conditions, and might play an importantly protective role to the pulp.4. Expression of organic acid metabolism related genes in pulp and peel, including citrate synthase (CS), isocitrate dehydrogenase (IDH), isocitrate lyase (ICL), aconitate hydratase (Acon) and malate dehydrogenase (MDH) were assayed in 2 harvest years. No obvious correlations were found between citric acid content and gene expression level, namely, no key gene controlling citric acid metabolism was obtained, which indicated that citric acid metabolism might be coregulated by multigenes.5. Differentional expression patterns of ethylene related genes were observed between ambient and cold stored fruits. After stored 30 days, ACC Synthase (ACS), ACC Oxidase (ACO), Ethylene insensitive 3 (Ein3 f-box), Ethylene Receptor 1 (ETR1) showed a significantly higher expression level in cold stored (4℃) fruits than that in ambient (20℃) samples. Especially, the the expression level of ACS gene showed seventy-fold higher at 4℃than that at 20℃, which indicated that ethylene related genes expression were induced by cold temperature; moreover, ethylene presumbely participated in the postharvest processes in citrus (a non-climacteric fruit).6. To identify the influence of storage temperature on acid and ethylene related gene expressions, alternate-temperature storage treatments were performed; and five genes (Aconitate Hydratase, Acon; 2-Oxoglutarate-dependent Dioxygenase, 2-ODD; Isocitrate Dehydrogenase, IDH; Isocitrate Lyase, ICL; ACC Synthase, ACS) were investigated. The results showed that three genes of IDH, ICL, and ACS were down regulated expression in alternate- temperature treatment fruits than that in fruits stored at 4℃or 20℃alone.. The result revealed that these gene expressions were repressed by alternate-temperature treatments. Moreover, our results indicated that metabolic rates in postharvest citrus might be slowdown by alternate-temperature treatment, which showed some applied potential in prolonging storage and shelf life in citrus. |
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