Research On The Rules Of The Peel Color Changes During ‘cutter Valencia’ Fruit Ripening

Peel color is one of the important indicators to measure its commodity value. The mature stage of the citrus in our country mostly concentrated in around November, which causes big pressure on the sales. Some early or late maturing varieties’ fruits were usually with poor coloration and ultimately impact the sale price. To solve this problem, fruit was usually undergone degreening treatment with exogenous ethylene to affect the peel color. However, little is yet known about regulation of citrus peel color.The Valencia Orange’ maturation is late and often regreen next February when the color of the peel becomes into green partly. However, the main parts of the fruit peel are yellowish, resulting in green-and-yellow as a mottled shape. In this study, we used cutter valcncia as material to understand the rules during fruit ripening at physiological and transcriptional levels. The main findings are as follows:1. The phenotypic changes of the fruits during developing and ripening are described. We observed the plastid development of the flavedo by the electron microscopy. The results indicated the plastid has experienced from the chloroplast to chromoplast and back to chloroplast during the whole developing stages. We detected the pigments’ content, the results indicated the content of total chlorophyll, chlorophyll a, chlorophyll b reduced to minimum and the content of total carotenoids reached the maximum on the 265 DAF. However, chlorophyll increased and carotenoids decreased significantly after 265 DAF. The maturity of the peel and the flesh was inconsistent during the fruits’ s developing and ripening. According to the above results, we divided the developing stages into two phases: the degreening stage(Degreening) and the regreening stage(Regreening).2. The primary metabolites of the flavedo during the developing and ripening were measured. As the degreening proceeded, the content of amino acid and organic acid and sugars were decreased and increased respectively; in the regreening stage, they showed opposite changes. The major components of amino acid and organic acid were aspartic acid, butanoic acid, threonic acid, malic acid and butanedioic acid. The major sugars were the fructose, glucose and sucrose. These results showed that sucrose and reducing sugar can promote degreening and inhibit regreening, and the role of organic acids and amino acid may be just the opposite.3. Real-time quantitative PCR was used to analyze the expression of the relevant genes in chlorophyll metabolism in flavedo during the developing. In the degreening stage, the expression of genes involving in chlorophyll biosythesis pathway(HEMA, GSA, HEMC, HEME1/2, HEMG1) and the expression of genes involving in chlorophyll degradating pathway(CHLI1/2, CHLD, CRD1, PORB/C) were up-and-down regulated. In the regreening stage, the expression of the downstream genes of the chlorophyll biosythesis pathway showed significantly increased. Furthermore, CAO, which regulates the proportion between chlorophyll a and b, was at low expression level at the early stage and subsequently increased significantly. Most of the genes participated in chlorophyll degradation pathway increased during the regreening stage. These results indicated that the degradation are active throughout fruit degreening and regreening processes, and that the accumulation of the chlorophyll during the regreening may be due to the conversion between chlorophyll a and b.4. Real-time quantitative PCR was used to analyze the expression of the relevant genes in carotenoids metabolism in flavedo during the developing. In degreening stage, a majority of carotengesis genes were up-regulated, including rate-limited gene(PSY) and upstream genes(HDS, PDS, LCY) and downstream genes(HYD, ZEP, NCED2/3, CCD1/4a/4b). By contrary, in the regreening stage, the expression of PSY and NCED2/3, CCD1/4a/4b, ZEP, HYD, HDR were reduced. These results indicated that the accumulation of the carotenoids during degreening was likely due to the increasing expression of the carotenoids biosynthesis genes, and the decreasing content of the carotenoids during regreening might be due to the increasing expression of the carotenoid degradation genes and the key gene PSY.