A Study Of Coloring Process And Its Physiology In Red Chinese Sand Pears

Chinese sand pear (P. pyrifolia Nakai) is the major cultivated group in south China and the ripen fruit usually appear green, yellow or brown (russet). Recently, some cultivars with red skinned fruit have been discovered in Yunnan Province and southern part of Sichuan Province. As red color meets the aesthetic values and consumer demands, red skinned pears distributed widely throughout China in a short time. Nevertheless, it appeared that red color of pear skin was not uniform in different regions and under different cultivated conditions. Thus, two red Chinese sand pear cultivars 'Meirensu' and 'Yunhongli No.1' were selected to investigate the characteristics of fruit development under natural conditions and effects of fruit bagging treatment on fruit development, especially with a view to fruit coloration.Under natural conditions, the fruit development showed in an "S" curve. Thirteen weeks after full bloom, limited amount of anthocyanin could be detected in the fruit peel, but the contents of chlorophyll, carotenoid, flavonoid, and total phenolics were high as well as the activities of PAL and UFGT. Besides, the content of malic acid and total organic acids were the highest and the content of soluble sugars increased rapidly. Then, with the fruit growth the content of chlorophyll decreased drastically and the content of carotenoid, flavonoid, and total phenolics and the enzymes activities also decreased but in a limited scale. However, the concentration of anthoycanin increased gradually and thereafter the skin became red from green. At the same time, the ratio of total soluble sugars to organic acids also increased, as the content of fructose and sucrose increased and the content of total organic acids decreased.Fruit covered with light-impermeable bags showed higher lightness (L* = 72.61 in 'Meirensu' and L* = 71.34 in 'Yunhongli No.1') indicating brighter skin than control. Besides, Fruit bagging promoted degradation of existing chlorophyll and inhibited anthocyanin synthesis entirely in fruit peel and thus the skin looked yellowy. Aiming at effects of fruit bagging treatment on color development and inner qualities of fruit, fruit bags were removed 3, 2 and 1 weeks before harvest and then the fruit were exposed or kept in white bags (80.31% Photo-permeability) or red bags (34.71% Photo-permeability). Results showed that both the visual qualities and the inner qualities were influenced by fruit bagging treatments. Fruit with re-exposed treatment had high anthocyanin and low chlorophyll contents, as well as the high lightness and chroma, which were responsible for the better visual quality than control. Considering the accumulation of chlorophyll and carotenoid, the best date for bag removal was 2 weeks before harvest. Fruit kept in red bag formed few anthocyanin even though the outer layer bag was removed 3 weeks before harvest. Limited anthocyanin production was induced by white bag treatment and was enhanced when treated early. With the same treatment, 'Yunhongli No.1' showed greater potential in anthocyanin formation than that in 'Meirensu'. Although there were no significant differences in soluble sugars between control and treatments, the composition of soluble sugars was changed and the concentration of sucrose was enhanced in both cultivars. Fruit bagging decreased the content of total organic acids in both cultivars.In order to study the mechanism of coloring, fruit re-exposed were collected at different times of 0, 1, 3, 5, 10, and 15 days after the bag removal and the anatomical structure of skin, the concentrations of pigments, the activities of enzymes and the contents of soluble sugars and organic acids were examined. The correlation analysis was conducted between anthocyanin concentration and enzyme activity, content of sugars and acids. Results of anatomical structure observation showed that the pattern of anthocyanin distribution in red Chinese sand pear involved three to four layers containing red cells where anthocyanin accumulated. Then, both the numbers of red cells and cell pigment concentrations increased and the fruit skin became red. After the bag removal, the contents of chlorophyll, carotenoid, flavonoid, and total phenolics were all accumulated, but the speed of anthocyanin accumulation was faster and thus the skin became redder and redder. With the increasing of anthocyanin content, PAL and UFGT activity also increased and the correlation analysis showed that there were close positive relationship. The increasing of anthocyanin concentration had the closest correlation with the change of sucrose (r = 0.92007** in 'Meirensu' and r = 0.99594** in 'Yunhongli No.1') and malic acid (r = -0.97993** in 'Meirensu' and r = -0.91827** in 'Yunhongli No.1').Based on the current results, we can conclude that anthocyanin biosynthesis in red Chinese sand pear commences with the fruit maturation and higher light intensity was required in red color development. The pattern of anthocyanin distribution in red Chinese sand pear involved three to four layers containing red cells where anthocyanin accumulated. In order to obtain red Chinese sand pear fruit with attractive appearance, fruit must be covered with light-impermeable bags at the early stage of fruit development and the bag should be removed totally at least 10 days before harvest.