Studies On Physiological Mechanism Of Sugar Transport And Accumulation In Satsuma Mandarin Fruit

Citrus is one of the most important fruits in China. With general increase in yields of all kinds of fruits and melons, the fruit production in our country has changed from a situation where supply and demand were basically balanced to an excess of supply over demand. After entry of China into WTO, our domestic citrus market will surely face the competition from abroad. Therefore, the breeding and production of high quality citrus fruit is urgent. Sugars play an essential part in flavor characteristics of the citrus fruit and are also a commercial measure of fresh fruit quality. Sugars are not only the substance that affect sweetness of fruit, but also form the material basis for synthesis of organic acids, carotenoids, and other nutrient and aromatic ingredients of citrus fruit. Studying sugar transporting and accumulating process in tissues of developing citrus fruit will supply an important basis for elucidating the mechanism of sugar transporting, metabolism and accumulation and is beneficial to scientific regulating citrus quality.To clarify the mechanism of sugar translation, partitioning and accumulation in citrus fruit, we studied the following physiological aspects of developing fruit using satsuma mandarin (Citrus unshiu Marc. cv. Miyagawa wase). (1) The trends of glucose, fructose, sucrose, dry weight, and fresh weight accumulation in various tissues at different stage fruit development. (2) Translocation and partitioning of leaf- and fruit-derived photosynthates in tissues of developing fruit. (3) Change in rate of peel photosynthesis and peel chlorophyll content and effects of fruit shading on peel pigment content, sugar content, partitioning of photosynthates within fruit and activity of sucrose metabolizing-enzyme in developing fruit. (4) The mechanism of postphloem sucrose transport in developing fruit. The results were summarized below.1) Sugar content and dry- or fresh weight of the fruit tissues increased steadily in the course of fruit development. In the peel, nearly equal amounts (in mg/g FW) of sucrose, fructose and glucose occurred in the peel and similar accumulation patterns of these sugars were observed. In juice sacs, segment epidermis and vascular bundles, however, sucrose level was substantially or moderately higher than that of total hexoses.2) Over 50% of the photosynthates transported to fruit were deposited in juice sacs at the primary stage of fruit enlargement, stage of rapid fruit enlargement and stage of fruit coloring even when the dry weight of peel exceeded that of juice sacs. At the full ripe stage, however, the amount of photosynthate obtained by juice sacs was about the same as that entering peel, vascular bundle or segment epidermis.3) Analysis of ethanol soluble I4C-photosynthates of various fruit tissues by TLC showed that over 75% of 14C-sugars recovered in fruit tissues were sucrose. This results suggested that most of photosynthates is transported as the form of sucrose within fruit, and most of photosynthates is in the form of sucrose entering into juice sacs where they were accumulated and metabolized.4) Specific I4C radioactivity acquired by juice sacs markedly decreased with fruit development, especially at the latest stage. Ratios of Specific MC radioactivity of vascular bundle to that ofsegment epidermis and of segment epidermis to that of juice sacs considerably increased with the1 progress of fruit development, the latter ratio being much higher. This result suggests thatpostphloem assimilate transport was the limiting step of assimilate translocation in the fruit.5) Peel photosynthetic rate and chlorophyll content, and the characteristics of translocation and distribution of radiolabelled assimilates within fruit were examined from primary stage of fruit enlargement up to fruit full ripe. Change in fruit photosynthetic rate was related to change in peel chlorophyll content. Fruit photosynthetic rate markedly declined as the fruit grew and chlorophyll degraded in peel. Before the fruit full ripe stage, photosynthates produced...