Abstract: [Objective] Phloem unloading and postphloem transport play pivotal parts in the regulation of the distribution of sucrose in the storage organs and maintenance the sink of fruit, and largely determine the yield and quality of the crop. The objective of this study was to clarify the mechanism of phloem unloading and glucose metabolism in the phloem of blueberry.[Method] Vaccinium corymbosum ‘Sierra’(5-year-old Highbush blueberry) was used for observation of the ultrastructure of the phloem of blueberry in each development period. Fluorescent dye tracer and laser scanning confocal microscopy were used to observe the unloading path of the phloem in the fruit timely. Determination of soluble sugar content and related metabolic enzyme activity in blueberry fruit by HPLC.[Result] An ultrastructural investigation of phloem tissue in blueberry fruit showed that the fruit phloem SE/CC complex is symplasmically isolated from surrounding parenchyma cells over fruit development, whereas a large number of piasmodesma exist between parenchyma cells and pulp cells. Confocal laser scanning images of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands,not spread to the surrounding parenchyma cells, during the whole fruit development.These results provided a clear evidence for the predominance method of the assimilation of blueberry fruit is the unloading pathway over blueberry fruit development and symplasmic pathway present in postphloem transport. A large number of cells are beneficial to the material exchange between parenchyma cells,parenchyma cells and flesh cells. Activity assay showed that,invertase activity remained at a high level, it provides evidence to confirm the apoplasmic phloem unloading pathway at blueberry fruit development. In addition,carbohydrate metabolism related enzyme activity assay shows that a great amount of glucose and fructose primarily accumulated during blueberry ripening and sucrose enzyme decomposition activity was always greater than synthetic activity. During the late stage of fruit development, the activity of soluble acid invertase and neutral invertase was increased, the activity of sucrose decomposition, transformation and storage in the fruit pulp cell was revealed,the accumulation process is positively correlated with the activities of invertase, sucrose synthase and sucrose phosphate synthase enzymes. In the process of fruit development, the comprehensive effect of sucrose metabolism related enzymes is an important factor affecting the accumulation of soluble sugar in blueberry fruit.[Conclusion] These results provided clear evidence for the predominance method of the assimilation of blueberry fruit is the unloading pathway over blueberry fruit development and symplasmic pathway present in postphloem transport. A large amount of glucose and fructose primarily accumulated in blueberry and sugar accumulation was positively correlated with invertase, sucrose synthase and sucrose phosphate synthase.

Key words: Vaccinium corymbosum, fruit, photosynthate, apoplasmic unloading, sucrose-metabolizing enzymes