Cytological Pathway Of Sugar Unloading Within Phloem In Jujube Fruit

The ultrastructures of phloem and its surrounding parenchyma cells in jujube (Zizyphus jujuba Mill.cv Dongzao and Lizao) fruit were systematically investigated using transmission electron microscopy during the whole developmental process of fruit. The pathway of phloem unloading was studied in developing jujube fruit by carboxyfluorescein real-time imaging. The acid invertase was localized at SE/ CC complex and its surrounding phloem parenchyma cells.Structural investigations showed that plasmodesmata existed at the thickened part of the cell wall between the sieve elements (SEs) and companion cells (CCs), with the plasmodesmata being furcated at the CC side and keeping a single oriface at the SE side. At the early stage and the late stage of jujube fruit development, there is almost no plasmodesmata between the SE/ CC complex and its surrouding phloem parenchyma cells, resulting in the symplastic isolation between them. At the middle stage of jujube fruit development, plasmodesmata were found numerous between the SE/ CC complex and its surrouding phloem parenchyma cells, making the phloem a symplastic integration.The CF imaging showed that phloem-transported carboxyflurescein remained confined to the phloem of jujube fruit and not into the surrounding parenchyma tissues even 48h after application in the treated petiole during the early and late stage of jujube fruit development. The results obtained from the main and minor veins of the different sample position and development stage were in agreement with the above point. This suggested the symplasmic isolation of SE/ CC complex from the surrounding parenchyma cells. But at the middle stage, CF was shown to be released from the phloem strand to the phloem parenchyma cells and this suggested symplastic connectjion of SE/ CC complex with the surrounding parenchyma cells.The acid invertase was localized at SE/ CC complex and its surrounding phloem parenchyma cells. The density of immunogold particles at the middle stage was obviously lower than that at early and late development stage. The results further confirmed the symplastic unloading pathway at the middle stage of jujube fruit development.Those results above provide strong evidence for two changes of the pathway of photoassimilates unloading, apoplastic- symplastic- apoplastic pathway in jujube. The apoplasmic unloading pathway of sugar dominates in jujube fruit at early and last stage development, whilet symplasmic unloading pathway of does at middle stage development.