Abstract:

The ultrastructure of phloem and its surrounding parenchyma cells in the developing apple (Malus domestica Borkh) fruit was for the first time systematically investigated through transmission electron microscopy during the entire developmental process of the fruit. The results showed that plasmodesmata exist at the thickened part of the cell wall between the sieve elements (SEs) and companion cells (CCs) both in the minor and main veins, with the plasmodesmata being furcated at the CC side and keeping a single oriface at the SE side. In the minor veins the SEs are smaller than the CCs, while the reverse seen in the main veins. The CCs contain electron-dense protoplasm and nucleus, and are enriched with mitochondria, endoplasmic reticulum and Golgi apparatus or dictyosomes in their cytoplasm; and they also contain some paramural bodies at the cell wall adjacent to SE and numerous vesicles in their vacuoles. In the minor vein there is a slightly invagination of the plasmalemma of CC into the protoplasm. Many vesicles and microfibril-like structures exist in the SEs of both minor and main veins. In the main veins the cell wall of SEs often become thickened to create a nacreous wall, and a great deal of starch-typed plastids are present in the SEs. There is almost no plasmodesmata between the SE/CC complex and its surrouding phloem parenchyma cells, resulting in the symplastic isolation between them, which may be associated to apoplastic transportation. Plasmodesmata were found numerous both between phloem parenchyma cells and between flesh parenchyma cells (non-phloem parenchyma cells), as well as between phloem parenchyma cell and flesh parenchyma cell. The relationships between the ultrastructure of the fruit phloem and its surrounding parenchyma cells and the photoassimilate unloading pathway in the developing apple fruit are discussed. It is considered that the apoplastic unloading may be the main pathway of the photoassimilates unloaded from SE/CC complex, and that the apoplastic and symplastic pathways may coexist during postphloem transport of photoassimilates in the developing apple fruit.

狭基线纹香茶菜中一个新的木脂素类化合物
姜北¹ 陈兴荣²  卢泽勤³ 孙汉董^1*

（1. 中国科学院昆明植物研究所植物化学开放研究实验室，昆明650204；
2. 云南大理医学院药学系，云南大理671000；2.  广西师范大学化学化工系，桂林541004）

Key words: apple fruit, phloem, ultrastructure, photoassimilate unloading