Ultrastructure and Its Function of Phloem Cell in Abdominal Vascular Bundle of Wheat Caryopsis

The ultrastructure of phloem cell in developing wheat( Triticum aestivum L )caryopsis was investigated and analyzed by transmission electron microscopy during the entire developmental process of the fruit. The results showed that, during the early and middle developmental stages of caryopsis, numerous plasmodesmata were found between phloem parenchyma cells and companion cells (CCs), between intermediary cells, and between phloem parenchyma cells. There was short of plasmodesma between SE and CC, between SE/CC complexes and between SEs and phloem parenchyma cells; but there were numerous plasmodesmata between SEs and intermediary cells, between CCs and intermediary cells. The SEs were rich in mitochondria, P type plastids, and some SEs had nacreous wall. CCs contained electron-dense protoplasm and nucleus. Intermediary cells also contained electron-dense protoplasm distribution along intermediary cell wall. Numerous plasmodesmata were found between intermediary cells and surrounding cells, and there were some furcated plasmodesmata channels between two intermediary cells, indicating that intermediary cells might act as some special companion cells that suited to symplastic transport. During the later developmental stage of caryopsis, the substance in SEs disappeared and the number of mitochondria decreased. The CCs and phloem parenchyma cells contained electron-light protoplasm, but intermediary cells still contained numerous mitochondria to form a similar structure to SE, suggesting that intermediary cell had an important function during later filling stage. The photoassimilate unloading and transport path in phloem was analyzed according to the ultrastructure of phloem cells and the distribution of plasmodesmata.