Ultrastructure of vascular bundles and fundamental parenchyma in relation to movement of photosynthate in leaf sheath of rice.

The ultrastructure of vascular bundles and fundamental parenchyma (parenchyma in fundamental tissue system) in the 8th leaf sheath of rice were examined with a light and an electron microscopes in reference to possible pathways for photosynthate between phloem and fundamental parenchyma. 1. In the phloem of small bundles, the sieve element-companion cell complexes located in the middle and adaxial side of phloem remained without degeneration after sheath elongation (Fig. 1). However, the degeneration of phloem in large bundles is earlier than that in small bundles, and only several sieve element-companion cell complexes that abut on xylem remained without degeneration in the second leaf sheath from the uppermost fully-expanded leaf (Fig. 5). 2. In the boundary of xylem and phloem in small and large bundles, the plasmodesmatal connections were found in each interface between sieve element-companion cell complexes and xylem parenchyma cells, and between parenchyma cells of xylem and phloem (Table 1, Fig. 1). These connections may play a role in transfer pathways to phloem for solutes absorbed from transpiration stream by xylem parenchyma cells. 3. Cell walls in phloem of large and small bundles were densely stained purple with toluidine blue O (Figs. 3 and 4) and the plasmodesmatal connections were rare in the walls between sieve element-companion cell complexs and phloem parenchyma cells (Table 1). The phloem parenchyma cells contained many mitochondria with well-developed cristae and remained without degeneration after sheath elongation (Figs. 1, 2, 6 and 7). These data support the view that sucrose moves in the apoplast between sieve element-companion cell complexes and phloem parenchyma cells. 4. Suberized lamellae occur in all walls of the mestome sheath cells of small and large bundles (Figs. 1, 7, 8 and 9). Aggregates of plasmodesmata were observed in the walls between phloem parenchyma cells and mestome sheath cells (Figs. 1 and 7), between mestome sheath cells and fundamental parenchyma cells (Figs. 8 and 9), and also between fundamental parenchyma cells (Figs. 10 and 13). Judging from these observations, it appears that sucrose moves in the symplast between phloem parenchyma cells and fundamental parenchyma cells. 5, Different types of plastids were contained in the fundamental parenchyma cells of leaf sheath. The plastids in the top of sheath were similar in structure to the chloroplast (Fig.11), whereas those in the base of sheath were the typical amyloplasts having large starch grains and a few thylakoids (Fig. 13). The plastids in the middle of sheath showed an intermediate structure of chloroplast and amyloplast (Fig. 12).