水稲葉身における横走維管束の微細構造と光合成産物の移動経路

抄録

The histology and ultrastructure of the transverse veins in the 8th leaf blade of rice were examined with a light and an electron microscopes in relation to possible pathways for photosynthate from mesophyll to the sieve element. 1. The transverse veins lack the parenchyma sheath and mestome sheath, accordingly the vascular elements are in contact with the mesophyll cells. The elliptical mesophyll cells are closely attached on both sides of the transverse veins with smaller number of vascular elements. Sieve elements of the transverse veins are entirely surrounded by the mesophyll cells and vascular elements, and the walls are rarely exposed to the intercellular space (Fig. 1). 2. The phloem of transverse veins consists of a single sieve element and one to three phloem parenchyma cells. The sieve elements contain a few mitochondria and have thin walls. Phloem parenchyma cells are classified into three types (A, B and C) according to the size and structure. The A-type cells are smaller than the cells of the other types and have abundant mitochondria, small vacuoles and dense cytoplasm. The B-type cells contain chloroplasts and all of the other organelles which are present in mesophyll cells, but the walls are thicker than mesophyll cells. The C-type cells contain plastids with undeveloped thylakoids and mitochondria with well-developed cristae, and have relatively thick walls (Figs. 2, 3, 4, 5 and 6). The xylem of transverse veins consists of a single vessel and one to three xylem parenchyma cells. The xylem parenchyma cells contain various organelles such as mitochondria, plastids, dictyosomes, rough endoplasmic reticulum and microbodies, and the wall that faces to the lumen of the vessel has a swollen and loosely fibrillar appearance (Figs. 8 and 9). 3. The mesophyll cells are interconnected by plasmodesmata (Fig. 4), and are connected by plasmodesmata with the B-, C-type cells of phloem parenchyma and the cells of xylem parenchyma, but connections between mesophyll cells and A-type phloem parenchyma cells are rare. Vascular parenchyma cells are interconnected by abundant plasmodesmata, in contrast, connections between sieve elements and vascular parenchyma cells are rare (Table 1). It is assumed, therefore, that photosynthate moves in the symplast from mesophyll cells to the A-type phloem parenchyma cells via the other parenchyma cells, and during this process sucrose may be released into the apoplast and finally be absorbed by the sieve element. 4. The sieve elements of the transverse veins are connected by the sieve plates to those in metaphloem of the longitudinal vascular bundles (Figs. 10 and 12). The vessels of the transverse veins are also connected to metaxylem of the longitudinal bundles, especially to the small vessels in the middle of longitudinal large bundles (Fig. 11).