Ultrastructure of elliptical and diffuse bundles in the vegetative nodes of rice.

The second and fourth nodes from the top of rice culm were harvested at the times of flag leaf emergence and heading respectively. Ultrastructural observation has been made on the elliptical and diffuse bundles in particular reference to solute movement in the node. 1. In the phloem of elliptical bundles, which are continuous with the large bundles of leaf, both the sieve elements and phloem parenchyma cells increased in number and showed a ratio of about one to two. A sieve element was connected by plasmodesmata with the plural number of phloem parenchyma cells (Fig. 1). In the border region between phloem and xylem, the parenchyma cells with dense cytoplasm were interconnected by plasmodesmata (Fig. 2). It is assumed that the solutes actively move in the symplast between phloem and xylem. 2. The elliptical bundles contain numerous xylem transfer cells, in which the wall ingrowths were formed on the walls contacting with tracheary elements. The mitochondria with developed cristae and the plastids with peripheral reticulums were contained in the xylem transfer cells (Figs. 3 and 4). A large amount of plasmodesmata was found in the walls between xylem transfer cells and bundle sheath cells, between bundle sheath cells and fundamental parenchyma cells and also between fundamental parenchyma cells (Figs. 4, 5, 6 and 7). It can be (considered that the solutes absorbed from transpiration stream by xylem transfer cells are transferred actively to fundamental parenchyma via bundle sheath cells. 3. In the phloem of diffuse bundles which are surrounding the elliptical bundles and are continuous with the large bundles of internode, the number of sieve elements was about 3.5 times as many as phloem parenchyma cells and the sieve elements were partitioned by numerous sieve plates (Fig. 8). The phloem parenchyma cells of diffuse bundle contained abundant mitochondria, plastids, rough endoplasmic reticulum and ribosomes (Figs. 9 and 10). It is suggested that the phloem parenchyma cells play an important role in climbing of photosynthate in the phloem of diffuse bundle through energy supply.