Varietal differences of the course and differentiation time of large vascular bundles in the rachis of rice

The course and differentiation time of large vascular bundles in the rachis of japonica (cv. Musashikogane) and indica (cv. IR36) rice were compared. Large vascular bundles in the rachis consisted of large vascular bundles entering from the primary rachis branch (V-p) and the secondary branch (V-p). In Musashikogane, one V-p entered from each primary branch to the rachis and travelled down to the neck internode without fusion. Usually, V-s doesn't enter the rachis. Therefore, the vascular ratio shown by the number of large vascular bundles in the neck internode against the number of primary rachis branches was about one. On the other hand, in IR36, one V-p entered from the primary rachis branch to the rachis at the upper position on the rachis. However two V-s and one V-p entered the rachis at the middle and lower positions on rachis. Some of the V-s fused together in the rachis so that the vascular ratio was about two. In both cultivars, V-p differentiated at the early stage of secondary rachis branch primordia differentiation and V-s at the early to middle stage of newer primordia differentiation. These results imply that the development of primary rachis branches as compared with that of rachis is faster in IR36 than Musashikogane.