Varietal differences in the number and cross area of large vascular bundles at the neck internode of rice

Varietal differences in the number and cross area of large vascular bundles (LVs) at the neck internode were investigated in 18 rice cultivars. There were two kinds of LVs in Chinese japonica (CJ), Japonica-indica hybrid (JI),Tall indica (TI) and Semidwarf indica (SDI) types, i.e. the large (L1) and the small (L2) bundles. The number of L2 varied with the cultivar group. The total number of LVs was in the decreasing order of SDI > TI > JI > CJ > JP (Japonica panicle weight type) > JN (Japonica panicle number type). The ratio of the number of LVs to the number of primary rachis branches (V/R ratio) was nearly 1.0 in JP and JN. However, the V/R ratios in JI, TI and SDI were higher than 1.0 because the number of L2 was added. Total or phloem areas of L1 were larger in TI and SDI than in JP and JN. The whole area of LVs and phloem areas were in the decreasing order of SDI > TI, JI > CJ > JP > JN because of the difference in the numbers of L2. The whole area of LVs and phloem areas of JP (cv. Koganemasari) and SDI (cv. Gui zhao 2) were increased by the increment of N application at the panicle formation stage. The increment of the area of LVs and the running of L2 at the neck internode are considered to be important indicators to strengthen the transportation system of assimilates by breeding.