Utility of Japanese native lowland rice varieties with high V/R ratio as breeding materials

Abstract

In order to identify the genetic resources of rice varieties with higher translocation efficiency, the ratio of a number of large vascular bundles in the peduncle (Vb) to that of the primary rachis branch (Rb) (V/R ratio) was investigated in 103 varieties of Japanese native lowland rice. We found 6 high V/R ratio (≤1.5) varieties, which consisted of two indica varieties and four temperate (Tm)-japonica varieties. The course of the Vb in the rachis of the Tm-japonica varieties with a high V/R ratio was compared with those of an indica variety, Habataki (V/R ratio=2), and of a Tm-japonica variety, Sasanishiki (V/R ratio=1). Varieties with a V/R ratio of 1.5 showed characteristics between Habataki and Sasanishiki in the course of Vb. Because of reproductive barriers between indica and japonica, and undesirable traits of indica, for example, the shattering habit of spikelet and grain shapes, it is difficult to use indica varieties directly as materials for introducing a high V/R ratio. In fact, high V/R ratio plants were segregated in higher frequency in the F₂ population of indica/Tm-japonica than that of the Tm-japonica/high V/R ratio Tm-japonica, but their seed fertilities were quite low. On the other hand, the F₂ population of the Tm-japonica/high V/R ratio Tm-japonica segregated some plants with a V/R ratio higher than 1.5 and normal fertility. Therefore, the Tm-japonica varieties with high V/R ratios identified in this study may be useful materials for breeding varieties with high yielding ability by improving the vascular bundle system.