Abstract
For practical use of F1 hybrid in Japonica rice it is necessary to increase natural out-crossing rate and seed set percentage on cytoplasmic male sterile lines in hybrid seed production field by reforming those floral characteristics which do not normally fit cross-pollination. Floral characteristics of some wild species and Indica cultivars of rice are estimated to fit natural outcrossing (OKA and MORISHIMA 1967, PARMAR et al. 1979a). Although introduction of these characteristics from wild rice or Indica to Japonica is one way of increasing natural outcrossing in Japonica rice, it is too difficult to introduce only these floral characteristics owing to the strong linkage between the floral characteristics and undesirable wild rice traits (IRRI 1985). In addition, Indica-Japonica hybrid shows hybrid sterility (MORINAGA 1968). Accbrdingly, we studied the ihtervarietal variations in the most important floral characteristics for increasing natural outcrossing viz. the number of protruding stigmas per spikelet and pstil length for seed parent, and the number of residual pollren at the anther protruding stage for pollen parent. The materials used in this experiment were Temperate (115 samples of 57 cultivars) and Tropical Japonica (18 samples of 18 cultivars) as shown in Table 1. The Temperate Japonica were classified into 14 improved cultivars and 43 local cultivars. The local cultivars were divided into group A and B. Group A means 16 cultivars which were those without femark rec6rded as "Matjiri", and group B meahs 85 samples qf 27 cultivars which were thbse with th same culrivar name and contained those with remark recorded as "Majiri". "Majiri" means contamintion. Natural outcrossin.g is considered as a cause of the occurente of the contamination.
