High yielding F1 hybrid carrying Ur1 (Undulate rachis-1) gene in Japonica rice

An incompletely dominant gene Ur1 (Undulate rachis -1) increases spikelet number per panicle owing to increase of secondary branches. This genic effect on spikelet number can increase grain yield in either the Ur1/Ur1 or Ur1/+ genotype by enlarging sink size. We examined whether the yield-increasing effect of Ur1 in the heterozygous genotype can be superimposed upon the high yield ability of a japonica F1 hybrid. Two F sub(1)s with and without Ur1 (H sup(U) and H sup(+)) were produced by the hybridization between 'Nishihikari' (common maternal parent, short-culm variety) and the isogenic line of Taichung 65 possessing both Ur1 and sd1 or that carrying sd1 only. In 1999. H sup(U), H sup(+) and 'Hinohikari', a leading variety in southern Japan, were grown under three fertilizer levels. 'Nishihikari' was included in the experiment only at the middle fertilizer level. In 2000, H sub(+) and 'Hinohikari' were grown at the twofold (heavy) fertilizer level. H sup(+) had a 21% higher yield than 'Nishihikari', revealing its heterobeltiosis. The yield of H sup(+) was 12 to 15% higher under the three fertilizer levels than that of the check variety 'Hinohikari'. H sup(+) had a higher ripened-grain percentage than either 'Hinohikari' or 'Nishihikari'. H sup(U) had a 7, 10 and 11% higher yield, respectively, at the low, middle and high fertilizer levels than H sup(+). Among the four yield components, spikelet number per panicle alone contributed to the higher yield of H sup(U), even though its ripened-grain percentage was lower than that of H sup(+). In yield, both F sub(1)s showed positive responses from the middle to high fertilizer levels but little response from the low to middle fertilizer levels. H sup(U) showed 22, 25 and 24% (116, 126 and 137 g/square m) higher yield, respectively, at the low, middle and high fertilizer levels in 1999 than 'Hinohikari'. H sup(U) gained the highest yield, 723 g/square m at the twofold fertilizer level in 2000 which was 140/0 (88 g/square m) higher than that of 'Hinohikari'. Thus, the yield-increasing effect of Ur1 was superimposed upon the high yield ability of H sup(+). Consequently, Ur1 could be utilized for developing high yielding F sub(1) varieties in the Ur1/+ genotype.