High Yielding F₁ Hybrid Carrying Ur1 (Undulate rachis -1) Gene in Japonica Rice

Abstract

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 F₁s with and without Ur1 (H^U and H⁺) 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^U, H⁺ 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^U and ‘Hinohikari’ were grown at the twofold (heavy) fertilizer level. H⁺ had a 21 % higher yield than ‘Nishihikari’, revealing its heterobeltiosis. The yield of H⁺ was 12 to 15 % higher under the three fertilizer levels than that of the check variety ‘Hinohikari’. H⁺ had a higher ripened-grain percentage than either ‘Hinohikari’ or ‘Nishihikari’. H^U had a 7, 10 and 11 % higher yield, respectively, at the low, middle and high fertilizer levels than H⁺. Among the four yield components, spikelet number per panicle alone contributed to the higher yield of H^U, even though its ripened-grain percentage was lower than that of H⁺. In yield, both F₁s showed positive responses from the middle to high fertilizer levels but little response from the low to middle fertilizer levels. H^U showed 22, 25 and 24 % (116, 126 and 137 g/m²) higher yield, respectively, at the low, middle and high fertilizer levels in 1999 than ‘Hinohikari’. H^U gained the highest yield, 723 g/m² at the twofold fertilizer level in 2000 which was 14 % (88 g/m ²) higher than that of ‘Hinohikari’. Thus, the yield-increasing effect of Ur1 was superimposed upon the high yield ability of H⁺. Consequently, Ur1 could be utilized for developing high yielding F₁ varieties in the Ur1/+ genotype.