Yield-increasing Effect of a Major Gene, Ur1 (Undulate rachis -1) on Different Genetic Backgrounds in Rice

Abstract

An incompletely dominant gene Ur1 (Undulate rachis -1) increases spikelet number per panicle, and can enhance grain yield by enlarging sink size. The yield-increasing effect of Ur1 was examined on five different genetic backgrounds. The Ur1 isogenic line of ‘Nishihikari’ (N^U) has been developed by successive backcrossing. ‘Nishihikari’ (N) has the highest lodging-resistance in southern Japan. This isogenic line, N and their F₁ possessing the Ur1/+ genotype (N^H) were grown in 2000. The yield of the three genotypes was in the order N^U > N^H > N, though the difference between N^U and N^H was not significant. Ur1 increased spikelet number per panicle by 29 and 24%, respectively, in N^U and N^H, whereas Ur1 decreased panicle number per m², ripened-grain percentage and 1000 grain weight by the ranges of 4 to 8% and 4 to 6%, respectively, in N^U and N^H. From the F₁ of N × the isogenic line of ‘Taichung 65’ carrying both Ur1 and sd1-d, four isogenic-line pairs (e.g. 27^U and 27⁺) were developed after maintaining the heterozygosity (Ur1/+) to F₈ or F₉ generation. These four number-named isogenic-line pairs, N^U and N were planted under a field condition in 2001. Analysis of variance indicated that the effects of Ur1, genetic background and the interaction were significant in yield. The five Ur1-carrying isogenic lines had a varying yield from 752 (N^U) to 542 (30^U) g/m². The yield increase by Ur1 ranged from 250 g/m² (61%) to 75 g/m² (16%). In spikelet number per panicle, the increase by Ur1 ranged from 62.1 to 39.2, while the percentage increase varied from 90 to 42%. The spikelet-increasing effect of Ur1 was greater on a small-panicled genetic background than on a large-panicled genetic background. Ur1 decreased ripened-grain percentage by 17% in 30^U, relative to 30⁺, whereas no significant decreases were noticed in the other four Ur1-carrying isogenic lines. Ur1 increased culm length in 27^U, 52^U and 74^U, whereas no increase was noticed in N^U and 30^U. It is concluded that N^U could be a prospective material for developing high-yielding japonica varieties carrying Ur1, from the viewpoint of lodging tolerance as well as yield.