Yield-increasing effect of a major gene, Ur1 (Undulate rachis-1) on different genetic backgrounds in rice [Oryza sativa]

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'(NU) 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(NH) were grown in 2000. The yield of the three genotypes was in the order NU NH N, though the difference between NU and NH was not significant. Ur1 increased spikelet number per panicle by 29 and 24%, respectively, in NU and NH, whereas Ur1 decreased panicle number per m(2), ripened-grain percentage and 1000 grain weight by the ranges of 4 to 8% and 4 to 6%, respectively, in NU and NH. From the F1 of N x the isogenic line of 'Taichung 65' carrying both Ur1 and sd1-d, four isogenic-line pairs (e.g. 27U and 27+) were developed after maintaining the heterozygosity (Ur1/+) to F8 or F9 generation. These four number-named isogenic-line pairs, NU 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 NU to 542 (30(U)) g/m(2). The yield increase by Ur1 ranged from 250 g/m(2) (61%) to 75 g/m(2) (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 30U, relative to 30+, whereas no significant decreases were noticed in the other four Ur1-carrying isogenic lines. Ur1 increased culm length in 27U, 52U and 74U, whereas no increase was noticed in NU and 30U. It is concluded that NU could be a prospective material for developing high-yielding japonica varieties carrying Ur1, from the viewpoint of lodging tolerance as well as yield.