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 F
1 hybrid. Two F
1s 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 F
1s 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/m
2) higher yield, respectively, at the low, middle and high fertilizer levels in 1999 than ‘Hinohikari’. H
U gained the highest yield, 723 g/m
2 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 F
1 varieties in the
Ur1/+ genotype.
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