The sd1 (dee-geo-woo-gen dwarf) gene has contributed to develop short-culm and high-yielding varieties widely grown in Southeast Asia and other areas. An incompletely dominant gene Ur1 (Undulate rachis -1) increases spikelet number per panicle, and can heighten grain yield in either the Ur1/Ur1 or Ur1/+ genotype by enlarging sink size. The effects of sd1 and Ur1 on lodging resistance and related traits were examined. The Ur1/Ur1, Ur1/+ and +/+ genotypes with sd1 (denoted by u, h and d, respectively, viz. three dwarfs) and those without sd1 (U, H and T, respectively, viz. three talls) under the common genetic background of a japonica variety Taichung 65 were grown in a field condition. sd1 reduced culm length by 29 to 37 % and changed the internode elongation pattern toward the upper internode elongation type, whereas Ur1 increased culm length by 12 to 25 % and changed toward the lower internode elongation type. The index of lodging was applied to the 4th internode, which was the 4th-panicle length (the total length from the 4th internode to panicle) × the 4th-top weight (the fresh weight including leaves at and above the 4th internode) / the breaking strength at the 4th internode. sd1 decreased the index of lodging by 47 to 59 % with and without Ur1, being caused by the reduction of the 4th-panicle length and the increase of breaking strength, although its effect on reducing the 4th-top weight was not significant. The effect of sd1 on the breaking strength made a greater contribution towards the lodging-resistance enhancement than its height-reducing effect. Ur1, in the Ur1/Ur1 genotype, increased the index of lodging by 90 and 61 % with and without sd1, respectively; while Ur1, in the Ur1/+ genotype, increased it by 73 and 32 % with and without sd1. The increase of the index (deterioration of lodging resistance) by Ur1 was due to the reduction of breaking strength and the increases of both the 4th-panicle length and 4th-top weight. It is suggested that genes with lodging-resistance-enhancing effect should be introduced together with sd1 when Ur1 is utilized to develop high-yielding varieties.

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