Prevention of Lodging of Rice Plants under Direct Sowing Culture on Well-Drained Paddy Field : I. Varietal differences in lodging resistance under dense sowing

Abstract

The lodging resistance as affected by dense sowing and the interrelationships of the traits related to the lodging resistance were examined in rice varieties under direct sowing culture on well-drained paddy field. The results obtained were summarized as follows: 1. Under sparse sowing condition (22.2 hills/m2), the severer lodgings were observed in longer culmed varieties. However, under dense sowing condition (200 hills/m2), the chief reason for lodging was the decrease in breaking strength of stem, and the additional one was the elongation of the basal internodes in long-culmed varieties. 2. The elongation of culm by dense sowing was attributed to the lengthening of lower internodes (N3∼N5). In some long-culmed varieties, the breaking internode came to one internode lower than usual by means of the elongation of the lower internode, and this resulted in the decrease of living leaf sheaths wrapped on the culm, which increased the stem strength. 3. By the research on the relationship between breaking strength and other characteristics, the following facts were made clear. (1) The decrease of breaking strength of the stem by dense sowing was related both to that of the culm and to that of the leaf sheath. Of the two, the latter gave larger influence than the former. There could be observed high positive correlation between breaking strength of stem (culm with leaf sheath) and that of leaf sheath. (2) The contribution rate of leaf sheath to breaking strength of stem amounted high, in some varieties attaining as much as 50-60%. This contribution value was shown larger in short-culmed varieties than in long-culmed varieties and greater in lodging resistant varieties than in lodging susceptible ones. Dense sowing made this rate decrease by diminishing the number of living leaf sheath wrapped on the basal internode. (3) Long diameter/short diameter ratio of culm was increased by dense sowing, and there could be observed negative correlation between this ratio and the breaking strength of the culm. Similar results were recognized also in the leaf sheath. (4) Starch grains were noticed in the culm of lodging resistant varieties, however, the effect of dense sowing on starch amount could not be made clear. 4. By multiple regression analysis, high multiple coefficients of correlation were shown between lodging resistance and four characters (plant height, fresh weight, breaking strength and length of lower internode). Also, high ones were observed between breaking strength of culm (also leaf sheath) and three characters (cross-sectional area, long diameter/short diameter ratio and amount of starch in culm), however, these multiple coefficients of correlation became low in long-culmed varieties under sparse sowing condition. 5. The results may be concluded as follows. One of the most effective method to hold lodging resistance larger is to increase both the living number of leaf sheath on the basal internode and the duration period of it, and the other is to inhibit the elongation of the basal internode, especially in long-culmed varieties.