The present investigation was undertaken to examine which physical and chemical characteristics of the culm associated with lodging resistance changed under different growth conditions using three varieties. Three different conditions were prepared: (1) ''Border'' (Border of the rice population was made by reaping several rows in the south side at the maximum tiller number stage, and rice plants were grown under strong light intensity and low humidity compared with ''Control''.), (2) ''Shade'' (50% Shade of the population was treated from the maximum tiller number stage to heading time) and (3) ''Control'' (Untreated rice population). Change in the degree of lodging and the lodging index was largely dependent on variation in the breaking strength of the basal internode. The breaking strength was much larger in ''Border'' and much smaller in ''Shade'' than that in ''Control'' for all varieties. The degree of change in the breaking strength was more pronounced in Koshihikari which has a small breaking strength, however, the change was smaller in Taichung 189 or Milyang 23 which have large breaking strength. With the exception of ''Shade'' in Milyang 23 with a large section modulus, these changes in the breaking strength were not due to the section modulus but rather to the bending stress. The density of glucose, xylose and lignin were correlated with the bending stress in each variety, and their correlations were the closest on Taichung 189. The degree of the increase in bending stress accompanied by a concomitant increase in the densities of xylose and lignin was much larger in Taichung 189 and Koshihikari than in Milyang 23, and the efficiency of the increase in bending stress to the increase in density of lignin was the highest in Taichung 189.
