ECOPHYSIOLOGICAL CHARACTERISTICS RELATED WITH LODGING TOLERANCE OF RICE IN DIRECT SOWING CULTIVATION .3. RELATIONSHIP BETWEEN THE CHARACTERISTICS OF ROOT DISTRIBUTION IN THE SOIL AND LODGING TOLERANCE

To elucidate the relationship between root distribution in each soil l ayer and root lodging tolerance, the effects of root prunning and the laying of porous membrane (non-woven fabric) between topsoil and subso il layer on pushing resistance were investigated using two root lodgin g tolerant USA rice cultivars (M-302 and Lemont) and two susceptible J apanese cultivars (Nipponbare and Hatsuboshi). Although pushing resist ance was affected by root prunning to 5 cm under soil surface with ins erting the steel plate, the reduction was more pronounced and signific ant when roots were pruned to a 10 cm depth. In addition, a remarkable decrease in pushing resistance was observed when a porous membrane wa s laid under the topsoil. This decrease was larger in the lodging-tole rant USA cultivars than susceptible Japanese cultivars. Comparison bet ween the variances of pushing resistance and root dry weight induced b y each treatment indicated that the contribution of unit root weight t o lodging tolerance was higher in deeper (including subsoil) than in s hallower soil layers. In the pot experiment, pushing resistance per un it dry weight of root was higher in rice plants grown on soil with hig her bulk density than with lower bulk density. These results confirmed that the higher ability of root formation in rice into subsoil with a higher bulk density was one of the important characteristics for root lodging tolerance.