ANALYSIS ON ROOT-SYSTEM MORPHOLOGY IN RIC E WITH REFERENCE TO VARIETAL DIFFERENCES AT RIPENING STAGE

The morphology of the rice root system was established by analyzing va rietal differences. Four rice varieties, Koshihikari, Dobashi 1, IR36 and Lemont, were grown in an experimental paddy under ordinary conditi on. Soil monolith were taken from the direct-foot of a hill and from t he midpoint among hills at the ripening stage. After washing out roots carefully, the length and the weight of roots at different soil depth s were measured. The number and the growth angle of nodal roots were a lso examined in the same four varieties. With these data, the root amo unt and the 'root depth index' as an index of root distribution were c alculated. The root system morphology of the four varieties were quant itatively characterized with these two indices : Koshihikari has a roo t system with a small amount and a shallow distribution, Lemont large amount and deep distribution, IR36 large amount and shallow distributi on, and Dobashi the moderate of these three other varieties, respectiv ely. Furthermore, it was shown that the root amount was determined by the combination of the number of nodal roots and the 'mean root length ' (the mean length of a nodal root including the lateral roots of any order). The 'root depth index', on the other hand, was determined by t he combination of the mean growth angle of nodal roots and the 'mean r oot length'. The contribution of these determining factors differed am ong cultivars. For instance, the large root amount depended on the lar ge number of nodal roots in IR36 and on the large 'mean root length' i n Lemont, respectively. The deep distribution of roots in Dobashi 1 wa s due to large 'mean root langth' whereas to the large mean growth ang le in Lemont.