Abstract
The morphology of the rice root system was established by analyzing varietal differences. Four rice varieties, Koshihikari, Dobashi 1, IR36 and Lemont, were grown in an experimental paddy under ordinary condition. Soil monolith were taken from the direct-foot of a hill and from the midpoint among hills at the ripening stage. After washing out roots carefully, the length and the weight of roots at different soil depths were measured. The number and the growth angle of nodal roots were also examined in the same four varieties. With these data, the root amount and the 'root depth index' as an index of root distribution were calculated. The root system morphology of the four varieties were quantitatively characterized with these two indices : Koshihikari has a root system with a small amount and a shallow distribution, Lemont large amount and deep distribution, IR36 large amount and shallow distribution, and Dobashi the moderate of these three other varieties, respectively. 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 the combination of the mean growth angle of nodal roots and the 'mean root length'. The contribution of these determining factors differed among cultivars. For instance, the large root amount depended on the large number of nodal roots in IR36 and on the large 'mean root length' in Lemont, respectively. The deep distribution of roots in Dobashi 1 was due to large 'mean root langth' whereas to the large mean growth angle in Lemont.
