Difference of the Finishing Time of Crown Roots Elongation among Rice Cultivars with Different Number of Leaves on the Main Stem

Abstract

In order to ascertain the previous consideration, the finishing time of crown roots elongation was investigated by using rice cultivars with different number of leaves on the main stem. Roots elongation finished about a week before the heading stage in cultivar Reiho which had twenty-two leaves on the main stem (Fig. 1, A and through discussion). The roots of cultivar Toyonishiki whose number of leaves on the main stem was sixteen reached their maximum length at the heading stage. In cultivar Ishikari with ten leaves on the main stem, the elongation of most roots ceased thirteen days after the heading stage. The difference in the finishing time of root elongation was not due to the difference of the elongating duration but to the difference of the time of the root appearance, that is, the difference of the number of days preceding the heading stage or that of the plant age in leaf number. And this difference was brought about by the difference of the number of shoot units without crown roots. From the result obtained here and the number of shoot units without crown roots reported before, it is considered that the roots of a rice cultivar with more than eighteen or nineteen leaves on the main stem tend to stop their elongation before the heading stage and that in a rice cultivar having less than twelve or thirteen leaves on the main stem root elongation ceases long after that stage, while in a rice cultivar which has the intermediate number of leaves between those mentioned above, root elongation may finish at about the heading stage. Several differences were found in the root elongation features of three cultivars as follows. 1. In cultivar Reiho and Toyonishiki, root elongation finished earlier and the final root length was much shorter in the upper roots than in the lower ones of the same shoot unit. But in cultivar Ishikari, root elongation ceased almost at the same time and the maximum length did not so differ between the upper roots and the lower ones in every shoot unit. 2. In cultivar Ishikari, vigorous elongation occured even in the roots of the secondary uppermost shoot unit with crown roots. 3. The roots of cultivar Reiho were the longest of all the longest lower roots in each of three cultivars. The latter one or two features would be of advantage to increase in the number of crown roots which continue to elongate till relatively late growth stage or to widen the root distribution extent. In the roots which developed from any shoot unit of three cultivars, the appearance of the secondary roots were terminated at about the finishing time of the primary root elongation and at the same time the tertiary root had begun to be formed on the most distal secondary root which develop the tertiary root. This indicates that the difference exists in the root development at the ripening stage among rice cultivars with different number of leaves on the main stem.