On the Number and the Diameter of Crown Root Primordia in Rice Plants

Abstract

The crown root primordia of rice plants appeared to undergo a "dormant" stage before their emergence. The number of these "dormant" root primordia was found to increase successively from 9th "shoot unit" (S.U.) to the 13th S.U. (Fig. 4). The total number of crown roots including these root primordia formed was found to increase successively in the upper S.U. However the development of upper roots was less regular than the lower ones (Fig. 5). The influence of growth conditions on total number of root primordia formed in the 9th S.U. was examined and it was revealed that in the plants grown under low nitrogen supply and low degree of light intensity, the formation of the total crown root primordia had considerably reduced, when compared to those of the control (Fig. 10). This decrease of total number of crown root primordia was due to the decreased number of upper root primordia, since different nitrogen and light intensity treatments did not show any significant variation in the formation of lower root primordia. The number of the upper root primordia which were formed away from the midrib had decreased considerably in the case of plants grown under low nitrogen supply and low degree of light intensity (Fig. 16). It might be concluded from the above study that the upper root primordia of any S.U. were more sensitive to the various physiological factors concerned with the root initiation than the lower ones of the same S.U. The root primordia which were formed away from the midrib were also appeared to be sensitive to the various physiological factors than those found nearer to the midrib. The diameter of the primordia, and stele, thickness of cortex and the number of cortical cell layers of the primordia of rice plants grown under different growth conditions have also been investigated. However, no significant differences were observed in this study (Tables 2, 3).