On the Direction of the Crown Root Growth of Rice Plants

Abstract

The direction of the crown root growth of rice plants cultured in a pot and in a paddy field was observed. In case of the culture in a 1/2, 000 a Wagner pot (diameter: 255 mm), the direction of the crown root growth was observed by a method of measuring a length of each crown root cut off vertically by a tin cylinder (diameter: 150 mm) (Fig. 1). In this case, it was found that each crown root grew in a linear pattern (Fig. 2), and it was certain that a length of the crown root having grown horizontally would be much shorter than that having grown vertically (Fig. 1). Meanwhile, in case of the culture in the paddy field with higher grain yield, the direction of the crown root was observed by means of an improved monolith method (Fig. 3). As a result of these observations, we could classify the crown roots of rice plants cultured both in the Wagner pot and in the paddy field into the following three types according to their growing directions. (1) The upper primary roots grew horizontally, regardless of the position on a stem of the unit they emerged. (2) The lower primary roots grew in various directions, i.e. horizontally, oblique downwards and vertically. (3) Most of the primary roots emerging from the prophyll unit of a tiller grew vertically, even when the tiller bud was dormant (Fig. 9). Here, a shoot of rice plants is designated as comprising the following two types of units (Fig.4A). One is a unit that consists of a leaf blade, a leaf sheath, a stem portion with upper and lower primary roots and a basal tiller bud (Fig. 4B, also refer to Kawata et al.3)). The other is a prophyll unit, located singly at the base of each tiller, that consists of a prophyll and an adjacent lower stem portion with primary roots and lacks a basal tiller bud (Fig. 4C). There was a remarkable difference in the growing direction of the lower primary roots depending on whether the plants were cultured in the pot or in the paddy field. Namely, the lower primary roots mostly grew horizontally in the pot, while they grew oblique downwards or vertically in the paddy field. There was generally no correlation between the diameter and the growing direction of the crown roots. Why did the direction of the crown root growth of rice plants vary in the above-mentioned three types? The results of our observation in this respect gave us many suggestions on the possible geotropic responses in the roots and further investigations are required to continue.