Relationships between the Growth Direction of Primary Roots and their Conductive Capacities in Rice Plants

1. In previous paper it was shown that the interphloem distance (=stele diameter×3.14/phloem pole number) was closely correlated to the growth direction of primary roots in rice plants. Then the interphloem distance was examined in this paper and high correlations were found between the interphloem distance and the number as well as the tangential width of interphloem pericycle cells. These relationships suggest that the interphloem distance may also be related to the number and the cross sectional area of other stele cells. 2. From the examination of the cross sectional area of the stele it was found that the stele area was more highly correlated to the total phloem area than to the mean phloem area or to the phloem pole number (total phloem area=mean phloem area×phloem pole number). These facts show that the roots of the same diameter may have variable pairs of the mean phloem area and the phloem pole number though the total phloem area are almost the same. And it is suggested, therefore, that the conductive capacities of the phloem in total are not always the same even if the root diameters are constant. 3. The relationships between the growth direction of primary roots and their morphological and functional characters were examined last. And the growth direction of primary roots, in general, was found to be more highly correlated to their conductive capacities than to other morphological characters of the roots. Here, the conductive capacities of each roots were estimated from the mean phloem area and the phloem pole number by means of Poiseuille's law. From these observations it is suggested that the root growth direction might be influenced by the amount of assimilates translocated from the shoot through the phloem.