The Apical Structure of Crown Roots in Rice Plants

Abstract

To clarify the geotropic behavior of crown roots of rice plants, the apical structures of various crown roots were examined in relation to their position on culms or to their stages of elongation. The diameters of actively growing root apices were always larger in the lower roots than in the upper ones of the same shoot unit, reaching the largest near the IXth shoot unit. Among the actively growing roots, the thicker ones always possessed the following histological characteristics as compared to the thinner ones, irrespective of the difference in their position of culm or on each shoot unit : (1) Cortical cell layers are more numerous. (2) Stele diameters are larger. (3) The cells, manifesting meristematic appearance, are more numerous either in stele or in cortical and dermal tissues. (4) Root caps are larger both in diameter and in length, having well developed columella. (5) Cells in columella containing starch grains are more numerous. In the roots of the later elongation stage, the cells of root caps were depleted completely of starch grains. The so-called lion's-tail-like roots or stunted roots had root caps degraded to various degrees, tending to complete absence. On the basis of these findings, the crown root apices of rice plants were found to have structural constructions closely related to their tip dimensions. Some possible implications of these apical structures of crown roots to their geotropic behaviors were discussed.