On the Development of the Crown Root Primordia in Rice Plants

Abstract

The initial cells of crown root primordia have been shown by the authors to have been found in the upper and lower parts of the shoot unit of rice plants12), but the developmental process from initiation to emergence of the crown root primordia have not been investigated. In the present study this process was found to be divided into the following 12 stages. At first, in the initial cells, the radial elongation of the cells and the enlargement of the nuclei and the nucleoli were observed, and the protoplasmic density of these cells became gradually higher (Stage 1). Subsequently, the initial cells began anticlinal and periclinal divisions. The outer cells adjacent to the crown root primordia showed the same cytological. features as the initial cells did in the stage 1, and these outer cells constituted a part of the primordia (Stage 2, Fig. 3). In the next stage the “epidermal-endodermal initials” were formed, by only anticlinal divisions of the cells in the outermost layer of the divided cells in the stage 2. The cells which had been added to the primordia at the stage 2 began to mainly make anticlinal divisions and formed the “root cap initials” (Stage 3, Fig. 4). After the stage 3, the epidermis and the endodermis of the primordia were differentiated by a periclinal division of all but two or three centrally located cells of the “epidermal-endodermal initials” (Stage 4, Fig. 5). Subsequently, these endodermal cells began to form the cortical cells of the primordia (Stage 5, Fig. 7). The cells of the “root cap initials” began at the 6th stage to form columella by periclinal. divisions, and the stelar cells of the primordia continued to divide anticlinally and priclinally to form “dome shape”. In this stage the essential pattern of apical organization of the primordia was accomplished as observed in the apices of the ordinary elongated crown roots. And the procambia of the “connecting phloem strands” which were linked with the transversely oriented procambia in the periphery of the stem were differentiated by periclinal and radial divisions of basal-peripheral cells of the stele (Stage 6, Fig. 8). In the central part of the stele, the meristematic strands of the “larger late meta-xylem. vessel initials” were observed. These showed active transversal divisions and transversal elongation of cells (Stage 7, Fig. 10). After the stage 7, the meristermatic strands of the: “primordial phloem initials” were observed in the peripheral part of the stele, which extended from the procambia of the “connecting-phloem strands”. These cells showed high protoplasmic density. The basal cells of the primordial endodermis continued to divide tangentially and formed the transversely oriented cortical cell files (Stage 8, Fig. 11). Subsequently, between these meristematic strands of the “primordial phloem initials” (observed in the transversal sections of the primordia), the strands of the “early meta-xylem. vessel initials” were differentiated. The cells of the “primordial phloem initials” began to divide tangentially and radially at this stage (Stage 9, Figs. 12 and 13). After the 9th stage, several series of sieve elements were observed to differentiate acropetally in the “connecting phloem strands”. The cells in the basal part of the cortex divided transversally and formed the longitudinally oriented cell files (Stage 10, Figs. 14 and 17). At the stage 11, the “connecting vessels” were differentiated between the “connecting phloem strands” as observed in the transversal sections of the primopdia. These vessels were differentiated from series of the basal stelar cells extended from a vessel element of the transversely oriented vascular bundles in the periphery of the stem to the base of the meristematic strands of the “early meta-xylem vessel initials”. In this stage, the cells in the most basal part of the cortex showed the commencement of vacuolation and of cell enlargement (Figs. 15 and 18). Subsequently, in the basal part of the ermerging roots, tissues showed conspicuous longitudinal elongation (Stage 12, Fig. 19). Thus the basal cell elongation was found to occur concurrently with the emergence of the crown roots. Therefore, it may be assumed that there is intensive correlation of the emergence of the crown roots with the accomplishment of the “connecting phloem strands” and/or the “connecting vessels” formation. © 1975, CROP SCIENCE SOCIETY OF JAPAN. All rights reserved.