A MORPHOMETRIC ANALYSIS OF CELLULAR DIFFERENTIATION IN THE ROOT CAP OF ZEA MAYS

In order to quantify the ultrastructural changes associated with cellular differentiation, we have performed a morphometric analysis of the ultrastructure of the calyptrogen, columella, and peripheral cells of the root cap of Zea mays. The relative volumes of the nucleus, nucleolus, and mitochondria in the protoplasm gradually decrease as a cell moves through the root cap. The relative volume of plastids increases 240% during the differentiation of calyptrogen cells into columella cells. This increase is transient, however, since the relative volume of plastids as well as starch in plastids decreases markedly as columella cells differentiate into peripheral cells. Dictyosomes and spherosomes increase more gradually than plastids, peaking in relative volume in the innermost peripheral cells (PCI). The relative volume of the vacuome decreases as calyptrogen cells differentiate into columella cells, after which it increases during the differentiation of peripheral cells. By the time the outermost peripheral cells (PCIII) are sloughed from the cap, the relative volume of the vacuome has almost tripled. These results indicate that each cell type comprising the root cap of Zea mays is characterized by a distinctive ultrastructure. Furthermore, the ultrastructural changes associated with the differentiation of these cells are organelle specific. The results of this study are discussed relative to the function of the various cell types of the root cap. THE ROOT CAP of Zea mays is an excellent system for investigating cellular differentiation. As cells are pushed out of the calyptrogen, they sequentially differentiate into columella statocytes and, finally, into peripheral cells. Concurrently, cells assume distinctive structures and functions during the course of their differentiation and movement through the cap. Although certain cell types have been described by various authors in separate papers (Juniper and French, 1970; Juniper and Roberts, 1965; Paull and Jones, 1976; Whaley, Kephart and Mollenhauer, 1959), there has been no comprehensive study of cellular differentiation in the root cap of Zea. This paper is the second in a continuing series aimed at elucidating the structure and function of the root tip in Zea mays. In a previous manuscript (Moore and McClelen, 1983) we presented a qualitative description of cellular differentiation in the Zea root cap. The maize root cap is composed of five distinctive types of cells: 1) calyptrogen cells, which comprise the cap meristem; 2) the columella, com1 Received for publication 16 March 1982; revision accepted 17 July 1982. Thanks are extended to Jim Mauseth for his excellent technical assistance with the statistical analyses and for critically reviewing the completed manuscript. This study was supported in part by a grant from the University Research Committee of Baylor University. posed of statocytes involved in geoperception; 3) peripheral cells I (PCI), located immediately adjacent to the columella and characterized by a pronounced increase in dictyosomes and spherosomes and an absence of sedimented amyloplasts; 4) peripheral cells II (PCII), characterized by the presence of mucilage between the plasmalemma and cell wall; and 5) peripheral cells III (PCIII), highly vacuolate cells separated from the root cap (i.e., in the process of being sloughed from the cap). This qualitative study (Moore and McClelen, 1983) suggested that the spatial and ultrastructural aspects of differentiation of these five types of cells comprising the corn root cap are more pronounced than previously believed. Thus, in order to better understand the differentiation of these cells, we have quantified the ultrastructure of each of the five cell types comprising the root cap of Zea mays. MATERIALS AND METHODS-Plant material and microscopy procedures-Grains of Zea mays L. (cv. "yellow dent") were germinated in a vertically oriented cylinder of moist filter paper. Three days after germination, roots were fixed in situ in 2% glutaraldehyde in 0.05 M sodium cacodylate buffer (pH 6.9) for 2 hr at room temperature. Following three washes of 10 min each in buffer, the tissue was postfixed in buffered 2% osmium tetroxide for 2 hr. Material was then dehydrated in an ethanol series