Calcium-accumulating cells in the meristematic region of grapevine root apices

Apical roots of grapevines were examined by cryo-SEM (scanning electron microscopy) and the intracellular distribution of Ca was demonstrated by X-ray microanalysis in different regions of the primary root. We show that large amounts of Ca are accumulated as raphide crystals in the vacuoles of specialised cortical cells (idioblast cells) of the root apex. These crystal idioblast cells appeared to form a discontinuous cone of cells in the outer region of the root meristem. The raphide crystals within these cells were less apparent in older regions of the root, 10–12 mm basipetal to the root tip. We suggest that the raphide crystals could initially act as another Ca sink involved in the regulation of Ca levels in root apices. In older regions of the root these cells are spaced at intervals around the periphery of the cortex and the subsequent disappearance of the raphides may be indicative of remobilisation, perhaps in the zone of elongation where cell wall synthesis occurs and Ca demand is high. Calcium-accumulating cells were also observed in the older regions of the root, forming endodermal protrusions extending into the cortex. These cells may play a part in regulating Ca delivery to the xylem stream by sequestration of Ca from the radial flow of water at the endodermis. The observed distribution of Ca in root apices was different from the other major cations (e.g. K) and anions (e.g. Cl) because high concentrations were localised to specific cells. We interpret the results in the context of a model of the dynamics of grapevine root growth and cell differentiation, and the temporal balance of solute supply from the protophloem and the external medium.

An SEM study of raphide crystal initials in the leaves of vitis (grape)

Many papers have been published on the structure of calcium oxalate crystals in plants, however, few deal with the early development of crystals. Large numbers of idioblastic calcium oxalate crystal cells are found in the leaves of Vitis mustangensis, V. labrusca and V. vulpina. A crystal idioblast, or raphide cell, will produce 150-300 needle-like calcium oxalate crystals within a central vacuole. Each raphide crystal is autonomous, having been produced in a separate membrane-defined crystal chamber; the idioblast''s crystal complement is collectively embedded in a water soluble glycoprotein matrix which fills the vacuole. The crystals are twins, each having a pointed and a bidentate end (Fig 1); when mature they are about 0.5-1.2 μn in diameter and 30-70 μm in length. Crystal bundles, i.e., crystals and their matrix, can be isolated from leaves using 100% ETOH. If the bundles are treated with H2O the matrix surrounding the crystals rapidly disperses.