Effects of salinity on ultrastructure and ion distributions in roots of Plantago coronopus

Root cell structure and ion distributions have been examined in Plantago coronopus L. grown in the absence or presence of 110–125 m M NaCl. In both salt‐treated and control plants, the inner cortical cells often had membrane whorls projecting into the vacuoles. These structures appeared to be continuous with the endoplasmic reticulum. In roots grown in saline conditions, the parenchyma cells surrounding the xylem vessels showed very uneven wall thickenings and corrugations. In control roots, X‐ray microanalysis of frozen hydrated bulk specimens showed that there were three levels at which discrete reciprocal changes in Na/K levels occurred: the outer‐middle cortex, the endodermis and the xylem vessels. The first two of these were associated with high Mg, and the last with high Ca percentages. In the salt‐treated roots, the overall Cl percentages were much lower than in the culture medium, being severely limited at the epidermis. Na and, to a lesser extent, Cl percentages decreased radially inwards, while those of K increased. The Na:Cl ratio decreased radially inwards across the cortex. The analytical validity of the data is discussed in relation to differential elemental losses during analysis, specimen topography and analytical spatial resolution. The results are interpreted in terms of the proposed involvement of Mg 2+ ‐ and Ca 2+ ‐ATPases in ion uptake and regulation of translocation in Plantago coronopus roots.