The regulation of ion levels within Plantago maritima roots: An examination of freeze-substituted and frozen hydrated specimens.

Abstract

Elemental distributions throughout the root radius in the absorption zone were measured by x-ray microanalysis of frozen hydrated bulk Plantago maritime root samples. Relative elemental concentrations in the cytoplasm were measured in freeze-substituted sections of undifferentiated cells and xylem parenchyma cells in the absorption zone. When grown in the presence of 200mM NaCl, relative levels of Na+ and K+ reaching the stele are adjusted from those in the culture solution principally by an initial enhancement of the K+ level in the epidermis, and a subsequent reciprocal decrease in Na+ and increase in K+ levels at the endodermis. Mean Cl- levels do not balance those of Na+ in any of the analysed cells. Together with differences observed between control and salt grown roots in the Mg and Ca levels in the inner cortex, the regulation of Na+ and K+ reaching the xylem vessels is discussed in terms of ion pumps and ATPases, and related to the overall salinity tolerance of P. maritima. The mean relative cytoplasmic concentration ratio of Na+: K+ was 0.6:1.0 in the undifferentiated cells and 1.1:1.0 in the xylem parenchyma; in the latter the mean relative concentrations of Na+, K+ and Cl- were appreciably higher than in the former. The absolute ion concentrations in the cytoplasm, and the extents of variations therein, rather than the Na+:K+ ratio, may be correlated with the overall salinity tolerance of the plant.