Longitudinal and transverse profiles of K+ and Cl− concentration in ‘low‐’ and ‘high‐salt’ barley roots

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Abstract
The concentrations of K+ and Cl- in meristematic and differentiated cells of barley (Hordeum vulgare. L. cv. California Mariout) roots were determined by means of quantitative X-ray microanalysis. In roots of seedlings grown in 0.5 mM CaSO4 (''low-salt'' roots), the mean cytoplasmic K+ concentration of meristematic cells was 194 mM, while the average vacuolar concentrations at 10, 50 and 100 mm from the root tip were 62, 25 and 22 mM, respectively. Such marked differences in vacuolar K+ concentration of differentiated cells were not found in roots of seedlings grown in full strength Johnson solution (''high-salt'' roots). Thus the maintenance of K+ at a relatively high concentration in the cytoplasm of meristematic cells of ''low-salt'' roots appears to be a priority requirement. The mean Cl- concentration in the cytoplasm of meristematic cells of ''low-salt'' roots was 4.6 mM, and the vacuolar Cl- concentration declined towards the root base. These measured concentrations are compatible with previous data, indicating that X-ray microanalysis of fully-hydrated samples, used together with colloidal graphite mixtures as standards, provides a reliable means of measuring ion concentrations at the subcellular level.