Potassium activities in cell compartments of salt-grown barley leaves

Abstract

Triple‐barrelled microelectrodes measuring K⁺ activity (a_K), pH and membrane potential were used to make quantitative measurements of vacuolar and cytosolic a_K in epidermal and mesophyll cells of barley plants grown in nutrient solution with 0 or 200 mM added NaCl. Measurements of a_K were assigned to the cytosol or vacuole based on the pH measured. In epidermal cells, the salt treatment decreased a_K in the vacuole from 224 to 47 mM and in the cytosol from 68 to 15 mM. In contrast, the equivalent changes in the mesophyll were from 235 to 150 mM (vacuole) and 79 to 64 mM (cytosol). Thus mechanisms exist to ameliorate the effects of salt on a_K in compartments of mesophyll cells, presumably to minimize any deleterious consequences for photosynthesis. Thermodynamic calculations showed that K⁺ is actively transported into the vacuole of both epidermal and mesophyll cells of salinized and non‐ salinized plants. Comparison of the values of a_K in K⁺‐replete, non‐salinized leaf cells with those previously measured in root cells of plants grown under comparable conditions indicates that cytosolic a_K is similar in cells of both organs, but vacuolar a_K in leaf cells is approximately twice that in roots. This suggests differences in the regulation of vacuolar a_K, but not cytosolic a_K, in leaf and root cells.