ACCUMULATION OF HIGH-LEVELS OF POTASSIUM IN THE DEVELOPING XYLEM ELEMENTS IN ROOTS OF SOYBEAN AND SOME OTHER DICOTYLEDONS

Potassium concentrations have been determined by cryoanalytical scanning microscopy in vacuoles of cells of the roots of soybean and six other dicotyledons. Developing vessel elements accumulate the highest concentration of potassium in any cell type in these roots, and those of the secondary xylem have more (median 190 mM normalized to 120%) than those of either the early (median 100 mM, normalized 80%) or late metaxylem (median 110 mM, normalized 100%). Potassium concentration in these developing vessels always exceeds those of their adjacent parenchyma (60-80%), which in turn are higher than those in cells of the cortex (30-40%), including the endodermis. Potassium concentration in the vessels increases during their development until cell death and maturation when it drops dramatically (normalized 4-12%). Developing vessel elements are clearly major sites of potassium accumulation in roots and need to be considered in any models of ion uptake, accumulation, circulation, or exudation.