Uptake and Distribution of Sodium and Potassium by Corn Seedlings

Abstract

The distribution of Na and K throughout corn (Z. mays L. [''A632'' .times. ''Crows 3640''] .times. ''Oh 43'')plants is not simply a matter of uptake by cortical cells and irreversible delivery to the xylem for upward transport. Na, but not K, accumulates in the mesocotyl of corn seedlings grown on NaCl medium. Upon transfer to NaCl-free medium, total Na is reduced by export through the roots but remains at high levels within the mesocotyl. Experiments which consider uptake from the xylem are reported. Shoots excised at the seed were allowed to transpire solutions containing 22Na and 42K. Potassium uptake within the mesocotyl was very sensitive to concentration, increasing 27-fold between 1 and 10 mM. Na uptake was dependent upon the square root of the concentration suggesting active accumulation. At Na concentrations < 1 mM, more than 80% of the Na in the plant was retained in the mesocotyl. Both the uptake by the retention within the mesocotyl were dependent upon transpiration rate as well as concentration. The limitations of measuring of uptake from a finite, depletable medium are discussed. The mesocotyl is a modified root with a cuticularized epidermis. The feasibility of using this plastic-coated root as a model for root transport studies is discussed.