Solute Accumulation in Plant Cells: II. The Progressive Uptake of Non-electrolytes and Ions in Carrot Explants as They Grow1

An earlier paper established the range of solute composition that may obtain in aseptically cultured carrot explants growing in different media which regulate either cell enlargement or cell division in the explants. This paper concentrates upon the most rapidly growing cultures, containing cells which individually pass through their cycle of division and enlargement and collectively trace a sigmoidal curve of growth for the explant as a whole. The time course of growth is interpreted in terms of the number and average size of the cells and, in different phases, in terms of the changes in solute uptake and content of the cells. These data are correlated with the concomitant metabolic characteristics of the tissue, notably its protein synthesis. In the early exponential growth phase of the explant, when the emphasis is on cells in division, the organic solutes which are absorbed are used to create form and complex substance; concomitantly, the cells develop a specific requirement for potassium, selectively preferred to sodium, and balanced by organic anions rather than halide. These relationships change as cells develop; the emphasis is then upon the maintenance of osmotic value in cells with enlarging vacuoles. The developing vacuoles preferentially store organic solutes but, later, these solutes may be replaced by natural salts (KC1, NaCl) when organic supplies are depleted. This latter accumulation of salts does not display as markedly the disparity between potassium, sodium, and chloride which was so evident in the cell multiplication phase of growth. Superimposed upon these relationships are certain clonal differences which are interpreted compatibly with the above concepts. The data obtained on many clones reinforce the view that the changes in ion relations of cells with growth, as noted above, are compensated by accumulation of organic solutes as cells build osmotically active concentrations of solutes in a system that primarily controls the internal activity of their water.