Solute Accumulation In Plant Cells IV. Effects of Ammonium lons on Growth and Solute Content

Procedures previously described were used to study growth and solute content of aseptically cultured carrot explants as affected by supplementary salts in the medium. The salts chosen (KC1, KNO3, NH4,Cl, and NH4,NO3) contrasted, with appropriate controls, the effects due to nitrate and ammonium. Growth was measured in terms of fresh weight, the number and average size of cells: solute concentrations were recorded for total solutes, sugars, soluble nitrogen compounds, and the electrolytes K+, Na+, C1−, NO−3, and organic acids. The time-response curves of the cultures were traced at a fixed concentration of the added salts and the effects due to the concentration of the supplementary salts were tested after a fixed time period, For the same nitrogen source the concentrations of metabolites and solutes in cells were very similar despite some clonal differences in their growth. When cells in a nitrate medium were small and dividing, the cultures had a low osmotic value, contained K+ as the principal cation balanced by organic acid, had relatively low sugar content, and their enriched total nitrogen content emphasized protein rather than soluble nitrogen compounds. Later, as the cells became older and larger, salts (K+, organic anions, Cl−) contributed substantially to their increased osmotic value but they accumulated sugar as their main, osmotically active solute, and the ratio of soluble to protein nitrogen declined as protein synthesis progressed. The extra nitrogen supplied by the additional potassium nitrate contributed more to protein and caused potassium, organic acids, and sugars to accumulate to higher levela. Supplementary ammonium salts required that more sugar be metabolized to organic nitrogen compounds (e.g. glutamine), contributed more to soluble than to protein nitrogen, and sharply reduced. both the osmotic value of the cells and the potassium linked to organic anions. The selectivity of the growing cells for K+ over Na+ and their discrimination. between alkali cations (Ka++Na+) and halides (C1−) were relaxed in the presence of ammonia. Attention is drawn to the implications of these results for the accumulation of solutes, organic and inorganic, by dividing and enlarging cells.