Osmotic adjustment and osmotic constituents in roots of mung bean seedlings

Osmotic adjustment in roots of mung bean seedlings (Vigna mungo (L.) Hepper) and the effect of cotyledon excision on the osmotic adjustment were investigated. The major osmotic constituents in roots of intact seedlings were K⁺, Cl^-, free amino acids and sugars (glucose, fructose and sucrose). All these intracellular concentrations distinctly increased under osmotic stress and contributed to about 80% of the intracellular osmotic pressure of the root cell sap. Cotyledon excision remarkably suppressed both the osmotic adjustment and the elongation in roots. However, the effect of cotyledon excision on intracellular K⁺ and Cl^- concentrations in roots was quite small. Two different mechanisms are likely for the osmotic adjustment in roots. One is the K⁺ and Cl^--dependent osmotic adjustment which is cotyledon-independent, and the other is the osmotic adjustment dependent on the supply of free amino acids and sugars from cotyledons.