Steady state osmotic adaptation inUlva lactuca.

The effects of hyper- and hypo-saline stresses on the levels of various inorganic and organic solutes inUlva lactuca have been recorded. Hypoosmotic stress decreased the tissue concentration of K(+), Na(+) and Cl(-) while hyper-osmotic stress caused a transient increase in Na(+) and a stable accumulation of K(+) and Cl(-). The tissue content of β-dimethylsulphoniopropionate (β-dimethylpropiothetin) responded to changes in salinity. The time course of hypersaline stress showed the β-dimethylsulophoniopropionate concentration rose as the Na(+) level fell. The levels of free sugars and amino acids, including proline, were relatively low in this alga and did not appear to be important in osmotic adjustment. The possibility that tertiary sulphonium dipolar ions have an analogous role in some algae to glycinebetaine and possibly other quaternary nitrogen compounds in higher plants as cytoplasmic osmotica is discussed briefly.