RESPONSE OF MELANTHERA-BIFLORA TO SALINITY AND WATER-STRESS

Melanthera biflora (Asteraceae) is a moderately salt-tolerant plant from the Indo-Pacific region. In laboratory studies its growth was inhibited by salt above 50 mol m-3, but it was able to survive salinities approaching that of seawater, namely 400 mol m-3. Shoot potassium concentrations were maintained over a range of salinities up to 400 mol m-3, while sodium and chloride accumulation followed closely the increase in external osmotic pressure. In contrast, the increase in osmotic pressure of the leaf sap of Melanthera biflora, subjected to water stress, was due mainly to a decrease in the ratio of fresh weight/dry weight. 3-dimethylsulphoniopropionate (3-DMSP) and glycinebetaine were identified by fast atom bombardment mass and H-1-NMR spectroscopy, with 3-DMSP being the main onium compound and glycinebetaine absent in some accessions. Onium (quaternary ammonium and/or tertiary sulphonium) compounds and proline increased during salt and water stress due mainly to a decrease in the fresh weight/dry weight ratio of tissue, although part of the increase in salt-stressed tissue was due to an increase in the accumulation of the onium compound. This salt-induced increase in 3-DMSP was inhibited in conditions of low sulphur supply and there was no compensatory increase in proline.