Interactive effects of salinity, nitrogen and sulphur on the organic solutes in Spartina alterniflora leaf blades

Glycinebetaine, proline, asparagine, sucrose, glucose, and dimethylsulphoniopropionate (DMSP) were the major organic solutes in Spartina alterniflora leaf blades. To investigate the physiological role(s) of these solutes, the effects of salinity, nitrogen, and sulphur treatments on leaf blade solute levels were examined. Glycinebetaine was the major organic solute accumulated in leaf blades grown at 500 mol m^-3 NaCl, although asparagine and proline also accumulated when the supply of nitrogen was sufficient. These solutes may play a role in osmotic adjustment. In contrast, DMSP levels either did not change or were reduced in response to the 500 mol m^-3 NaCl treatment. Furthermore, elevated nitrogen supply decreased leaf blade DMSP levels, which was opposite to the response of glycinebetaine, proline, and asparagine. A 1000-fold increase in external sulphate concentration had no effect on the leaf blade levels of DMSP, glycinebetaine, proline, or asparagine. These findings suggest that the major physiological role of DMSP in S. alterniflora leaf blades is not for osmotic adjustment, even under conditions of nitrogen deficit and excess sulphur. Instead, DMSP which was present at 45-130 μmol g^-1 dry weight, may play a role as a constitutive organic osmoticum.