Accumulation of proline in a submerged aquatic halophyte, Ruppia L.

Osmoregulation in the aquatic halophyte Ruppia was examined as part of a wider study of mechanisms of salt tolerance within the genus. The tissue fluids of the three species R. megacarpa Mason, R. polycarpa Mason and R. tuberosa Davis and Tomlinson, were always hypertonic to the surrounding water. The organic solute proline was accumulated, possibly as a mechanism to counteract the effects of high external salinities. Proline content increased with the salinity of the habitat in all three species. In general proline content increased while plant growth was active and declined later in the growing season. If the assumptions that proline is confined to the cytoplasm and that the cytoplasm comprises 10% of the cell volume are made for these submerged aquatic plants, proline is estimated to generate a solute concentration in this compartment equivalent to up to 50% of the total solute concentration of the external habitat.