Tolerance of extreme salinity in two stem-succulent halophytes (Tecticornia species)

Communities of Tecticornia on the margins of ephemeral salt lakes in Australia often exhibit species zonation, such as at Hannan Lake (Western Australia) where Tecticornia indica subsp. bidens (Nees) K.A.Sheph. and Paul G.Wilson occupies the less saline dune habitat on lake margins and Tecticornia pergranulata (J.M.Black) K.A.Sheph. and Paul G.Wilson subsp. pergranulata occupies both the dunes and the more saline and moist lake playa. Here we tested the hypothesis that these two species differ in tolerance to extreme salinity. Plants were grown in drained sand cultures with treatments of 10-2000mM NaCl for 85 days. Both species were highly salt tolerant, maintaining growth at treatments of up to 2000mM NaCl, although the death of two replicates of T. indica at 2000mM NaCl suggests this salinity is close to the species tolerance limit. Both Tecticornia species maintained a favourable gradient in tissue water potential via osmotic adjustment as external salinity increased, also with reduced tissue water content at very high external salinity. Regulated accumulation of Na+ and Cl-, maintenance of net K+ to Na+ selectivity, high tissue concentrations of glycinebetaine and presumed cellular solute compartmentation, would have contributed to salt tolerance. The growth rate of T. pergranulata was 11-29% higher than T. indica suggesting, in addition to these moderate differences in salinity tolerance, other factors are likely to contribute to species zonation at salt lakes. The higher water use efficiency of the C4 T. indica compared with the C3 T. pergranulata may provide an advantage in the drier dune habitat on salt lake margins. An additional experiment confirmed the hypothesis that survival of T. pergranulata seedlings is enhanced by the duration of reduced salinity after germination, as would occur following significant rainfall, as older seedlings maintained higher growth rates during subsequent increases in salinity.