Ecophysiological responses to changes in salinity and water availability in stem-succulent halophytes (Tecticornia spp.) from an ephemeral salt lake

Louis Moir-Barnetson

    Research output: ThesisDoctoral Thesis

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    Abstract

    Stem-succulent halophytes of the genus Tecticornia (commonly known as samphires; sub-family Salicornioideae; family Amaranthaceae, formerly Chenopodiaceae) dominate the margins of the numerous ephemeral salt lakes of inland Australia. These are stressful environments, often typified by aridity, extreme soil salinity and episodes of overland flash floods. Despite the ecological importance of Tecticornia communities to salt lake systems of inland Australia, knowledge about their ecophysiology is very limited.
    Studies conducted at the Fortescue Marsh, an ephemeral salt lake in the semi-arid region of north-west Australia, revealed that flooding, soil salinity and drought interact and jointly determine species distribution and community structure of this halophytic vegetation. Based on population size structure and field observations of seedlings in the dominant Tecticornia species, it was shown that population renewals are infrequent and dependent on specific flooding and post-flooding conditions. A seasonal study revealed that Tecticornia medusa, which occurs at the lowest end of the vegetated zone into the marsh, is exposed to prolonged submergence during the wet season and extreme reductions in soil water potential during the dry season, whilst two other species occupying progressively higher elevated sites (T. auriculata and T. indica respectively) experience more moderate seasonal fluctuations.
    Glasshouse experiments revealed that differences in the tolerance of Tecticornia species to extreme soil salinity and also to the submergence of shoots in low-salinity water (as occurs following cyclonic events), are likely to influence patterns of species zonation at Fortescue Marsh. At extreme salinity (2000 mM NaCl), T. medusa and T. auriculata survived whilst T. indica died, whereas in response to submergence, T. medusa survived whilst T. auriculata and T. indica died. These responses were consistent with the hypothesis that interspecies differences in tolerances of salinity and flooding contribute to species distributions, as inability to tolerate extreme salinity and submergence in T. indica and submergence only in T. auriculata, may reduce the competitiveness of these species in low elevated marsh habitats dominated by T. medusa.
    Physiological tolerance to salinity in Tecticornia was based on the accumulation in succulent stem tissues of Na+ and Cl–, and of the compatible solute glycinebetaine, and the maintenance of tissue K+ concentrations above critical levels by increasing the net K+ to Na+ selectivity to the shoot. Submergence tolerance in T. medusa was associated with the ability to maintain tissue integrity despite the high mechanical stresses caused by water uptake from the surrounding submergence solution due to steep osmotic gradients. The relatively weaker succulent tissues of T. auriculata and T. indica ruptured due to excessive swelling caused by submergence, leading to mortality.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - Jun 2014

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