Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Carbon acquisition in variable environments : aquatic plants of the River Murray, Australia

Auteur : Barrett, Melissa S.

Université de soutenance : University of Adelaide

Grade : Doctor of Philosophy (PhD) 2008

Résumé
This thesis considers the implications of changes in the supply of resources for photosynthesis, with regard for modes of carbon acquisition employed by aquatic plants of the River Murray. Carbon supplies are inherently more variable for aquatic plants than for those in terrestrial environments, and variations are intensified for plants in semi-arid regions, where water may be limiting. In changeable environments the most successful species are likely to be those with flexible carbon-uptake mechanisms, able to accommodate variations in the supply of resources. Studies were made of plants associated with wetland habitats of the Murray, including Crassula helmsii, Potamogeton tricarinatus, P. crispus and Vallisneria americana. The aim was to elucidate the mechanisms of carbon uptake and assimilation employed, and to determine how flexibility in carbon uptake and/​or assimilation physiology affect survival and distribution. Stable carbon isotopes were used to explore the dynamics of carbon uptake and assimilation, and fluorescence was used to identify pathways and photosynthetic capacity. The studies suggest that physiological flexibility is adaptive survival in changeable environments, but probably does not enhance the spread or dominance of these species. V. americana is a known bicarbonate-user, and it is shown here that it uses the Crassulacean Acid Metabolism (CAM) photosynthetic pathway under specific conditions (high light intensity near the leaf tips) concurrently with HCO₃⁻ uptake, while leaves deeper in the water continue to use the C₃ pathway, with CO₂ as the main carbon source. However, V. americana does not use CAM when under stress, such as exposure to high light and temperature. The diversity of carbon uptake and assimilation mechanisms in this species may explain its competitive ability in habitats associated with the Murray. In this way it is able to maximise use of light throughout the water column. In shallow, warm water, where leaves are parallel to the surface, CAM ability is likely to be induced along the length of the leaf, allowing maximal use of carbon and light. The amphibious C. helmsii is shown to use CAM on submergence, even where water levels fluctuate within 24 hours. This allows continued photosynthesis in habitats where level fluctuations prevent access to atmospheric CO₂. It appears that stable conditions are most favourable for growth and dispersal, and that the spread of C. helmsii is mainly by the aerial form. Carbon uptake by P. tricarinatus under field conditions is compared with that of P. crispus to demonstrate differences in productivity associated with aqueous bicarbonate and atmospheric CO₂ use. P. tricarinatus uses HCO₃⁻ uptake to promote growth toward the surface, so that CO₂ can be accessed by floating leaves. Atmospheric contact provides access to light and removes the limitation

Mots clés : aquatic photosynthesis ; crassulacean acid metabolism ; bicarbonate uptake ; vallisneria ; crassula

Présentation (National Library of Australia)

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