Wild plants can improve their rhizosphere chemistry in acid soils
Abstract: Growth conditions on acid soils may be hampered by low nutrient availability and phytotoxic Al (aluminium) concentrations. In this thesis I present my studies on adaptation mechanisms of wild plants to acid soils in the close root environment, the rhizosphere. The studies showed that plants have developed several mechanisms that may work simultaneously to face the complex soil chemistry. A first greenhouse and field study indicated that wild plants use soil heterogeneity to exploit nutrient rich patches and maybe also to avoid elevated Al concentrations. Further experiments concentrated on the ability of wild plants to exude organic acids in response to elevated Al concentrations. Organic acids chelate Al and render it thereby non-toxic to plants. Deschampsia flexuosa increased the malic acid exudation in a rhizotron experiment but not in hydroponics. Of ten studied species in hydroponics Rumex acetosella and Viscaria vulgaris increased the exudation of oxalic acid and Galium saxatile and Veronica officinalis the exudation of citric acid in response to Al. In studies over one week the exudation pattern was dependent on Al concentration. All species needed at least four hours before exudation of organic acids was increased. Veronica officinalis was able to raise the pH in the rhizosphere when grown in nutrient solutions with Al. Biodegradation of organic acids was studied to estimate to what extent it lowers the chelating of Al under natural conditions. About 70 % of added organic acids were respired after 24 h. Al concentrations at levels of possible phytotoxicity to plants did not seem to inhibit organic acid degradation. Plants were able to maintain a several fold higher organic acid concentration in the rhizosphere than in the bulk soil solution, despite of the high biodegradation rates. My results showed that wild plants use several mechanisms to cope with acid soils, e.g. organic acid exudation, pH changes and exploitation of nutrient rich patches and that the mechanisms vary with plant species, time, plant age and growth medium.
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