Abstract
In order to investigate the effects of Al on nodule formation and function in the Casuarina-Frankia symbiosis, inoculated plants were grown in sand culture at five nominal Al concentrations (0-880 μM Al) at pH 4.0. There was an Al-free control at pH 6.0 to assess the effects of pH 4.0 treatments. Mean N concentration of nodules was significantly less at pH 4.0 (1.83%) than at pH 6.0 (2.01%). There were nodulated plants at all Al levels, though there were fewer nodulated plants at 440 and 880 μM Al. Dry weights of nodules, shoots and roots were not reduced by Al concentrations at or below 220 μM Al, but were decreased by Al concentrations at or above 440 μM Al. Nodule weight expressed as a percentage of total weight did not differ significantly with respect to an Al-free control at pH 4. N concentrations of shoots and whole plants were significantly reduced at 440 μM Al. Nodular specific acetylene reduction activity (ARA) did not differ significantly among Al treatments. However, N2-fixation efficiency was decreased from 0.20 to 0.10 mg N fixed mg nodule dry weight−1 at 880 μM Al.
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