Photosynthesis in the Aquatic Macrophyte Egeria densa. II. Effects of Inorganic Carbon Conditions on 14C Fixation

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Abstract
In short-term labelling experiments, tripling the concentration of total inorganic carbon (TIC) did not significantly increase the high rates of 14C fixation reported in an earlier paper [19.0 μmol C (g dry wt)-1 min-1 at pH 6.8, [TIC] = 1 mM]. However, either decreasing [TIC] or increasing the pH caused the fixation rate to fall markedly. Thus at pH 6.8, [TIC] = 38 μM and pH 10.2, [TIC] = 1.0 mM, photosynthesis was 2.3 and 1.3 μmol g-1 min-1, respectively. Time courses of the distribution of photosynthetic intermediates indicated that the Calvin cycle remained the predominant pathway of carbon fixation, irrespective of the ambient conditions of TIC and pH. When the rate of photosynthesis was reduced by decreasing [TIC] or increasing pH, the proportion (but not the absolute amount) of label found in malate increased. At pH 6.8, [TIC] = 2.9 mM, μ-carboxylation accounted for only 2.7% of the total carbon fixed, compared with 9% at air levels of CO2 (pH 4.5, [TIC] = 13 μM). Egeria does not appear to exhibit C4 photosynthesis under any of the conditions studied, but malate may be a significant product of photosynthesis whenever the fixation rate is reduced by carbon availability.