Comparison of the Photosynthetic Characteristics of Three Submersed Aquatic Plants 1

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Light- and CO2-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum, and Myriophyllum spicatum were 50 to 60 μmol O2/mg Chl·hr at 30 C. At air levels of CO2, the rates were less than 5% of those achieved by terrestrial C3 plants. The low photosynthetic rates correlated with low activities of the carboxylation enzymes. In each species, ribulose 1,5-diphosphate carboxylase was the predominant carboxylation enzyme. The apparent Km(CO2) values for photosynthesis were 150 to 170 μm at pH 4, and 75 to 95 μm at pH 8. The Km(CO2) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 μm at pH 8. Optimum temperatures for the photosynthesis of Hydrilla, Myriophyllum, and Ceratophyllum were 36.5, 35.0, and 28.5 C, respectively. The apparent ability of each species to use HCO3− ions for photosynthesis was similar, but at saturating free CO2 levels, there was no indication of HCO3− use. Increasing the pH from 3.1 to 9.2 affected the photosynthetic rate indirectly, by decreasing the free CO2. With saturating free CO2 (0.5 mm), the maximum photosynthetic rates were similar at pH 4 and 8. Carbonic anhydrase activity, although much lower than in terrestrial C3 plants, was still in excess of that required to support HCO3− utilization.

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