Light- and CO 2 -saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum , and Myriophyllum spicatum were 50 to 60 μmol O 2 /mg Chl·hr at 30 C. At air levels of CO 2 , the rates were less than 5% of those achieved by terrestrial C 3 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 K m (CO 2 ) values for photosynthesis were 150 to 170 μ m at pH 4, and 75 to 95 μ m at pH 8. The K m (CO 2 ) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 μ m 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 HCO 3 − ions for photosynthesis was similar, but at saturating free CO 2 levels, there was no indication of HCO 3 − use. Increasing the pH from 3.1 to 9.2 affected the photosynthetic rate indirectly, by decreasing the free CO 2 . With saturating free CO 2 (0.5 m m ), the maximum photosynthetic rates were similar at pH 4 and 8. Carbonic anhydrase activity, although much lower than in terrestrial C 3 plants, was still in excess of that required to support HCO 3 − utilization. Hydrilla and Ceratophyllum had CO 2 compensation points of 44 and 41 μl/l, respectively, whereas the value for Myriophyllum was 19. Relatively high CO 2 compensation points under 1% O 2 indicated that some “dark” respiration occurred in the light. The inhibition of photosynthesis by O 2 was less than with terrestrial C 3 plants. Glycolate oxidase activity was 12.3 to 27.5 μmol O 2 /mg Chl·hr, as compared to 78.4 for spinach. Light saturation of photosynthesis occurred at 600 to 700 μeinsteins/m 2 ·sec in each species grown under full sunlight. Hydrilla had the lowest light compensation point, and required the least irradiance to achieve the half-maximal photosynthetic rate. Field measurements in a Hydrilla mat indicated that in the afternoon, free CO 2 dropped to zero, and O 2 rose to over 200% air saturation. Most photosynthetic activity occurred in the morning when the free CO 2 was highest and O 2 and solar radiation lowest. The low light requirement of Hydrilla probably provides a competitive advantage under these field conditions.
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