Comparison of the Photosynthetic Characteristics of Three Submersed Aquatic Plants

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Abstract
Light- and CO2-saturated photosynthetic rates of the submersed aquatic plants Hydrilla verticillata, Ceratophyllum demersum and Myriophyllum spicatum were 50-60 .mu.mol O2/mg Chl [chlorophyll] .cntdot. h at 30.degree. 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 150-170 .mu.M at pH 4, and 75-95 .mu.M at pH 8. The Km(CO2) of Hydrilla ribulose 1,5-diphosphate carboxylase was 45 .mu.M at pH 8. Optimum temperatures for the photosynthesis of Hydrilla, Myriophyllum, and Ceratophyllum were 36.5.degree., 35.0.degree. and 28.5.degree. 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 3.1-9.2 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. Hydrilla and Ceratophyllum had CO2 compensation points of 44 and 41 .mu.l/l, respectively, whereas the value for Myriophyllum was 19. Relatively high CO2 compensation points under 1% O2 indicated that some dark respiration occurred in the light. The inhibition of photosynthesis by O2 was less than with terrestrial C3 plants. Glycolate oxidase activity was 12.3-27.5 .mu.mol O2/mg Chl .cntdot. h, as compared to 78.4 for spinach. Light saturation of photosynthesis occurred at 600-700 .mu.einsteins/m2 .cntdot. s 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 CO2 dropped to zero, and O2 rose to over 200% air saturation. Most photosynthetic activity occurred in the morning when the free CO2 was highest and O2 and solar radiation lowest. The low light requirement of Hydrilla probably provides a competitive advantage under these field conditions.