C4 Acid Metabolism and Dark CO2 Fixation in a Submersed Aquatic Macrophyte (Hydrilla verticillata)

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Abstract
The CO2 compensation point of the submersed aquatic macrophyte H. verticillata varied from high (above 50 .mu.l/l) to low (10-25 .mu.l/l) values, depending on the growth conditions. Plants from the lake in winter or after incubation in an 11.degree. C/9 h photoperiod had high values, whereas summer plants or those incubated in a 27.degree. C/14 h photoperiod and low values. The plants with low CO2 compensation points exhibited dark 14CO2 fixation rates that were up to 30% of the light fixation rates. This fixation reduced respiratory CO2 loss, but did not result in a net uptake of CO2 at night. The low compensation point plants also showed diurnal fluctuations in titratable acid, such as occur in Crassulacean acid metabolism plants. Dark fixation and diurnal acid fluctuations were negligible in Hydrilla plants with high CO2 compensation points. Exposure of the low compensation point plants to 20 .mu.M 14CO2 resulted in 60% of the 14C being incorporated into malate and aspartate, with only 16% in sugar phosphates. At a high CO2 level, the C4 acid label was decreased. A pulse-chase study indicated that the 14C in malate, but for asparate, decreased after a long (270 s) chase period; thus, the C4 acid turnover was much slower than in C4 plants. Phosphoenolpyruvate carboxylase activity was high (330 .mu.mol/mg chlorophyll per h), as compared to ribulose bisphosphate carboxylase (20-25), in the plants with low compensation points. These plants also had a pyruvate, Pi dikinase activity in the leaves of 41 .mu.mol/mg chlorophyll per h, which suggests they are not C3 plants. NAD- and NADP+-malate dehydrogenase activities were 6136 and 24.5 .mu.mol/mg chlorophyll per h, respectively. Of the 3 decarboxylating enzymes assayed, the activities of NAD- and NADP-malic enzyme were 104.2 and 23.7 .mu.mol/mg chlorophyll per h, while phosphoenolpyruvate carboxykinase was only 0.2. Low compensation point Hydrilla plants fix some CO2 into C4 acids, which can be decarboxylated for later refixation, presumably into the Calvin cycle. Refixation would be advantageous in summer lake environments where the CO2 levels are high at night but low during the day. Hydrilla does not fit any of the present photosynthetic categories, and may have to be placed into a new group, together with other submersed aquatic macrophytes that have environmentally variable CO2 compensation points.