Incubation under water in a 30 C/14-hour or 12 C/10-hour photoperiod caused the CO 2 compensation points of 10 aquatic macrophytes to decrease below 25 or increase above 50 microliters CO 2 per liter, respectively. Submerged and aerial leaves of two amphibious angiosperms ( Myriophyllum brasiliense and Proserpinaca palustris ) maintained high compensation points when incubated in air but, when the submerged or aerial leaves of Proserpinaca were incubated under water, the compensation points dropped as low as 10. This suggests that, in addition to temperature and photoperiod, some factor associated with submergence regulates the compensation point of aquatic plants. In the high-compensation point plants, photorespiration, as a percentage of net photosynthesis, was equivalent to that in terrestrial C 3 plants. For Hydrilla verticillata, the decreasing CO 2 compensation points (110, 40, and 10) were associated with reduced photorespiration, as indicated by decreased O 2 inhibition, decreased rates of CO 2 evolution into CO 2 -free air, and increased net photosynthetic rates. The decrease in the CO 2 compensation points of Hydrilla, Egeria densa, and Cabomba caroliniana was accompanied by an increase in the activity of phosphoenolpyruvate, but not of ribulose bisphosphate, carboxylase. In Hydrilla, several C 4 enzymes also increased in activity to the following levels (micromoles per gram fresh weight per hour): pyruvate Pi dikinase (35), pyrophosphatase (716), adenylate kinase (525), NAD and NADP malate dehydrogenase (6565 and 30), NAD and NADP malic enzymes (239 and 44), and aspartate and alanine aminotransferases (357 and 85), whereas glycolate oxidase (6) and phosphoglycolate and phosphoglycerate phosphatases (76 and 32) showed no change. Glycolate dehydrogenase and phosphoenolpyruvate carboxykinase were undetectable. The reduced photorespiration in these plants may be due to increased CO 2 fixation via a C 4 acid pathway. However, for three Myriophyllum species, some other mechanism appears operative, as phosphoenolpyruvate carboxylase was not increased in the low compensation point state, and ribulose bisphosphate carboxylase remained the predominant carboxylation enzyme.
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