Photosynthetic inorganic carbon acquisition in 30 freshwater macrophytes

- 21 of the 30 species tested can use HCO3− as a carbon source.
- Ottelia alismoides and Deinostema violaceum may possess C4 or CAM characteristics.
- Low inorganic carbon incubation of O. alismoides and D. violaceum significantly increased the PEPC activity.

Photosynthetic inorganic carbon (Ci) acquisition mechanisms were surveyed in 30 freshwater macrophytes grown under uniform, nutrient-sufficient, low Ci conditions where Ci is a potentially limiting factor on plant photosynthesis. In pH-drift experiments there was evidence for HCO3− use in 21 of the 30 species that were able to raise the solution pH above 10.0. Titratable acidity had distinct diel fluctuations in the leaves of Nechamandra alternifolia, Egeria densa, Vallisneria spinulosa, Deinostema violaceum, and Isoetes sinensis (aerial and submersed), which suggests some degree of Crassulacean Acid Metabolism (CAM) in these species. Incubation of D. violaceum under low Ci conditions substantially increased the amount of acidity and the difference between night and day. The phosphoenolpyruvate carboxylase (PEPC) activity in Hydrilla verticillata, V. spinulosa, E. densa, D. violaceum, Ottelia acuminata, and O. alismoides ranged from 32.4 ± 6.6 to 156.8 ± 5.3 μmol mg−1protein h−1 and the ratio of PEPC to ribulose bisphosphate carboxylase-oxygenase (Rubisco) activity ranged from 1.0 to 2.9. When O. alismoides and D. violaceum were incubated under conditions that greatly reduced daytime Ci, the PEPC activity was around 1.5 times higher than in untreated leaves. A high PEPC to Rubisco activity ratio, and the increased ratio under low Ci conditions, is consistent with a facultative C4 or CAM system. These results indicated that most of the freshwater macrophytes tested can use HCO3− as a carbon source, while O. alismoides, and D. violaceum may possess C4 or CAM characteristics like H. verticillata and I. howellii, and thus are worthy of further study.