Inorganic carbon source for photosynthesis in the aquatic macrophytes Potamogeton natans and Ranunculus fluitans
Photosynthetic carbon uptakes of floating and submersed leaves of the pondweed Potamogeton natans L. and submersed leaves of Ranunculus fluitans Lam. were studied in a number of KHCO3 media of varying pH (7–9.5) and concentration (1–12 mM). Photosynthetic rates of the floating and submersed leaves of P. natans correlated with free-CO2 concentrations and were independent of HCO3− concentrations in the media. The CO2 affinity of submersed leaves was greater than that of floating leaves, but floating leaves showed marked increases in CO2 affinity after submergence at low CO2. R. fluitans clearly has the ability to use HCO3− for photosynthesis. The mechanism of HCO3− utilization in relation to leaf forms within the genus Potamogeton is discussed.