Carbon isotope fractionation in species of the torrenticolous families Podostemaceae and Hydrostachyaceae

Carbon isotope ratios of herbarium material from members of the fresh-water families Podostemaceae and Hydrostachyaceae (Rosidae) were analyzed. The levels of 13C were highly variable (Podostemaceae −12.8‰ to −38.55‰; Hydrostachyaceae −10.78‰ to −30.42‰), across as well as within species and across a wide geographic range.We suggest that the high variance observed is due neither to a constant attribute of the species like the photosynthetic CO2-carboxylase (in water plants with very high discrimination of the 13CO2 probably Rubisco) nor to the constant structural peculiarities of these species. Rather, it is likely due to the ‘diffusional resistance’ for the CO2-flux from the turbulent and/or fast flowing water, causing a very variable boundary layer on the plant surface.