Affinity for CO2 in relation to the ability of freshwater macrophytes to use HCO3-

1. The affinity of photosynthesis for CO2 is calculated here as the initial slope of net-photosynthetic rate against concentration of CO2. The affinity for CO2 for pairs of freshwater macrophytes with similar leaf morphology but able or unable to use HCO3- as a carbon source was compared. 2. Species restricted to CO2 had a higher affinity for CO2 than species that were also able to use HCO3- when rates were expressed on the basis of area, dry mass and content of chlorophyll a. 3. Published values for the affinity for CO2 and the concentration of CO2 which half-saturated rate of photosynthesis were compiled and compared. Despite a large range of values, affinity for CO2 was greater for species restricted to CO2 than for those also able to use HCO3- and statistically different when the slope was expressed on the basis of dry mass and chlorophyll a content. 4. The difference in affinity is consistent with predicted benefits of a high permeability to CO2 for species relying on passive diffusion of CO2 and a lower permeability for species able to use HCO3- in order to reduce efflux of CO2 from a high internal concentration generated by active transport. 5. The implications of the different affinities are discussed in terms of species distribution.