Photosynthetic carbon assimilation by Crassula helmsii

Photosynthesis of Crassula helmsii, an amphibious aquatic macrophyte weed species, has been measured with respect to pH and irradiance. C. helmsii shows a marked diel fluctuation in titratable acidity, which can be accounted for by changing levels of malic acid. C. helmsii is unable to use HCO _inf3^sup- for photosynthesis and exhibits generally low photosynthetic rates when CO₂ is not limiting. The photon flux density at which the onset of light saturation of photosynthesis is reached (E _K ) is low for aquatic macrophytes. Some advantages conferred on C. helmsii by the possession of crassulacean acid metabolism are an extension of the period of assimilation of dissolved inorganic carbon, resulting in a reduction in the limitation imposed on photosynthesis in aquatic environments by a very high CO₂ diffusion resistance.