Photosynthesis by Aquatic Plants: Effects of Unstirred Layers in Relation to Assimilation of Co2 and Hco3−and to Carbon Isotopic Discrimination

S ummary We review and reassess experimental evidence which indicates that the unstirred layers of solution bathing aquatic plant cells or organs represent a major factor limiting their rate of photosynthesis under natural conditions. Unstirred layers may also limit membrane transport of HCO 3 − where this occurs. Some authors have suggested that aquatic plants carry out C 4 photosynthesis and cite in support measurements of discrimination between 13 C and 12 C. We propose that this discrimination can also be a manifestation of the presence of unstirred layers or membrane transport or both.