Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model

Based on measurements of delta O-18 and delta C-13 in organic matter of C-3 -plants, we have developed a conceptual model that gives insight into the r elationship between stomatal conductance (g(1)) and photosynthetic capacity (A(max)) resulting from differing environmental constraints and plant-inte rnal factors. This is a semi-quantitative approach to describing the long-t erm effects of environmental factors on CO2 and H2O gas exchange, whereby w e estimate the intercellular CO2 concentration (c(i)) from delta C-13 and t he air humidity from delta O-18. Assuming that air humidity is an important factor influencing g(1), the model allows us to distinguish whether differ ences in c(i) are caused by a response of g(1) or of A(max). As an applicat ion of the model we evaluated the isotope data from three species in plots differing in intensity of land use (hay meadows and abandoned areas) at thr ee sites along a south north transect in the Eastern Alps. We found three d ifferent delta O-18-delta C-13 response patterns in native and planted gras sland species (cultivated in the greenhouse). After preliminary confirmatio n by gas-exchange measurements we conclude that the proposed model is a pro mising tool for deriving carbon water relations in different functional gro ups from delta O-18 and delta C-13 isotope data.