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 relationship between stomatal conductance (g(1)) and photosynthetic capacity (A(max)) resulting from differing environmental constraints and plant-internal factors. This is a semi-quantitative approach to describing the long-term effects of environmental factors on CO2 and H2O gas exchange, whereby we estimate the intercellular CO2 concentration (c(i)) from delta C-13 and the air humidity from delta O-18. Assuming that air humidity is an important factor influencing g(1), the model allows us to distinguish whether differences in c(i) are caused by a response of g(1) or of A(max). As an application 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 three sites along a south north transect in the Eastern Alps. We found three different delta O-18-delta C-13 response patterns in native and planted grassland species (cultivated in the greenhouse). After preliminary confirmation by gas-exchange measurements we conclude that the proposed model is a promising tool for deriving carbon water relations in different functional groups from delta O-18 and delta C-13 isotope data.