A global survey of carbon isotope discrimination in plants from high altitude.

Carbon 13/12 isotope ratios have been determined from leaves of a hundred C plant species (or ecotypes) from all major mountain ranges of the globe, avoiding drought stressed areas. A general increase in C content was found with increasing altitude, i.e. overall discrimination against the heavy isotope is reduced at high elevation. The steepest decline of discrimination is observed in taxa typically ranging to highest elevations (e.g. the genus Ranunculus). Mean δ C for all samples collected between 2500 and 5600 m altitude is-26.15‰ compared to the lowland average of-28.80‰ (P<0.001). Forbs from highest elevations reach-24‰. According to theory of C discrimination this indicates decreasing relative limitation of carbon uptake by carboxylation. In other words, we estimate that the ratio of internal to external partial pressure of CO (p /p )in leaves of high elevation plants is lower than in leaves of low altitude. These results confirm recent gas exchange analyses in high and low elevation plants.