The effect of internal CO2 conductance on leaf carbon isotope ratio.

We estimated internal CO2 conductance (gi) for C3 plant species of different life forms (annual herbs, deciduous trees and evergreen trees) grown in a variety of environments to examine the effect of gi on their leaf carbon isotope ratio (delta13C). The purpose of this study was to test the validity of using delta13C as an index of photosynthetic water use efficiency (WUE). When comparing deciduous tree species grown in contrasting light environments, there was a strong positive relationship between delta13C and WUE. Similarly, delta13C was positively correlated to WUE when comparing the different species of evergreen trees. However, the difference in WUE between evergreen and deciduous tree species did not relate to that in leaf delta13C. In addition, WUE was similar between highland and lowland herbaceous plants, although the former had a much higher delta13C. The positive relationship between delta13C and WUE did not hold across different life forms and different altitudes when differences in gi did not relate to those in stomatal conductance, resulting in independence of chloroplast CO2 partial pressure from intercellular CO2 partial pressure.