Is the sensitivity of leaf water and cellulose δ18O values sufficient for detecting effects of increasing atmospheric CO2 on stomatal conductance in plants? 

Abstract

The oxygen isotope composition of cellulose (δ18O values) has been suggested to contain information on stomatal conductance (gs) responses to rising pCO2. The extent by which pCO2 affects leaf water and cellulose δ18O values (δ18OLW and δ18OC) and the isotope processes that determine pCO2 responses of gs in δ18OLW and δ18OC are, however, unknown. We tested the effects of pCO2 on gs, δ18OLW and δ18OCin a greenhouse experiment, where six herbaceous plant species were grown under pCO2 levels ranging from 200 to 500 ppm. An increase in pCO2 caused a decline in gs. The effects of gs on δ18OLW were caused by direct and indirect mechanisms but were generally small. The model parameter effective path length (Lm) was unaffected by changes in pCO2. pCO2 effects on δ18OLW were not directly transferred to plant δ18OC but were attenuated in grasses and amplified in dicotyledonous herbs and legumes. This is likely because of functional group specific pCO2 effects on the model parameter pxpex. Our study removes critical uncertainties for using δ18OC as a proxy for gs. At the same time, our study shows that gs effects on δ18OLW and δ18OC are rather small, possibly too small to be detected in natural settings.