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

The oxygen isotope composition of cellulose (δ¹⁸O values) has been suggested to contain information on stomatal conductance (g_s) responses to rising pCO₂. The extent by which pCO₂ affects leaf water and cellulose δ¹⁸O values (δ¹⁸O_LW and δ¹⁸O_C) and the isotope processes that determine pCO₂ responses of g_s in δ¹⁸O_LW and δ¹⁸O_C are, however, unknown. We tested the effects of pCO₂ on g_s, δ¹⁸O_LW and δ¹⁸O_Cin 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 g_s. The effects of g_s on δ¹⁸O_LW were caused by direct and indirect mechanisms but were generally small. The model parameter effective path length (L_m) was unaffected by changes in pCO₂. pCO₂ effects on δ¹⁸O_LW were not directly transferred to plant δ¹⁸O_C 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 p_xp_ex. Our study removes critical uncertainties for using δ¹⁸O_C as a proxy for g_s. At the same time, our study shows that g_s effects on δ¹⁸O_LW and δ¹⁸O_C are rather small, possibly too small to be detected in natural settings.