Exploring the potential of Δ(17)O in CO(2) for determining mesophyll conductance

Mesophyll conductance to CO(2) from the intercellular air space to the CO(2)–H(2)O exchange site has been estimated using δ(18)O measurements (g(m18)). However, the g(m18) estimates are affected by the uncertainties in the δ(18)O of leaf water where the CO(2)–H(2)O exchange takes place and the degree of equilibration between CO(2) and H(2)O. We show that measurements of Δ(17)O ([Formula: see text]) can provide independent constraints on g(m) (g(mΔ17)) and that these g(m) estimates are less affected by fractionation processes during gas exchange. The g(m) calculations are applied to combined measurements of δ(18)O and Δ(17)O, and gas exchange in two C(3) species, sunflower (Helianthus annuus L. cv. ‘sunny’) and ivy (Hedera hibernica L.), and the C(4) species maize (Zea mays). The g(m18) and g(mΔ17) estimates agree within the combined errors (P-value, 0.876). Both approaches are associated with large errors when the isotopic composition in the intercellular air space becomes close to the CO(2)–H(2)O exchange site. Although variations in Δ(17)O are low, it can be measured with much higher precision compared with δ(18)O. Measuring g(mΔ17) has a few advantages compared with g(m18): (i) it is less sensitive to uncertainty in the isotopic composition of leaf water at the isotope exchange site and (ii) the relative change in the g(m) due to an assumed error in the equilibration fraction θ(eq) is lower for g(mΔ17) compared with g(m18). Thus, using Δ(17)O can complement and improve the g(m) estimates in settings where the δ(18)O of leaf water varies strongly, affecting the δ(18)O (CO(2)) difference between the intercellular air space and the CO(2)–H(2)O exchange site.