Environmental effects on oxygen isotope enrichment of leaf water in cotton leaves

The oxygen isotope enrichment of bulk leaf water (Δ_b) was measured in cotton (Gossypium hirsutum) leaves to test the Craig-Gordon and Farquhar-Gan models under different environmental conditions. Δ_b increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (e_a/e_i) and to stomatal conductance (g_s). The oxygen isotope enrichment of lamina water relative to source water (Δ₁), which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. The Craig-Gordon model overestimated Δ_b (and Δ₁), as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Δ_b and Δ₁ with an L of 7.9 mm, much less than the total radial effective length L_r of 43 mm. The fitted values of L for Δ₁ of individual leaves showed little dependence on VPd and temperature, supporting the assumption that the Farquhar-Gan formulation is relevant and useful in describing leaf water isotopic enrichment.