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

The oxygen isotope enrichment of bulk leaf water (Delta(b)) was measured in cotton (Gossypium hirsutum) leaves to test the Craig-Gordon and Farquhar-Gan models under different environmental conditions. Delta(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 (Delta(1)), 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 Delta(b) (and Delta(1)), as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Delta(b) and Delta(1) 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 Delta(1) 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.