Leaf temperature impacts canopy water use efficiency independent of changes in leaf level water use efficiency.

Canopy water use efficiency (above-ground biomass over lifetime water loss, WUE) can influence yield in wheat and other crops. Breeding for WUE is difficult because it is influenced by many component traits. For example, intrinsic water use efficiency (WUE), the ratio of net carbon assimilation (A) over stomatal conductance, contributes to WUE and can be estimated from carbon isotope discrimination (Δ). However, Δ is not sensitive to differences in the water vapor pressure deficit between the air and leaf (VPD). Alternatively, measurements of instantaneous leaf water use efficiency (WUE) are defined as A over transpiration and can be determined with gas exchange, but the dynamic nature of field conditions are not represented. Specifically, fluctuations in canopy temperature lead to changes in VPD that impact transpiration but not A. This alters WUE and in turn affects WUE. To test this relationship, WUE was measured in conjunction with WUE, WUE, and canopy temperature under well-watered and water-limited conditions in two drought-tolerant wheat cultivars that differ in canopy architecture. In this experiment, boundary layer conductance was low and significant changes in leaf temperature occurred between cultivars and treatments that correlated with WUE likely because of the effect of canopy temperature on VPD driving T. However, deviations between WUE, WUE, and WUE were present because measurements made at the leaf level do not account for variations in leaf temperature. This uncoupled the relationship of measured WUE and WUE from WUE and emphasizes the importance of canopy temperature on carbon uptake and transpired water loss.