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_canopy) can influence yield in wheat and other crops. Breeding for WUE_canopy is difficult because it is influenced by many component traits. For example, intrinsic water use efficiency (WUE_i), the ratio of net carbon assimilation (A_net) over stomatal conductance, contributes to WUE_canopy 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_leaf). Alternatively, measurements of instantaneous leaf water use efficiency (WUE_leaf) are defined as A_net 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_leaf that impact transpiration but not A_net. This alters WUE_leaf and in turn affects WUE_canopy. To test this relationship, WUE_canopy was measured in conjunction with WUE_i, WUE_canopy, 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_canopy likely because of the effect of canopy temperature on VPD_leaf driving T. However, deviations between WUE_i, WUE_leaf, and WUE_canopy were present because measurements made at the leaf level do not account for variations in leaf temperature. This uncoupled the relationship of measured WUE_leaf and WUE_i from WUE_canopy and emphasizes the importance of canopy temperature on carbon uptake and transpired water loss.