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. [ABSTRACT FROM AUTHOR]