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

中文翻译：

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叶温影响冠层水分利用效率与叶面水分利用效率的变化无关

冠层水分利用效率（地上生物量在整个生命周期的失水量，WUEcanopy）会影响小麦和其他作物的产量。WUEcanopy 的育种很困难，因为它受许多组成性状的影响。例如，内在用水效率 (WUEi)，即净碳同化 (Anet) 与气孔导度的比率，对 WUEcanopy 有贡献，并且可以通过碳同位素鉴别 (Δ) 进行估算。然而，Δ对空气和叶子（VPDleaf）之间的水蒸气压差的差异不敏感。或者，瞬时叶水分利用效率 (WUEleaf) 的测量值被定义为蒸腾作用的 Anet，可以通过气体交换来确定，但没有表示田间条件的动态特性。具体来说，冠层温度的波动会导致 VPDleaf 的变化影响蒸腾作用，但不会影响 Anet。这会改变 WUEleaf，进而影响 WUEcanopy。为了测试这种关系，在两个树冠结构不同的耐旱小麦品种中，WUEcanopy 与 WUEi、WUEcanopy 和树冠温度在浇水充足和水分有限的条件下一起测量。在该实验中，边界层电导率较低，并且与 WUEcanopy 相关的品种和处理之间的叶片温度发生了显着变化，这可能是由于冠层温度对 VPDleaf 驱动 T 的影响。然而，WUEi、WUEleaf 和 WUEcanopy 之间存在偏差是因为在叶级进行的测量不考虑叶温的变化。