Apparent Contradictions in 13 C Discrimination, Normalized Transpiration Efficiency and Wheat Water Use Efficiency.

Both transpiration efficiency and ¹³C discrimination (Δ) are related to the partial pressure of CO₂ in the leaf, relative to ambient. Field observations demonstrate a positive relationship of seasonal water use efficiency (kg biomass/ha mm) and Δ, which appears to contradict the negative relationship of normalized transpiration efficiency (nTE) and Δ. Deriving, from theory, the transpiration fraction of field crop water use (CWU), this contraction appears to be resolved. A dual-source energy balance algorithm is utilized to evaluate the derived transpiration fraction of CWU and to examine the relationship of nTE to Δ.