Transpiration efficiency (TE), defined as the ratio of biomass produced to water transpired, has long been a research target for improving crop water use efficiency and eventually yield. One avenue that has been proposed to increase TE is to harness the capacity of certain genotypes across species to restrict transpiration under high vapour pressure deficit (VPD), i.e. at the time of the day when the water cost of photosynthesis is the highest. A simulation study showed indeed that this trait enhanced yield and TE, particularly under water-limited conditions. This seminal study has triggered extensive experimental work across species to look for genotypic variation for this trait. However, experimental results testing the link between transpiration restriction and TE are scant and show an inconsistent link. This presentation will briefly take stocks of this past work and of the benefits of the trait. The presentation will then surveys existing reports on the TE vs transpiration restriction relationship and propose possible interpretations for the absence of a significant relationship between transpiration restriction and TE in about half of the cases. These interpretations are supported by recent evidence showing the importance of plant spacing when assessing transpiration restriction or TE, for this relationship. Lower leaves in a plant canopy experience lower VPD and differential light exposure, compared to individual plants in which all/most leaves are exposed to air VPD and incident light. Here we argue for the use of the Penman-Monteith reference evapotranspiration (ETref) as a composite measure of the evaporative demand instead of VPD only for a better representation of the environmental condition the plants can experienced, particularly in areas characterised by high solar radiation conditions. To support our approach, we will draw on recent evidence supporting a link between 3D architecture traits, transpiration restriction under high ETref, light penetratio