Transpiration response to vapour pressure deficit, transpiration efficiency (TE), and their putative link: retrospectives and prospectives on new opportunities to increase TE.

Transpiration efficiency (TE) is the ratio of biomass produced to water transpired, and has been a research target for improving crop water-use efficiency and yield. One way to increase TE is the capacity of certain genotypes to restrict transpiration and then reduce water loss under high vapour pressure deficit (VPD). A simulation study predicted that this trait would enhance yield and TE, particularly under water-limited conditions. This seminal study has triggered extensive experimental work across species to identify genotypic variation in this trait. However, only a few of these studies have assessed the link between transpiration restriction (TR) and TE, and have reported a positive association in only about half of the cases. This review explains the benefits of the trait, highlighting the importance of plant spacing when interpreting the TE vs TR relationship. Lower leaves in a canopy may experience lower VPD and differential light exposure, thereby modifying the drivers of transpiration and the mean VPD to which photosynthesising leaves are exposed. Because crops are exposed to changing light conditions, we propose using the Penman-Monteith reference evapotranspiration (ETref) as a composite measure of evaporative demand, accounting for solar radiation rather than VPD alone. We argue for considering canopy architecture and its effect on light penetration to interpret the TE vs TR relationship, which opens new opportunities to enhance TE in crops. We suggested revisiting the use of a 'big-leaf' model to represent a crop canopy, in favour of a bi- or multi-layer representation.

© 2026 The Author(s) (or their employer(s)). Published by CSIRO Publishing.