Plant Water‐Use Efficiency

Abstract The loss of water vapour from aerial tissues of terrestrial plants as they assimilate CO 2 from the atmosphere for photosynthesis is biophysically unavoidable. Photosynthesis and transpiration co‐vary with the aperture and density of stomatal pores, and are co‐determined by the leaf area, which is involved in light capture and water loss. Yet, the strength of this coupling varies depending on the genotype and the environmental conditions. This has prompted plant scientists to define water‐use efficiency (WUE) as the ratio of carbon gains to water losses. WUE is genetically variable and displays high plasticity to environmental cues including CO 2 , drought and vapour pressure deficit (VPD). Breeding crops with high WUE is a key challenge in the face of climate change, but this must be achieved without penalties on growth or yield. In this view, strategies exploiting the physiological and genetic bases of the spatial and/or temporal variability between carbon gain and water loss offer promising avenues. Key Concepts Plant water‐use efficiency (WUE) reflects the balance between carbon gains and water losses. WUE can be expressed at different scales, from seconds to months and from the stomata to the whole plant and the whole crop. Phenotyping methods for WUE are an active field of development, from gas‐exchange devices to measure photosynthesis and stomatal conductance, to phenotyping platforms and imaging methods to access biomass gain and water loss over several weeks or months. A wide genetic variability has been reported for WUE, both between and within species. WUE varies in response to environmental factors because these differentially affect water loss and carbon gain. The effects of atmospheric CO 2 concentration, VPD, light and ozone have been well described. WUE responses to drought and temperature are complex because many underlying processes are affected. To maintain production in the face of climate change, improved WUE associated with growth maintenance is a key breeding target. In this view, physiological and genetic leeway rely on exploiting the spatial and/or temporal decoupling between carbon gain and water loss.