The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants' WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization.

Chair of Forest Botany Institute of Forest Botany and Forest Zoology Technical University of Dresden Pienner Str 7 01737 Tharandt Germany

Département des Sciences de l'environnement Université du Québec à Trois Rivières Trois Rivières QC G8Z 4M3 Canada

Department of Phytology Faculty of Forestry Technical University in Zvolen T G Masaryka 24 960 01 Zvolen Slovakia

Institute of Botany Czech Academy of Sciences Lidická 971 602 00 Brno Czech Republic

Karlsruhe Institute of Technology Kreuzeckbahnstraße 19 82467 Garmisch Partenkirchen Germany

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