Coming of leaf age: control of growth by hydraulics and metabolics during leaf ontogeny

Summary Leaf growth is the central process facilitating energy capture and plant performance. This is also one of the most sensitive processes to a wide range of abiotic stresses. Because hydraulics and metabolics are two major determinants of expansive growth (volumetric increase) and structural growth (dry matter increase), we review the interaction nodes between water and carbon. We detail the crosstalks between water and carbon transports, including the dual role of stomata and aquaporins in regulating water and carbon fluxes, the coupling between phloem and xylem, the interactions between leaf water relations and photosynthetic capacity, the links between Lockhart's hydromechanical model and carbon metabolism, and the central regulatory role of abscisic acid. Then, we argue that during leaf ontogeny, these interactions change dramatically because of uncoupled modifications between several anatomical and physiological features of the leaf. We conclude that the control of leaf growth switches from a metabolic to a hydromechanical limitation during the course of leaf ontogeny. Finally, we illustrate how taking leaf ontogeny into account provides insights into the mechanisms underlying leaf growth responses to abiotic stresses that affect water and carbon relations, such as elevated CO 2 , low light, high temperature and drought. Contents Summary 349 I. Leaf growth: volume, structures, water and carbon 349 II. Coupling water and carbon limitations through the Lockhart model? 350 III. ABA signalling pathway as a hub to coordinate water and carbon relations 353 IV. Leaf venation: just a two‐way pipe network? 354 V. Leaf ontogeny orchestrates the actors involved in the control of leaf growth 355 VI. The growing leaf in a changing world 360 VII. Conclusion 361 Acknowledgements 362 References 362