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-Since about twenty years, hydraulic architecture (h.a.) is, doubtless, the major trend in the domain of plants (and especially trees) water relations. This review encompasses the main concepts and results concerning the hydraulic of architecture of trees. After a short paragraph about the definition of the h.a., the qualitative and quantitative characteristics of the h.a. are presented. This is an occasion to discuss the pipe model from the h.a. point of view. The second part starts with the central concept of embolism and give a review of important experimental results and questions concerning summer and winter embolism. The last part deals with the coupling between hydraulic and stomatal conductances. It discusses the theoretical and experimental relationships between transpiration and leaf water potential during a progressive soil drought, the increase of soil-root resistance and its consequences in term of xylem vulnerability, the factors controlling the daily maximum transpiration and how stomates can prevent "run away embolism". In conclusion different kinds of unsolved questions of h.a., which can be a matter of future investigations, are presented in addition with a classification of trees behaviour under drought conditions. To end, an appendix recalls the notions of water potential, pressure and tension.hydraulic architecture / cohesion-tension theory / summer embolism / winter embolism / drought resistance Résumé -Architecture hydraulique des arbres : concepts principaux et résultats. Sans aucun doute, depuis une vingtaine d'années, l'architecture hydraulique (a.h.) est devenue une approche majeure dans le domaine des relations plantes-eau (et particulièrement pour les arbres). Cette revue présente les principaux concepts et résultats concernant l'a.h. Après un bref paragraphe sur la définition de l'a.h., les caractéristiques qualitatives et quantitatives définissant l'a.h. sont passées en revue. À cette occasion le « pipe model » est discuté du point de vue de l'a.h. La seconde partie commence avec le concept central d'embolie et continue avec une présentation des principaux résultats et questions touchant l'embolie estivale et l'embolie hivernale. La dernière partie analyse le « couplage » entre les conductances hydraulique et stomatique. Il y est discuté des relations théoriques et expérimentales entre la transpiration et le potentiel hydrique foliaire durant la mise en place d'une sécheresse progressive du sol, de l'augmentation de la résistance sol-racines et de ses conséquences en terme de vulnérabilité du xylème, des facteurs contrôlant la transpiration maximale journalière et de quelle manière les stomates peuvent prévenir l'emballement de l'embolie. La conclusion fait état de différentes questions non résolues, qui pourraient faire l'objet de recherches futures et esquisse une classification du comportement des arbres vis-à-vis de la sécheresse. Pour finir, un appendice rappelle les notions de potentiel hydrique, de pression et de tension.architecture hydraulique / théorie de la cohé...

Trees of Juglans regia L. shed leaves when subjected to drought. Before shedding (when leaves are yellow), the petioles have lost 87% of their maximum hydraulic conductivity, but stems have lost only 14% of their conductivity. This is caused by the higher vulnerability of petioles than stems to water‐stress induced cavitation. These data are discussed in the context of the plant segmentation hypothesis.

The onset of the growing season of trees has been earlier by 2.3 days per decade during the last 40 years in temperate Europe because of global warming. The effect of temperature on plant phenology is, however, not linear because temperature has a dual effect on bud development. On one hand, low temperatures are necessary to break bud endodormancy, and, on the other hand, higher temperatures are necessary to promote bud cell growth afterward. Different process-based models have been developed in the last decades to predict the date of budbreak of woody species. They predict that global warming should delay or compromise endodormancy break at the species equatorward range limits leading to a delay or even impossibility to flower or set new leaves. These models are classically parameterized with flowering or budbreak dates only, with no information on the endodormancy break date because this information is very scarce. Here, we evaluated the efficiency of a set of phenological models to accurately predict the endodormancy break dates of three fruit trees. Our results show that models calibrated solely with budbreak dates usually do not accurately predict the endodormancy break date. Providing endodormancy break date for the model parameterization results in much more accurate prediction of this latter, with, however, a higher error than that on budbreak dates. Most importantly, we show that models not calibrated with endodormancy break dates can generate large discrepancies in forecasted budbreak dates when using climate scenarios as compared to models calibrated with endodormancy break dates. This discrepancy increases with mean annual temperature and is therefore the strongest after 2050 in the southernmost regions. Our results claim for the urgent need of massive measurements of endodormancy break dates in forest and fruit trees to yield more robust projections of phenological changes in a near future.