Hydraulic architecture of trees:: main concepts and results

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.