DEVELOPMENTAL CONTROL OF XYLEM HYDRAULIC RESISTANCES AND VULNERABILITY TO EMBOLISM IN FRAXINUS-EXCELSIOR L - IMPACTS ON WATER RELATIONS

The hydraulic properties and leaf gas exchanges of Fraxinus excelsior L. branches differing by their age and their vertical crown position, but in comparable ambient air conditions (vapour pressure deficit and global radiation) were compared. The variations in leaflet water poten tial psi(leaflet), leaflet stomatal conductance and transpiration rate , E, were small between different branches of the same crown. Whole br anch hydraulic resistances (r(branch)), and partitioning between leaf (r(leaf)) and xylem resistance (r(xylem)) were assessed with a high pr essure flowmeter, r(leaf) represented 90% and 10% of r(branch) for upp er and lower crown branches, respectively. The changes resulted from i ncreases in r(xylem) caused by the formation of short shoot internodes mostly located in secondary axes. However, leaf area-specific branch resistances (r(branch)=r(branch) x LA) were nearly constant throughou t the crown. This was consistent with the vertical variations in psi(l eaflet) because r(branch) x E represents the water potential drop fro m the trunk to the leaves. Because r(xylem) was higher, lower psi(xyle m) values were predicted in lower crown rachises However, rachises fro m lower crown branches were less vulnerable to embolism than in upper branches (psi(xylem) at onset of embolism, psi(cav), were -3 and -2 MP a, respectively). It was concluded that r(xylem) increased with branch age, but r(branch) remained constant because LA decreased. As a cons equence, E was maximized and psi(xylem) remained above psi(cav). This suggested that, in Fraxinus, leaf gas exchanges and leaf areas were co upled with xylem hydraulic capacities probably through a control of bu d activity.