Developmental control of xylem hydraulic resistances and vulnerability to embolism in Fraxinus excelsior L.: impacts on water relations

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Abstract
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 potential ψleaflet, leaflet stomatal conductance and transpiration rate, E, were small between different branches of the same crown. Whole branch hydraulic resistances (rbranch), and partitioning between leaf (rleaf) and xylem resistance (rxylem) were assessed with a high pressure flowmeter. rleaf represented 90% and 10% of rbranch for upper and lower crown branches, respectively. The changes resulted from increases in rxylem caused by the formation of short shoot internodes mostly located in secondary axes. However, leaf area-specific branch resistances (r*branch = rbranch × LA) were nearly constant throughout the crown. This was consistent with the vertical variations in ψleaflet because r*branch × E represents the water potential drop from the trunk to the leaves. Because rxylem was higher, lower ψxylem values were predicted in lower crown rachises. However, rachises from lower crown branches were less vulnerable to embolism than in upper branches (ψxylem at onset of embolism, ψcav were -3 and -2 MPa, respectively). It was concluded that rxylem increased with branch age, but r*branch remained constant because LA decreased. As a consequence, E was maximized and ψxylem remained above ψcav. This suggested that, in Fraxinus, leaf gas exchanges and leaf areas were coupled with xylem hydraulic capacities probably through a control of bud activity.

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