Effects of the experimental blockage of the major veins on hydraulics and gas exchange of Prunus laurocerasus L. leaves

The impact of leaf vein blockage on leaf hydraulic conductance (K-L), gas exchange (g(L)) and water potential (Psi(L)) was studied in Prunus laurocerasus L., a broad-leaved evergreen. For this purpose, leaves were measured for the three variables above, either with an intact leaf blade (controls) or with the midrib cut a third of the way up (cut a), or with the midrib cut at three different points and the first-order veins cut through near their insertion to the midrib (cut b), or with the midrib cut at 2 mm from the leaf base (cut c). All the cut surfaces were sealed with cyanoacrylate. A serial decrease of K-L was recorded from cut a to cut c with respect to that measured for the controls, i.e. a K-L loss of about 37% (cut a), 57% (cut b) and 87% (cut c). A positive linear relationship appeared to exist between g(L) and K-L with a high correlation coefficient (r(2)=0.99) and a high statistical significance (P <0.01). Even under a severe drop in K-L (as that induced by cut c), leaf water potential remained approximately constant and not statistically different from Ψ(L) measured for the controls. In fact, Ψ(L) ranged between -0.83 and -0.98 MPa, i.e. within the cavitation threshold of leaves in terms of the critical Ψ(L) inducing a significant production of ultrasound acoustic emissions which was -0.94 +/- 0.09 MPa. The conclusion was that stomata were very sensitive to changes in K-L and that stomatal closure led to the homeostatic maintenance of Ψ(L) and cavitation avoidance.