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

Abstract

The impact of leaf vein blockage on leaf hydraulic conductance (KL), gas exchange (gL) and water potential (ΨL) 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 KL was recorded from cut a to cut c with respect to that measured for the controls, i.e. a KL loss of about 37% (cut a), 57% (cut b) and 87% (cut c). A positive linear relationship appeared to exist between gL and KL with a high correlation coefficient (r2=0.99) and a high statistical significance (P <0.01). Even under a severe drop in KL (as that induced by cut c), leaf water potential remained approximately constant and not statistically different from ΨL measured for the controls. In fact, ΨL ranged between -0.83 and -0.98 MPa, i.e. within the cavitation threshold of leaves in terms of the critical ΨL 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 KL and that stomatal closure led to the homeostatic maintenance of ΨL and cavitation avoidance.