Xylem cavitation in the leaf of Prunus laurocerasus and its impact on leafhydraulics

This paper reports how water stress correlates with changes in hydraulic co nductivity of stems, leaf midrib, and whole leaves of Prunus laurocerasus. Water stress caused cavitation-induced dysfunction in vessels of P. lauroce rasus. Cavitation was detected acoustically by counts of ultrasonic acousti c emissions and by the loss of hydraulic conductivity measured by a vacuum chamber method. Stems and midribs were approximately equally vulnerable to cavitations. Although midribs suffered a 70% loss of hydraulic conductance at leaf water potentials of -1.5 MPa, there was less than a 10% loss of hyd raulic conductance in whole leaves. Cutting and sealing the midrib 20 mm fr om the leaf base caused only a 30% loss of conduction of the whole leaf. A high-pressure flow meter was used to measure conductance of whole leaves an d as the leaf was progressively cut back from tip to base. These data were fitted to a model of hydraulic conductance of leaves that explained the abo ve results, i.e. redundancy in hydraulic pathways whereby water can flow ar ound embolized regions in the leaf, makes whole leaves relatively insensiti ve to significant changes in conductance of the midrib. The onset of cavita tion events in P. laurocerasus leaves correlated with the onset of stomatal closure as found recently in studies of other species in our laboratory.