Stomatal conductance and xylem-sap abscisic acid (ABA) in adult oak trees during a gradually imposed drought

Thirty- to 40-year-old oak trees (Quercus petraea Matt Liebl and Q robur L) growing in a forest stand near Champenoux (Nancy, France) were submitted to an imposed drought in a lysimeter during two successive summers. Xylem sap was extracted from leafy twigs of two trees per species at regular intervals during the onset of drought, and of two controls in parallel. Predawn leaf water potential, soil water potential at different depths and midday stomatal conductance of sun-exposed leaves were recorded at the same pace. Concentrations of abscisic acid (ABA) and of the glucose ester of ABA (ABA-GE) were measured with an ELISA technique, after purification of the sap samples using high performance liquid chromatography (HPLC). Mean concentrations oi ABA recorded in the absence of drought constraint were around 30 mu mol m(-3), independently from year and species, and 45 mu mol m(-3) for ABA-GE. No significant drought-related increase in either of these concentrations could be recorded while predawn leaf water potential dropped below -1.5 MPa, and stomatal conductance down to 10% oi the values of controls. A good correlation between stomatal conductance and predawn leaf water potential was detected, and even a better one between stomatal conductance and soil water potential at 25 cm. It may be concluded from these results that, even ii a close correlation exists between stomatal aperture and soil moisture, the existence of a root signalling process based solely on increased ABA or ABA-GE transport in the xylem sap may be questioned. Stomatal conductance under natural conditions is probably controlled by a rather complex chain of processes which has still to be elucidated.