Water stress in an ABA-unresponsive hybrid poplar (Populus koveana x tvichocavpa cv Peace) - Response

We compared the xylem hydraulic properties of potted cuttings of two poplar clones (Populus koreana x trichocarpa cv. Peace and P.x euramericana cv. Robusta) known to differ in their sensitivity to ABA and in their ability to close their stomata during a period of drought. 'Robusta' can control stomatal aperture, whereas 'Peace' can only close the stomata of immature leaves. 'Peace' did not exhibit any specific hydraulic property that could compensate for its lack of response to a water deficit. The two clones did not differ in their petiole or internode xylem vulnerability to embolism, both being highly vulnerable (onset of embolism observed at -1.2 MPa minimum leaf water potential, Psi(wm)). Both clones became embolised rapidly when pots were dehydrated, but the onset of embolism was slightly delayed in 'Robusta'. 'Peace' was nevertheless capable of maintaining low xylem embolism near its apex and we associated this feature with the active stomatal control occurring in immature leaves. We conclude that efficient stomatal regulation is essential for maintaining the integrity of xylem sap flow under drought conditions and that 'Peace' shows an exception to the general rule of stomatal control of embolism.