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We compared the xylem hydraulic properties of potted cuttings of two poplar clones (Populus koreana × trichocarpa Cv. Peace and P. × 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, ψwm). 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 stomata! 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. [ABSTRACT FROM AUTHOR]

The poplar cultivar 'Peace' (Populus trichocarpa × koreana) displays abnormal stomatal behaviour with a lack of sensitivity to exogenous abscisic acid (ABA) in mature leaves. We report details of the leaf-age dependency of this feature, and present the responses of stomata to diverse closing stimuli in addition to ABA. Soil water depletion induced complete stomatal closure of the youngest leaves, whereas the oldest exhibited almost no closure. By contrast, no such age effect was observed in stomatal sensitivity to ABA: a complete lack of closure was observed on all leaves, even with 10-3 M ABA. Moreover, a preconditioning by drought did not restore the ability of stomata to close in response to ABA. Supplying detached leaves with Ca2+ (5 × 10-2 M) was effective in inducing stomatal closure in the youngest leaves, and reproduced the age-dependent drought response. These results support the hypothesis that drought control of stomatal conductance in 'Peace' is ABA-independent, and could involve calcium ions. A loss of stomatal sensitivity to calcium, with leaf maturation, could explain the dependency of stomatal closure on leaf age. Xylem sap concentrations of calcium were lower than those required to close stomata, and no drought-induced increase was recorded. The mature leaves were also insensitive to increased ambient CO2, but darkness promoted partial stomatal closure. [ABSTRACT FROM AUTHOR]