WATER-STRESS IN AN ABA-UNRESPONSIVE HYBRID POPLAR (POPULUS-KOVEANA X TVICHOCAVPA CV PEACE) - RESPONSE

The poplar cultivar 'Peace' (Populus trichocarpaxkoreana) displays abn ormal stomatal behaviour with a lack of sensitivity to exogenous absci sic acid (ABA) in mature leaves. We report details of the leaf-age dep endency of this feature, and present the responses of stomata to diver se closing stimuli in addition to ABA. Soil water depletion induced co mplete stomatal closure of the youngest leaves, whereas the oldest exh ibited almost no closure. By contrast, no such age effect was observed in stomatal sensitivity to ABA: a complete lack of closure was observ ed 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+ (5x10(-2) M) was effectiv e in inducing stomatal closure in the youngest leaves, and reproduced the age-dependent drought response. These results support the hypothes is that drought control of stomatal conductance in 'Peace' is ABA-inde pendent, and could involve calcium ions. A loss of stomatal sensitivit y to calcium, with leaf maturation, could explain the dependency of st omatal closure on leaf age. Xylem sap concentrations of calcium were l ower than those required to close stomata, and no drought-induced incr ease was recorded. The mature leaves were also insensitive to increase d ambient CO2, but darkness promoted partial stomatal closure.