Leaf conductance, water relations, growth, and abscisic acid (ABA) concentrations in xylem sap, root apices and leaves were assessed in oak seedlings (Quercus robur L.) grown with a root system divided between two compartments and subjected to one of four treatments: (a) well watered, WW; (b) half of root system exposed to soil drying and half kept well watered, WD; (c) whole root system exposed to drought, DD; and (d) half of root system severed, RE. Sharp decreases in plant stomatal conductance, leaf water potential, hydraulic conductance and leaf growth were observed during DD treatment. No significant differences in plant leaf water potential and stomatal conductance were detected between the WW and WD treatments. Nevertheless, the WD treatment resulted in inhibition of leaf expansion and stimulation of root elongation only in the well-watered compartment. Abscisic acid concentrations did not change in leaves, root tips, or xylem sap of WD- compared to WW-treated plants. Increased concentrations of ABA were observed in xylem sap from DD-treated plant roots, but the total flux of ABA to shoots was reduced compared to that in WW-treated plants, because of decreases in transpiration flux. Similar plant responses to the WD and RE treatments indicate that the responses observed in the WD-treated plants were probably not triggered by a positive signal originating from drying roots.

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SUMMARY 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 leafage. 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.