Responses to water stress in an ABA ‐ unresponsive hybrid poplar (Populus koreana×trichocarpa cv. Peace) I. Stomatal function

Abstract

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.

Responses to water stress in an ABA‐unresponsive hybrid poplar (Populus koreana×trichocarpa cv. Peace) III. Consequences for photosynthetic carbon assimilation

“…This difference might be at least partly due to the fact that leaves of ' Peace' present a very small stomatal density on the adaxial surface, whereas both cultivars display similar densities on the abaxial side. In ' Peace ', conductance of the adaxial side represented c. 10% of the leaf conductance, whereas in 'Robusta' it was c. 40% (Ridolfi et al, 1996). Likewise, CO2 assimilation rates {A) were c. 12-7+ 0-93 //mol m"^ s'^ in leaves of ' Robusta' and only 9-9 + 0-9 in ' Peace' (P = 0-0002).…”

“…A lower leaf stomatal conductance to CO2 (g,,) was observed in ' Peace' during the experiment at moderate irradiance, but not during the second, under low irradiance. A lower conductance can be partly explained by the low stomatal density on the adaxial leaf side, which contributes to only 10 °o of ^^ in 'Peace' vs. 40% in 'Robusta' (Ridolfi et al, 1996). Reich (1984) showed that the difterences in leaf conductance between three hybrid poplars were related to their respective stomatal densities.…”

“…In the healthy leaf areas stomatal conductance nevertheless remained high (data not shown). Leaf shedding followed, and only the three to four youngest leaves that displayed complete stomatal closure survived (Ridolfi et al, 1996). …”

“…Peace exhibits a lack of stomatal sensitivity to exogenously applied abscisic acid (ABA) (P\irukawa, Park & Fujinuma, 1990;Ridolfi et al, 1996). Some ecotypes of black cottonwood {Popuhis trichocarpa) originating from the coastal area of the northwest USA (Hinckley & Braatne, 1994) display a similar lack of sensitivity to ABA.…”

Responses to water stress in an ABA‐unresponsive hybrid poplar (Populus koreana×trichocarpa cv. Peace) II. Hydraulic properties and xylem embolism

“…Peace) has stomata unresponsive to exogenous abscisic acid (Furukawa, Park & Fujinuma, 1990), a plant growth regulator supposed to be one of the major molecules triggering stomatal closure in response to soil water deficit. Ridolfi et al (1996), working on the same hybrid, complemented the first observation of Furukawa et al (1990) by demonstrating a leaf-age dependency of stomatal response to drought for this hybrid: the youngest apical leaves exhibit stomatal regulation but partly lose this capacity when they mature. Correlatively, droughted cuttings rapidly shed their mature leaves while the youngest ones survive the drought.…”