This material is published by Oecologia (2005, vol 143, pp 652-660).

Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO2.

Penny J. Tricker, Harriet Trewin, Olevi Kull, Graham J.J. Clarkson, Eve Eensalu, Matthew J. Tallis, Alessio Colella, C. Patrick Doncaster, Maurizio Sabatti and Gail Taylor

Using a free-air CO2 enrichment (FACE) experiment, poplar trees (Populus x euramericana clone I214) were exposed to either ambient or elevated [CO2] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm-2) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year five, measurements were also taken of leaf stomatal conductance (gs, µmol m-2 s-1), photosynthetic CO2 fixation (A, µmol m-2 s-1), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO2 (Ci:Ca). Elevated [CO2] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO2] had no detectable effects on stomatal frequency. In contrast, even after 5 years of exposure to elevated [CO2], gs was reduced, A/E was stimulated, and Ci:Ca was reduced relative to ambient [CO2]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use (A/E) under elevated [CO2]. In contrast, A/E was stimulated by the persisting reductions in stomatal conductance. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO2], although the mechanism for this remains obscure.

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