title: Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO2.
abstract: 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 5, measurements were also taken of leaf stomatal conductance (gs, micromol m(-2) s(-1)), photosynthetic CO2 fixation (A, mmol 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 numbers or index. 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 under elevated [CO2]. 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.
authors.affiliation: School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton, SO16 7PX, UK.
authors.forename: Penny J
authors.initials: PJ
authors.lastname: Tricker
authors.forename: Harriet
authors.initials: H
authors.lastname: Trewin
authors.forename: Olevi
authors.initials: O
authors.lastname: Kull
authors.forename: Graham J J
authors.initials: GJ
authors.lastname: Clarkson
authors.forename: Eve
authors.initials: E
authors.lastname: Eensalu
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Tallis
authors.forename: Alessio
authors.initials: A
authors.lastname: Colella
authors.forename: C Patrick
authors.initials: CP
authors.lastname: Doncaster
authors.forename: Maurizio
authors.initials: M
authors.lastname: Sabatti
authors.forename: Gail
authors.initials: G
authors.lastname: Taylor
mesh_terms: D000704:Analysis of Variance; D002245:Carbon Dioxide / Q000378:metabolism* / Q000494:pharmacology; D007558:Italy; D010788:Photosynthesis / Q000187:drug effects / Q000502:physiology; D018515:Plant Leaves / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D018526:Plant Transpiration / Q000502:physiology*; D032107:Populus / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide