title: Differences in the response sensitivity of stomatal index to atmospheric CO2 among four genera of Cupressaceae conifers.
abstract: OBJECTIVE
  The inverse relationship between stomatal density (SD: number of stomata per mm(2) leaf area) and atmospheric concentration of CO2 ([CO2]) permits the use of plants as proxies of palaeo-atmospheric CO2. Many stomatal reconstructions of palaeo-[CO2] are based upon multiple fossil species. However, it is unclear how plants respond to [CO2] across genus, family or ecotype in terms of SD or stomatal index (SI: ratio of stomata to epidermal cells). This study analysed the stomatal numbers of conifers from the ancient family Cupressaceae, in order to examine the nature of the SI-[CO2] relationship, and potential implications for stomatal reconstructions of palaeo-[CO2]. Methods Stomatal frequency measurements were taken from historical herbarium specimens of Athrotaxis cupressoides, Tetraclinis articulata and four Callitris species, and live A. cupressoides grown under CO2-enrichment (370, 470, 570 and 670 p.p.m. CO2).
  
  
  RESULTS
  T. articulata, C. columnaris and C. rhomboidea displayed significant reductions in SI with rising [CO2]; by contrast, A. cupressoides, C. preissii and C. oblonga show no response in SI. However, A. cupressoides does reduce SI to increases in [CO2] above current ambient (approx. 380 p.p.m. CO2). This dataset suggests that a shared consistent SI-[CO2] relationship is not apparent across the genus Callitris. Conclusions The present findings suggest that it is not possible to generalize how conifer species respond to fluctuations in [CO2] based upon taxonomic relatedness or habitat. This apparent lack of a consistent response, in conjunction with high variability in SI, indicates that reconstructions of absolute palaeo-[CO2] based at the genus level, or upon multiple species for discrete intervals of time are not as reliable as those based on a single or multiple temporally overlapping species.
authors.affiliation: School of Biology & Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland. matthew.haworth@ucd.ie
authors.forename: Matthew
authors.initials: M
authors.lastname: Haworth
authors.forename: James
authors.initials: J
authors.lastname: Heath
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: McElwain
mesh_terms: D001272:Atmosphere; D002245:Carbon Dioxide / Q000378:metabolism*; D013045:Species Specificity; D064028:Tracheophyta / Q000145:classification / Q000378:metabolism*
chemical_list: D002245:Carbon Dioxide