title: Stomatal density and aperture in non-vascular land plants are non-responsive to above-ambient atmospheric CO2 concentrations.
abstract: OBJECTIVE
  Following the consensus view for unitary origin and conserved function of stomata across over 400 million years of land plant evolution, stomatal abundance has been widely used to reconstruct palaeo-atmospheric environments. However, the responsiveness of stomata in mosses and hornworts, the most basal stomate lineages of extant land plants, has received relatively little attention. This study aimed to redress this imbalance and provide the first direct evidence of bryophyte stomatal responsiveness to atmospheric CO2.
  
  
  METHODS
  A selection of hornwort (Anthoceros punctatus, Phaeoceros laevis) and moss (Polytrichum juniperinum, Mnium hornum, Funaria hygrometrica) sporophytes with contrasting stomatal morphologies were grown under different atmospheric CO2 concentrations ([CO2]) representing both modern (440 p.p.m. CO2) and ancient (1500 p.p.m. CO2) atmospheres. Upon sporophyte maturation, stomata from each bryophyte species were imaged, measured and quantified.
  
  
  RESULTS
  Densities and dimensions were unaffected by changes in [CO2], other than a slight increase in stomatal density in Funaria and abnormalities in Polytrichum stomata under elevated [CO2].
  
  
  CONCLUSIONS
  The changes to stomata in Funaria and Polytrichum are attributed to differential growth of the sporophytes rather than stomata-specific responses. The absence of responses to changes in [CO2] in bryophytes is in line with findings previously reported in other early lineages of vascular plants. These findings strengthen the hypothesis of an incremental acquisition of stomatal regulatory processes through land plant evolution and urge considerable caution in using stomatal densities as proxies for paleo-atmospheric CO2 concentrations.
authors: Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK and Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK k.field@sheffield.ac.uk.
authors: Katie J
authors: KJ
authors: Field
authors: Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK and Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors: Jeffrey G
authors: JG
authors: Duckett
authors: Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK and Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors: Duncan D
authors: DD
authors: Cameron
authors: Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK and Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK.
authors: Silvia
authors: S
authors: Pressel
mesh_terms: D001272:Atmosphere / Q000737:chemistry*; D044002:Bryophyta / Q000502:physiology* / Q000528:radiation effects / Q000648:ultrastructure; D002245:Carbon Dioxide / Q000494:pharmacology*; D010802:Phylogeny; D054046:Plant Stomata / Q000166:cytology / Q000187:drug effects* / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D002245:Carbon Dioxide
keywords: Anthoceros punctatus; Atmospheric CO2; Funaria hygrometrica; Mnium hornum; Phaeoceros laevis; Polytrichum juniperinum; bryophytes; carbon dioxide; evolution; hornworts; mosses; palaeo-atmospheric environment; stomatal density