Single-phylogram scenario illustrating key land plant lineages (bold text) and moss genera and the appearance of stomata in modern plants. Dashed lines indicate absence of stomata and solid black lines their presence. This phylogram with liverworts basal (Qiu et al., 2006, 2007; Liu et al., 2014) indicates a single origin of stomata and multiple losses, whereas an alternative topology with hornworts basal implies multiple origins (Haig, 2013; Wickett et al., 2014).
Stomatal density and aperture in non-vascular land plants are non-responsive to above-ambient atmospheric CO2 concentrations
by Field K.J., Duckett J.G., Cameron D.D., Pressel S. (2015)
in Annals of Botany, 115 (6). 915 – 922. – http://dx.doi.org/10.1093/aob/mcv021
http://eprints.whiterose.ac.uk/86084/ – http://aob.oxfordjournals.org/content/115/6/915
Light (D, F, G, I–Q) and cryoscanning electron (A–C, H, R) micrographs of moss and hornwort stomata. (A, B) Physcomitrella patens: 12–14 stomata slightly irregularly spaced (e.g. the paired stomata in B) and randomly orientated around the capsule base; pores are round and subsidiary cells absent. (C, D) In the closely related F. hygrometrica the numerous stomata are axially orientated and regularly spaced. Also note the radial arrangement of the epidermal cells around the long-pored stomata (D); compare with hornworts (R). (E–G) Mnium hornum stomata sunk in deep pits. Note the liquid-filled subtending intercellular spaces (*) in (E). Stomata are often irregularly spaced [see the paired stomata in (F)] and have small round pores (F, G). (H–Q) Polytrichum juniperinum (H–K, grown at 440 p.p.m. [CO2]; L–Q, grown at 1500 p.p.m. [CO2]). Note the predominately axially arranged long-pored stomata frequently occurring in multiple groups (H–K). Abnormalities occur on almost all sporophytes and these increase under elevated CO2, as does the size of some of the apertures (L–Q). (J) A pair of stomata with a shared pore. (M–P) Stomata with abnormal pores. (O) Stoma with massive aperture. (P) Stoma with four guard cells. (R) Sporophyte of the hornwort A. punctatus. Note the regularly spaced axial stomata lacking subsidiary cells. Scale bars: (C, H, R) = 200 µm; (A) = 100 µm; (D–G, I–Q) = 50 µm; (B) = 20 µm.
BACKGROUND AND AIMS:
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.
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. KEY 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].
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.