Stomata are one of the crucial adaptations in the evolution of the land flora and the development of the terrestrial landscape and atmosphere on Earth. These microscopic pores on the plant epidermis first appeared in the fossil record more than 400 million years ago, some 50–60 million years after the first land plants. Today they are found on the sporophyte generations of all land plant groups with exceptions only in the liverworts, the earliest moss lineages and a few derived hornwort clades. Exposure to high atmospheric CO2 concentration ([CO2]) has been shown to consistently result in a reduction of stomatal density (number of stomata per mm2) and index (ratio of stomata to epidermal cells) in the newly developed leaves of many vascular plant species. The general consensus view is that stomatal morphology is conserved throughout land plants and that their primary function is related to the regulation of gas and water exchange, [CO2] being a key activator of stomatal frequency. However, a crucial missing piece in the jigsaw of stomatal evolution and function is the responsiveness of bryophyte stomata to [CO2]. In the only previous studies on the effects of [CO2] on stomata in non-vascular plants recorded larger apertures in the mosses Physcomitrella and Funaria grown in the absence of CO2.
A recent paper in Annals of Botany challenges the widely accepted dogma that the responsiveness of stomata to [CO2] in terms of density and opening is conserved across the land plant phylogeny through careful experimentation and cytological observation and asks: (1) Are stomatal numbers on moss and hornwort sporophytes affected in the same way as those in angiosperms by elevated [CO2] representative of atmospheric concentrations in the Palaeozoic? (2) Do guard cell lengths and apertures in bryophytes change when subjected to representative Palaeozoic [CO2] throughout development?
Field, K. J., Duckett, J. G., Cameron, D. D., & Pressel, S. (2015) Stomatal density and aperture in non-vascular land plants are non-responsive to above-ambient atmospheric CO2 concentrations. Annals of Botany April 8, 2015 doi: 10.1093/aob/mcv021
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. 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.