Abstract
We investigated stomatal conductance (gs) and mesophyll conductance (gm) in response to atmospheric CO2 concentration [CO2] in two primitive land plants, the fern species Pteridium aquilinum and Thelypteris dentata, using the concurrent measurement of leaf gas exchange and carbon isotope discrimination. [CO2] was initially decreased from 400 to 200 μmol mol−1, and then increased from 200 to 700 μmol mol−1, and finally decreased from 700 to 400 μmol mol−1. Analysis by tunable diode laser absorption spectroscopy (TDLAS) revealed a rapid and continuous response in gm within a few minutes. In most cases, both ferns showed rapid and significant responses of gm to changes in [CO2]. The largest changes (quote % decrease) were obtained when [CO2] was decreased from 400 to 200 μmol mol−1. This is in contrast to angiosperms where an increase in gm is commonly observed at low [CO2]. Similarly, fern species observed little or no response of gs to changes in [CO2] whereas, a concomitant decline of gm and gs with [CO2] is often reported in angiosperms. Together, these results suggest that regulation of gm to [CO2] may differ between angiosperms and ferns.
Highlights
Atmospheric CO2 is a substrate for leaf photosynthesis in land plants, and CO2 availability at the carboxylation site is one of the most important limiting factors for leaf photosynthesis
We developed a custom-designed gas exchange system using a concurrent measurement of gas exchange and carbon isotope ratio using tunable diode laser absorption spectroscopy (TDLAS), to quantify the rapid, continuous responses in gm in fern species in response to changes in [CO2] with a time resolution of a few minutes (Tazoe et al 2009, 2011)
Photosynthesis rate (A) was light saturated at a photosynthetic photon flux density (PPFD) of 500 μmol m−2 s−1 for both P. aquilinum and T. dentata (Fig. 4a)
Summary
Atmospheric CO2 is a substrate for leaf photosynthesis in land plants, and CO2 availability at the carboxylation site is one of the most important limiting factors for leaf photosynthesis. In the process of leaf photosynthesis in C3 land plants, CO2 diffuses from the atmosphere through stomata, intercellular air spaces, and the leaf mesophyll to the site of carboxylation in the chloroplasts. CO2 concentration in the chloroplast is lower than that in the atmosphere because of significant resistance to CO2 diffusion through this diffusional pathway, i.e., limitations in CO2 diffusion strongly reduce leaf photosynthesis. Atmospheric CO2 levels fell abruptly during the Cretaceous period (Kuypers et al 1999), which coincides with a major diversification in the fern group (Pryer et al 2004).
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