Evolution of stomatal function: New perspectives and application to the fossil record
by McElwain J. C. (2012)
JENNIFER C. McELWAIN,
School of Biology and Environmental Science, University College Dublin, Belfield, Dublin, Ireland
Presentation at New Phytologist Symposium Nr. 29 on Stomata 2012 –
Vascular plants have evolved a suite of different strategies to optimize carbon uptake against water loss via developmental, physiological and cellular level control of stomatal function.
Alterations to the development of stomatal number and size in response to atmospheric CO2 concentration enable some species to regulate stomatal conductance on time-scales of weeks to decades.
Other species demonstrate physiological control of stomatal aperture allowing for rapid control of stomatal function in response to abiotic factors on time scales of seconds to minutes.
Competing hypotheses have been proposed for the evolution of stomatal function in land plants with suggestions on one side that stomatal function is highly conserved across the plant phylogeny and the alternative view that stomatal function has increased incrementally across vascular plants from monilophytes to spermatophytes.
Observations of stomatal response to both instantaneous and long-term exposure to elevated atmospheric CO2 in UCD PÉAC (Programme for Experimental Atmospheres and Climate) do not completely support either competing hypothesis. Rather they suggest that the evolution of stomatal function is more complex and does not display a strong phylogenetic signature.
We show that loss of one aspect of stomatal function in response to an external stimulus can be readily compensated by improved functionality in another.
Experiments also show that there may be a trade-off between physiological control of stomatal conductance via stomatal aperture opening/closing and morphological control of conductance through developmental alteration to stomatal density and pore size.
This “stomatal trade- off hypothesis” is supported by the observation of a significant negative correlation between the magnitude of response of conductance to instantaneous change in CO2 and the magnitude and sign of response of stomatal density to long term elevated CO2 exposure. In other words, species which show rapid instantaneous changes in stomatal conductance do not alter stomatal density inversely under elevated CO2 and vice versa.
This has obvious and important implications for future development and application of the stomatal-CO2 proxy method which uses the inverse relationship between SD and CO2 to reconstruct palaeo-CO2 concentration in the geological past.
This will be discussed in relation to new estimates of palaeo-CO2 spanning the Eocene-Oligocene boundary between 40 and 25 million years ago when the Earth transitioned between an ice-free (greenhouse) and glaciated (icehouse) state.