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Stomatal Density (Response to CO2 - Herbaceous Plants) -- Summary
As the air's CO2 content rises, many plants reduce their stomatal apertures, because with more CO2 in the air, they don't need to open their stomates as wide as they do at lower atmospheric CO2 concentrations to allow for sufficient inward diffusion of CO2 for use in photosynthesis.  As a result, plants growing in CO2-enriched air typically exhibit reduced rates of transpirational water loss, smaller yield losses attributable to the indiscriminate uptake of aerial pollutants, and increased water-use efficiency.  For much the same reason (and producing similar effects), many plants also reduce the density of stomates on their leaves at higher atmospheric CO2 concentrations.  In this brief summary, we review the findings of studies that have addressed this subject in non-woody herbaceous plants.

In a study that appears at first glance to challenge the norm, Case et al. (1998) collected twelve wild radish genotypes with a variety of stomatal indices and guard cell lengths in Maine, USA, and grew them to maturity in greenhouses maintained at atmospheric CO2 concentrations of 370 and 680 ppm, finding that the elevated CO2 did not significantly affect stomatal index or guard cell length.  In fact, across all genotypes investigated, leaf surface characteristics were essentially unchanged by elevated CO2.  In the case of C4 maize plants grown for 30 days in Plexiglass chambers maintained at ambient or triple-ambient concentrations of atmospheric CO2, on the other hand, Maroco et al. (1999) determined that the leaves of the CO2-enriched plants displayed approximately 10% fewer stomates per unit leaf area than the leaves of the control plants growing in ambient air.

Why one of these plants exhibited no change in stomatal density as the air's CO2 content was increased above the current ambient value while the other exhibited a decrease may perhaps be explained by the results of the study of Gray et al. (2000), who identified a gene of the small mustard plant Arabidopsis thaliana that prevents decreases in the number density of leaf stomata in response to atmospheric CO2 enrichment above a critical value of atmospheric CO2 concentration.  It can be appreciated, for example, that decreases in stomatal density and conductance in response to rising atmospheric CO2 concentrations (which are typically beneficial) cannot go on indefinitely, for there would ultimately come a point (likely different for different species) where further decreases in these plant properties become counterproductive, leading to a situation where (1) the enhanced air-to-leaf CO2 concentration gradient could not overcome the increased resistance of CO2 entry into the leaf, causing the plant to die of carbon starvation, and (2) transpiration is reduced to such a degree that leaf evaporative cooling cannot prevent the occurrence of plant death due to increased thermal stress, as we note in our editorial of 27 Dec 2000.

Viewed in this light, the difference between the stomatal density responses of wild radish and maize likely derives from genetically-programmed species-specific differences in the critical value of atmospheric CO2 concentration at which the oft-observed decline in stomatal density with increasing atmospheric CO2 concentration is genetically terminated. Commenting on this situation, Serna and Fenoll (2000) say that "plants seem to be well armed to cope with a further enrichment in atmospheric CO2," and that genes such as the one discovered by Gray et al. - denoted HIC for high carbon dioxide - "should ensure that, at high CO2 concentrations, changes in stomatal indices are kept to a minimum."

Case, A.L., Curtis, P.S. and Snow, A.A.  1998.  Heritable variation in stomatal responses to elevated CO2 in wild radish, Raphanus raphanistrum (Brassicaceae).  American Journal of Botany 85: 253-258.

Gray, J.E., Holroyd, G.H., van der Lee, F.M., Bahraml, A.R., Sijmons, P.C., Woodward, F.I., Schuch, W. and Hetherington, A.M.  2000.  The HIC signaling pathway links CO2 perception to stomatal development.  Nature 408: 713-716.

Maroco, J.P., Edwards, G.E. and Ku, M.S.B.  1999.  Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide.  Planta 210: 115-125.

Serna, L. and Fenoll, C.  2000.  Coping with human CO2 emissions.  Nature 408: 656-657.

Last updated 13 April 2005