During the process of photosynthesis, CO2 diffuses into plant leaves through open stomata where it ultimately enters chloroplasts and is converted into organic sugars to support plant growth and development. While CO2 is diffusing inward, however, water vapor is exiting leaves through stomata during the process of transpiration. Thus, many plants regulate their degree of stomatal opening to optimize carbon uptake per unit of water loss. It is important, therefore, to determine if plant stomatal regulation will be affected by the increasing CO2 content of the air. In this summary, we review the responses of leaf stomatal densities to atmospheric CO2 enrichment.
For the most part, the recent literature indicates that plants reduce their leaf stomatal densities in response to atmospheric CO2 enrichment. Maroco et al. (1999), for example, grew corn at an atmospheric CO2 concentration of 1100 ppm for 30 days and reported that leaves of such plants contained 10% fewer stomates per unit leaf area than leaves of control plants grown at 350 ppm CO2. Likewise, after reviewing contemporary and historic data for 60 tree, shrub and herb species from temperate woodlands of the United Kingdom, Beerling and Kelly (1997) determined that the rising CO2 content of the air over the past 70 years caused significant reductions in stomatal density for all 60 species. In a study by Case et al. (1998), however, elevated CO2 had no effect on leaf stomatal densities of wild radish.
Nonetheless, many wild or native species can and do respond to elevated CO2 by reducing their leaf stomatal densities. In central Italy, for example, white oak trees have been growing naturally for several centuries near CO2-emitting springs, providing an ideal situation to determine the long-term effects of atmospheric CO2 enrichment on leaf stomatal density in an unmanaged species. In this case, the stomatal densities on white oak leaves were decreased by a factor of 1.5 over an atmospheric CO2 concentration gradient ranging from 350 to 750 ppm (Paoletti et al., 1998). In another related study, Gingko saplings grown at 560 ppm CO2 for three years displayed significant reductions in their leaf stomatal densities that approached values obtained from fossilized Gingko leaves dating back to the Triassic and Jurassic time periods, when the atmospheric CO2 concentration was somewhat higher than it is today (Beerling et al., 1998).
In conclusion, these observations suggest that elevated CO2 concentrations tend to reduce leaf stomatal densities in most plant species. As a consequence of these reductions, it is likely that less water will be lost from plant leaves during transpiration, thereby increasing plant water-use efficiency as the air's CO2 content continues to rise.
References
Beerling, D.J. and Kelly, C.K. 1997. Stomatal density responses of temperate woodland plants over the past seven decades of CO2 increase: A comparison of Salisbury (1927) with contemporary data. American Journal of Botany 84: 1572-1583.
Beerling, D.J., McElwain, J.C. and Osborne, C.P. 1998. Stomatal responses of the 'living fossil' Ginkgo biloba L. to changes in atmospheric CO2 concentrations. Journal of Experimental Botany 49: 1603-1607.
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.
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.
Paoletti, E., Nourrisson, G., Garrec, J.P. and Raschi, A. 1998. Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments. Plant, Cell and Environment 21: 1071-1075.