As the air's CO2 content continues to rise, many plants will respond by reducing their stomatal apertures. This phenomenon commonly occurs at elevated CO2 concentrations because with more CO2 in the air, plants 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 consequence of this phenomenon, plants typically exhibit reductions in transpirational water loss, smaller yield losses attributable to the indiscriminate uptake of aerial pollutants, and increases in water-use efficiency. In this summary we review some of the recent scientific literature pertaining to this far-reaching effect of elevated CO2 on the stomatal conductances of agricultural crops.
With respect to stomatal conductance itself, in the FACE experiment of Garcia et al. (1998) a 190-ppm increase in the air's CO2 concentration reduced the average mid-day stomatal conductance in spring wheat by 28% over the entire growing season. Similarly, Hakala et al. (1999) reported that average stomatal conductances in spring wheat grown at twice-ambient levels of atmospheric CO2 were about 25% lower than those observed in ambiently-grown control plants, regardless of a concomitant exposure to an elevated air temperature treatment (3°C greater than ambient air temperature). Likewise, twice-ambient levels of CO2 generally decreased stomatal conductances in wheat, regardless of whether the elevated CO2 exposure was maintained on a 12- or 24-hour basis (Heagle et al., 1999). In addition, a 400-ppm increase in the air's CO2 concentration reduced stomatal conductances in hydroponically-grown peanuts by 44% (Stanciel et al., 2000), while a 750-ppm CO2 increase reduced the stomatal conductances of a C4 maize crop by 71% (Maroco et al., 1999).
With respect to the consequences of CO2-induced reductions in stomatal conductance, Smart et al. (1998) reported reduced rates of transpirational water loss for wheat grown at 1000 ppm CO2 for 23 days in controlled environment chambers. McKee et al. (2000) additionally found that a 310-ppm increase in the air's CO2 concentration, which reduced stomatal conductances in spring wheat by about 50%, also completely alleviated high-O3-induced reductions in leaf rubisco content and activity. In a somewhat similar study of soybeans, Heagle et al. (1998) observed that O3-induced foliar injuries decreased with increasing atmospheric CO2 concentration as a consequence of CO2-induced reductions in stomatal conductance. Likewise, Malmstrom and Field (1997) noted that twice-ambient levels of atmospheric CO2 caused greater reductions in stomatal conductances in oats infected with a pathogenic virus than in control plants that were unaffected (50% vs. 34%). Consequently, infected oats displayed the greatest CO2-induced percentage increases in both biomass production and water-use efficiency.
In summary, it appears that elevated CO2 reduces the stomatal conductances of nearly all agricultural plants under many different growing conditions, including unfavorable circumstances characterized by elevated air temperature, elevated O3 level, and the presence of pathogenic viruses. Thus, as the air's CO2 content continues to rise, agricultural crops should display ever-increasing reductions in transpirational water loss and yield losses that result from various diseases and aerial pollutants, while simultaneously exhibiting increases in water-use efficiency.
References
Garcia, R.L., Long, S.P., Wall, G.W., Osborne, C.P., Kimball, B.A., Nie, G.Y., Pinter Jr., P.J., LaMorte, R.L. and Wechsung, F. 1998. Photosynthesis and conductance of spring-wheat leaves: field response to continuous free-air atmospheric CO2 enrichment. Plant, Cell and Environment 21: 659-669.
Hakala, K. Helio, R., Tuhkanen, E. and Kaukoranta, T. 1999. Photosynthesis and Rubisco kinetics in spring wheat and meadow fescue under conditions of simulated climate change with elevated CO2 and increased temperatures. Agricultural and Food Science in Finland 8: 441-457.
Heagle, A.S., Booker, F.L., Miller, J.E., Pursley, W.A. and Stefanski, L.A. 1999. Influence of daily carbon dioxide exposure duration and root environment on soybean response to elevated carbon dioxide. Journal of Environmental Quality 28: 666-675.
Heagle, A.S., Miller, J.E. and Booker, F.L. 1998. Influence of ozone stress on soybean response to carbon dioxide enrichment: I. Foliar properties. Crop Science 38: 113-121.
Malmstrom, C.M. and Field, C.B. 1997. Virus-induced differences in the response of oat plants to elevated carbon dioxide. Plant, Cell and Environment 20: 178-188.
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.
McKee, I.F., Mulholland, B.J., Craigon, J., Black, C.R. and Long, S.P. 2000. Elevated concentrations of atmospheric CO2 protect against and compensate for O3 damage to photosynthetic tissues of field-grown wheat. New Phytologist 146: 427-435.
Smart, D.R., Ritchie, K., Bloom, A.J. and Bugbee, B.B. 1998. Nitrogen balance for wheat canopies (Triticum aestivum cv. Veery 10) grown under elevated and ambient CO2 concentrations. Plant, Cell and Environment 21: 753-763.
Stanciel, K., Mortley, D.G., Hileman, D.R., Loretan, P.A., Bonsi, C.K. and Hill, W.A. 2000. Growth, pod and seed yield, and gas exchange of hydroponically grown peanut in response to CO2 enrichment. HortScience 35: 49-52.