With respect to stomatal conductance itself, in the Free-Air-CO2-Enrichment (FACE) study of Garcia et al. (1998) a 190-ppm increase in the atmosphere'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. And 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%). And as a result, infected oats displayed the greatest CO2-induced percentage increases in both biomass production and water-use efficiency.

Moving on to the 21st century, enough work in this area had already been conducted for Kimball et al. (2002) to produce a review article on the subject based on Free-Air-CO2-Enrichment studies conducted in agricultural fields, where a 300 ppm increase in the air's CO2 concentration was employed. And in their review, they said that, on average, wheat experienced a 51% decrease in stomatal conductance at ample water and nitrogen, plus a 66% decrease at low nitrogen. Likewise, they found that sorghum experienced a 56% stomatal conductance decrease at ample water and nitrogen; while cotton and grapes each experienced decreases of 22% at ample water and nitrogen.

In further discussing these and several other observations, Kimball et al. noted that "growth stimulations were as large or larger under water-stress compared to well-watered conditions." They also reported that "roots were generally stimulated more than shoots," and that although growth stimulations of non-legumes were reduced at low-soil nitrogen, "elevated CO2 strongly stimulated the growth of the clover legume both at ample and under low nitrogen conditions."

Contemporaneously, Lawson et al. (2002) grew potatoes (Solanum tuberosum L.) in open-top chambers maintained at atmospheric CO2 concentrations of 380, 550 and 680 ppm. In addition, the chambers were fumigated with air containing ambient and elevated levels of ozone (O3). And what did they find? Overall, elevated CO2 had a greater effect on gas exchange than did elevated ozone. Plants grown at 680 ppm CO2, for example, exhibited stomatal conductances that were 62 and 47% lower than those displayed by plants growing at 380 ppm CO2 while exposed to ambient and elevated concentrations of ozone, respectively. Neither did elevated CO2 nor ozone impact rates of carbon fixation to any significant degree, except for plants growing at 550 ppm CO2 and ambient ozone, where there was an 80% increase in net photosynthesis due to atmospheric CO2 enrichment. Nevertheless, elevated CO2 progressively increased instantaneous water use efficiency, regardless of ozone exposure. Thus, as the air's CO2 content continues to rise, it is highly likely that potatoes will be able to better deal with water stress, enabling them to be grown in more arid regions of the globe.

Also experimenting concurrently - and with potatoes - Olivo et al. (2002) grew two species (Solanum curtilobum cv. Ugro Shiri, from high altitude, and S. tuberosum cv. Baronesa, from low altitude) in pots placed within open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm for 30 days following the onset of reproductive growth in order to study the effects of elevated CO2 on gas exchange and biomass production in these two potato species adapted to different altitudes. This work revealed that doubling the atmosphere's CO2 content increased rates of net photosynthesis by 56 and 53% in the high- and low-altitude potato species, respectively, while reducing their stomatal conductances by 55 and 59% and increasing their instantaneous water-use efficiencies by 90 and 80%, respectively. In addition, it increased tuber dry mass production by 85 and 40% in the high- and low-altitude potato species, respectively.

One year later, Agrawal and Deepak (2003) grew two cultivars of wheat (Triticum aestivum L. cv. Malviya 234 and HP1209) in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm alone and in combination with 60 ppb SO2 to study the interactive effects of elevated CO2 and this major air pollutant on the growth and yield of this important crop. And in doing so, they determined that exposure to elevated CO2 significantly increased photosynthetic rates by 58 and 48% in M234 and HP1209, respectively. In contrast, fumigation with elevated SO2 did not significantly impact rates of photosynthesis in either cultivar. However, plants grown in the combined treatment of elevated CO2 and elevated SO2 displayed photosynthetic rates that were 42 and 38% greater than those measured in control plants for M234 and HP1209, respectively.

The wheat plants grown in elevated CO2 also displayed an approximate 20% reduction in stomatal conductance, while those grown in elevated SO2 exhibited an average conductance increase of 15%. When exposed simultaneously to both gases, however, the wheat plants displayed an average 11% reduction in stomatal conductance. Consequently, this phenomenon contributed to an approximate 32% increase in water-use efficiency for plants simultaneously exposed to both gases, whereas those exposed to elevated SO2 alone displayed an average decrease in water-use efficiency of 16%.

Contemporaneously, Kyei-Boahen et al. (2003) grew well watered and fertilized plants of four carrot (Daucus carota var. sativus L.) cultivars (Cascade, Caro Choice, Oranza and Red Core Chantenay) from seed in 15-cm-diameter plastic pots in a controlled environment facility for 30 days past emergence, whereupon leaf stomatal conductance (Gs) was measured at 100-ppm intervals of short-term (5-minute) atmospheric CO2 enrichment yielding absolute CO2 concentrations (Ca) stretching from 50 to 1050 ppm; and in doing so, they found that "increasing Ca from 50 to 350 ppm increased Gs to a maximum and thereafter Gs declined by 17% when Ca was increased to 650 ppm," while "a three-fold increase in Ca from 350 to 1050 ppm decreased Gs by 53%."

Moving ahead another year, Cardoso-Vilhena et al. (2004) grew individual spring wheat (Triticum aestivum L. cv. Hanno) plants in 3-dm³ pots in controlled environment chambers for 77 days at atmospheric CO2 concentrations of either 350 or 700 ppm and at ozone (O3) concentrations of either less than 5 or 75 ppb, while gas exchange measurements of leaves number 4 and 7 on the plants' main shoots were made at regular intervals throughout the study, after which the plants were harvested and their total dry weights determined, while in parallel with these measurements Rubisco activity and chlorophyll fluorescence were also assessed throughout the experiment.

This work revealed that in air of less than 5 ppb O3, the doubling of the air's CO2 concentration increased total plant dry weight by 66%; while in air of 75 ppb O3, it increased total plant dry weight by 189%. Over the lifespans of leaves 4 and 7, elevated CO2 also reduced cumulative O3 uptake by 10 and 35%, respectively, due to the decrease it caused in leaf stomatal conductance, while it protected against the decline in apparent quantum yield of CO2 assimilation caused by high O3 in the ambient CO2 treatment. In addition, elevated CO2 protected against the reduction in the maximum in vivo rate of Rubisco carboxylation induced by high O3 in both leaves 4 and 7.

In light of these findings, the five scientists said their data revealed that "rising atmospheric CO2 concentrations are likely to afford protection against the adverse effects of O3 on plant growth and photosynthesis, with the effect due, at least in part, to the decline in stomatal conductance triggered by increases in atmospheric CO2." In addition, they wrote that their study suggested that "rising atmospheric CO2 concentrations may also enhance the tolerance of leaf tissue to O3-induced oxidative stress," remarking that "this finding is consistent with reported shifts in the antioxidant status of leaves under the combined influence of elevated CO2+O3 (Rao et al., 1995)." And when considering the bottom line of total plant dry weight production, it can once again be seen that atmospheric CO2 enrichment more than ameliorated the deleterious effects of ozone pollution.

In a concomitant study of rice, Yoshimoto et al. (2005) grew Oryza sativa L. cv. Akita-Komachi from hand-transplanting to harvest (May to September) under normal paddy culture near Shizukuishi, Iwate, Japan, within FACE rings maintained at either ambient or ambient + 200 ppm CO2 for 24 hours per day, over which period they measured a number of micrometeorological parameters and plant characteristics. This work revealed, as they described it, that "elevated CO2 reduced stomatal conductance by 13% in upper leaves and by 40% in lower leaves at the panicle initiation stage," but that the reduction declined thereafter. In addition, they observed that "stomata closed more in the elevated CO2 plot as vapor pressure deficit increased," i.e., during drier conditions. And so it was that averaged over the entire growing season, the Japanese researchers determined that the total water used by the crop was 268.7 mm in the ambient CO2 treatment and 246.7 mm in the elevated CO2 treatment, thanks to the CO2-induced increase in plant stomatal conductance.

Advancing two more years, Bernacchi et al. (2007) wrote as background for their study that "with very few exceptions, deceased stomatal conductance (gs) is one of the most consistent and conserved responses of leaves to growth at elevated CO2," but they went on to note that less well understood was the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET); and, therefore, they said that it "is not certain that a decrease in gs will decrease ET in rain-fed crops."

To try to provide some certainty in this regard was thus the primary purpose of their study; and to do so, the five researchers grew soybean (Glycine max) under field conditions in Free-Air CO2 Enrichment (FACE) plots maintained at ambient (~375 ppm) and elevated (~550 ppm) atmospheric CO2 concentrations for four complete growing seasons at the SoyFACE facility in central Illinois (USA), making season-long measurements of both leaf gs and canopy ET once complete canopy closure had been achieved, after which they conducted a number of analyses of the data thereby obtained.

Based on their real-world field observations, Bernacchi et al. were able to report that "elevated CO2 caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield," and that "ecosystem ET was linked with gs of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in gs results in a 8.6% decrease in ET."

In discussing the implications of their meticulous and voluminous field observations, the researchers confirmed that their findings "are consistent with model and historical analyses that suggest that ... decreased gs of upper canopy leaves at elevated CO2 results in decreased transfer of water vapor to the atmosphere." They also noted that their findings were "consistent with the recent mechanistic modeling and statistical fingerprinting analysis of global trends in increasing river discharge across the 20th century," which suggested, last of all, that "suppression of plant transpiration due to the effect of rising CO2 on stomatal closure is the most consistent factor in explaining observed [river discharge] changes (Gedney et al., 2006)." And this conclusion, in turn, finally confirmed what was suggested to be the case well over two decades earlier by Idso and Brazel (1984) in a paper entitled "Rising atmospheric carbon dioxide concentrations may increase streamflow."

Moving on three more years, Shimono et al. (2010) wrote as background for their study that "by 2050, the world's population will have increased by about 37%, from the current level of 6.7 billion to an estimated 9.2 billion (UN, 2009), with a corresponding increase in global food demand." They also stated that "about 0.6 billion Mg of rice is produced annually from an area of 1.5 million km², making rice one of the most important crops for supporting human life," especially, as noted by Pritchard and Amthor (2005), since it supplies the planet's human population with an estimated 20% of their energy needs (on a caloric basis) and 14% of their protein requirements (on a weight basis).

Within this context, the six scientists further noted that "rice production depends heavily on water availability," stating that "irrigated lowlands account for 55% of the total area of harvested rice and typically produce two to three times the crop yield of rice grown under non-irrigated conditions (IRRI, 2002)." And because the demand for ever greater quantities of water will continue to rise, due to our need to adequately feed humanity's growing numbers, they concluded that "efficient use of water will thus be essential for future rice production."

As a result, and in an attempt to determine how the agricultural enterprise may be impacted in this regard by the ongoing rise in the air's CO2 content, the Japanese researchers conducted a two-year free-air CO2 enrichment or FACE study in fields at Shizukuishi, Iwate (Japan) to learn how elevated CO2 may reduce crop water use via its impact on the leaf stomatal conductance (gs) of three varieties of rice (Oryza sativa L.): early-maturing Kirara397, intermediate-maturing Akitakomachi, and latest-maturing Hitomebore.

In response to the 53% increase in daytime atmospheric CO2 concentration employed in their experiments, Shimono et al. determined that "the reduction in gs due to elevated CO2 was similar across measurements, averaging around 20% in the morning, 24% around noon and 23% in the afternoon across all growth stages." And they added that "there was no significant CO2 x cultivar interaction." Therefore, with the concomitant increase in grain yield that also results from atmospheric CO2 enrichment, it should be apparent to all that a continuation of the historical and still-ongoing rise in the air's CO2 content will play a major role in enabling humanity to meet its food needs at the mid-point of the current century, without having to lay claim to all of the planet's remaining fresh water resources and much of its undeveloped land and thereby driving many of the species - with which we share the terrestrial biosphere - to extinction for lack of land and water to meet their needs.

Rounding out this review of CO2 effects on plant stomatal conductance is the paper of Burkart et al. (2011). Working in a 20-hectare field near the German city of Braunschweig in southeastern Lower Saxony, they began a free-air CO2 enrichment (FACE) experiment in 1999 by enriching the air above portions of the field to a CO2 concentration of 550 ppm. This study included two rotation cycles (six years in total) of a typical local crop rotation that went from winter barley (Hordeum vulgare) to a cover crop of Italian ryegrass (Lolium multiflorum) to sugar beet (Beta vulgaris) to winter wheat (Triticum aestivum), during which time they measured a number of soil, plant and atmospheric properties, while managing the crops in accordance with current local farming practices in which "field irrigation was applied in order to avoid drought stress during the main growing season, keeping soil water content above 50% of maximum plant available soil water content."

Averaged across the two rotation cycles, Burkart et al. determined that the approximate 47% increase in the air's CO2 content they supplied to portions of the field reduced canopy stomatal conductance by 9%, 17% and 12% in barley, sugar beet and wheat, respectively, and that it likewise reduced canopy transpiration by 9%, 18% and 12% in the same three crops. And as a consequence of the lower canopy transpirational water loss, they found that the CO2 increase "increased plant water available soil water content in the course of the season by ca. 15 mm ... for all crops and years."

In discussing the implications of their findings, the five German scientists thus said that the cumulative increase of soil moisture due to CO2 enrichment found in their study suggests that in a future atmosphere, "CO2-related water savings may improve crop water status and reduce the need for irrigation in Central Europe," which could prove to be a huge boon to the inhabitants of that continent.

In succinctly stating the main message of the experimental findings described in this summary document, it would appear that atmospheric CO2 enrichment reduces the stomatal conductances of nearly all agricultural plants under many different growing conditions, including unfavorable ones characterized by elevated air temperatures, elevated ozone concentrations, and the presence of pathogenic viruses. Thus, as the air's CO2 content continues to rise, agricultural crops should exhibit ever-increasing reductions in transpirational water losses and yield losses that result from various diseases and aerial pollutants, while simultaneously experiencing significant increases in crop water-use efficiency.

References
Agrawal, M. and Deepak, S.S. 2003. Physiological and biochemical responses of two cultivars of wheat to elevated levels of CO2 and SO2, singly and in combination. Environmental Pollution 121: 189-197.

Bernacchi, C.J., Kimball, B.A., Quarles, D.R., Long, S.P. and Ort, D.R. 2007. Decreases in stomatal conductance of soybean under open-air elevation of [CO2] are closely coupled with decreases in ecosystem evapotranspiration. Plant Physiology 143: 134-144.

Burkart, S., Manderscheid, R., Wittich, K.-P., Lopmeier, F.J. and Weigel, H.-J. 2011. Elevated CO2 effects on canopy and soil water flux parameters measured using a large chamber in crops grown with free-air CO2 enrichment. Plant Biology 13: 258-269.

Cardoso-Vilhena, J., Balaguer, L., Eamus, D., Ollerenshaw, J. and Barnes, J. 2004. Mechanisms underlying the amelioration of O3-induced damage by elevated atmospheric concentrations of CO2. Journal of Experimental Botany 55: 771-781.

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.

Gedney, N., Cox, P.M., Betts, R.A., Boucher, O., Huntingford, C. and Stott, P.A. 2006. Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439: 835-838.

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.

Idso, S.B. and Brazel, A.J. 1984. Rising atmospheric carbon dioxide concentrations may increase streamflow. Nature 312: 51-53.

IRRI (International Rice Research Institute). 2002. Rice Almanac: Source Book for the Most Important Economic Activity on Earth. CABI Publishing, Oxnon, United Kingdom.

Kimball, B.A., Kobayashi, K. and Bindi, M. 2002. Responses of agricultural crops to free-air CO2 enrichment. Advances in Agronomy 77: 293-368.

Kyei-Boahen, S., Astatkie, T., Lada, R., Gordon, R. and Caldwell, C. 2003. Gas exchange of carrot leaves in response to elevated CO2 concentration. Photosynthetica 41: 597-603.

Lawson, T., Craigon, J., Black, C.R., Colls, J.J., Landon, G. and Weyers, J.D.B. 2002. Impact of elevated CO2 and O3 on gas exchange parameters and epidermal characteristics in potato (Solanum tuberosum L.). Journal of Experimental Botany 53: 737-746.

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.

Olivo, N., Martinez, C.A. and Oliva, M.A. 2002. The photosynthetic response to elevated CO2 in high altitude potato species (Solanum curtilobum). Photosynthetica 40: 309-313.

Pritchard, S.G. and Amthor, J.S. 2005. Crops and Environmental Change. Food Production Press, New York, New York, USA.

Rao, M.V., Hale, B.A. and Ormrod, D.P. 1995. Amelioration of ozone-induced oxidative damage in wheat plants grown under high carbon dioxide. Plant Physiology 109: 421-432.

Shimono, H., Okada, M., Inoue, M., Nakamura, H., Kobayashi, K. and Hasegawa, T. 2010. Diurnal and seasonal variations in stomatal conductance of rice at elevated atmospheric CO2 under fully open-air conditions. Plant, Cell and Environment 33: 322-331.

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.

UN (United Nations). 2009. The 2006 World Population Prospects. The 2008 Revision Population Database. United Nations, New York, New Your, USA. [WWW document]. URL http://esa.un.org/unpp/.

Yoshimoto, M., Oue, H. and Kobayashi, K. 2005. Energy balance and water use efficiency of rice canopies under free-air CO2 enrichment. Agricultural and Forest Meteorology 133: 226-246.