Garcia et al. (1998) grew spring wheat in the field near Maricopa, Arizona, USA, from emergence to maturity in FACE plots maintained at ambient and 1.5 x ambient CO2 to determine the effects of elevated CO2 on photosynthesis and stomatal conductance.  Their data indicated that in these field plots, where soil nutrition was good and rooting volume was not limiting to growth, the approximate 50% enhancement of the air's CO2 content increased the average rate of midday net photosynthesis by 28% over control plants during the course of the growing season.  In addition, it reduced average midday stomatal conductance by 36%; and, as a result, the water-use efficiency of the CO2-enriched plants was enhanced by about a third on a whole crop basis.

In the same study, Osborne et al. (1998) measured net photosynthesis and photosynthetic protein concentrations in leaves located at different depths in the plant canopy.  They found that the elevated CO2 led to a certain degree of photosynthetic acclimation or down regulation of photosynthesis by reducing leaf rubisco and nitrogen levels, and that this phenomenon intensified with depth in the canopy.  However, the acclimation was not complete; and elevated CO2 still enhanced photosynthesis at all canopy levels relative to similarly located leaves in the ambient-air treatment, leading the team of scientists to conclude that this situation could only result from a CO2-induced "increased efficiency of leaf photosynthetic nitrogen use," which phenomenon is typically deemed to be a positive thing, because it allows plants to allocate excess leaf nitrogen to support larger reproductive structures, thereby enabling the plants to produce and sustain the larger yields that are commonly reported for crops exposed to elevated concentrations of atmospheric CO2.

In a two-year study conducted at the same site, Kimball et al. (1999) once again grew spring wheat in FACE plots maintained at atmospheric CO2 concentrations of ambient and 1.5 x ambient, as well as at low and high soil nitrogen concentrations, to study the effects of these two factors on evapotranspiration.  Their research indicated that the elevated CO2 consistently tended to conserve water, with the CO2-enriched plots experiencing daily water losses that were about 7 and 20% less than those experienced in the control plots under the high and low levels of soil nitrogen, respectively, which findings suggest that wheat crops in a high-CO2 world of the future will likely fare better under drought conditions than they do today.

Also exploring the joint effects of atmospheric CO2 and soil nitrogen status on spring wheat growth in this two-year study at the Arizona FACE facility were Brooks et al. (2000).  They found that although plants receiving low nitrogen inputs to their soils experienced reduced seasonal carbon accumulation in both CO2 treatments, plants grown in the plots exposed to 1.5 x ambient CO2 accumulated 8 and 16% more carbon than plants exposed to ambient air under low and high soil nitrogen treatments, respectively.  As the air's CO2 content continues to rise, therefore, it is likely that spring wheat yields will also increase, due to enhanced season-long carbon accumulation in both low and high soil nitrogen environments.  Although much of this increase will likely result from reductions in photorespiration and increased carboxylation efficiency of rubisco, it is possible that the increasingly more planar distribution of foliage observed in the CO2-enriched plants will also contribute to the growing productivity of wheat.

Wechsung et al. (1999) studied the effects of water stress (a 50% reduction in irrigation water) and atmospheric CO2 enrichment (a 50% increase in CO2) on root growth of wheat at the Arizona FACE site.  They found that the 50% increase in the air's CO2 content increased in-row root dry weight by an average of 22% over the growing season in both wet and dry irrigation regimes.  In addition, during vegetative growth, atmospheric CO2 enrichment increased inter-row root dry weight by 70%, indicating that plants grown in CO2-enriched air developed greater lateral root systems than did plants grown in ambient air.  Similarly, during the reproductive growth phase, elevated CO2 stimulated branching of lateral roots into inter-row areas, but only when water was limiting to growth.  The vertical extension of roots in the soil profile was also enhanced by elevated CO2, with the CO2-enriched plants displaying greater root dry weights at all depths.  Hence, it can be anticipated that as the CO2 content of the air continues to rise, spring wheat crops will likely develop larger and more extensively branching root systems that may help them to better cope with conditions of soil water-stress, which should allow them to be successfully grown in regions where mild water stress may be incurred.

In the same experiment, Li et al. (2000) found that the 50% increase in CO2 increased final grain weights in the upper and lower sections of the wheat crop's main stems by 10 and 24%, respectively, under water-stressed conditions.  Under well-watered conditions, in contrast, the elevated CO2 increased final grain weights only in the lower sections of the main stems and only by 14%.  Thus, elevated CO2 had a greater positive impact on final grain weights under less-then-optimal water-stressed conditions than under well-watered conditions, once again demonstrating that atmospheric CO2 enrichment is often more important to stressed plants than it is to non-stressed plants.

In yet another study associated with the same experiment, Wall (2001) found that as the amount of moisture in the soil decreased, leaf water potentials of CO2-enriched plants were always higher (less negative) than those of ambiently-grown plants, due to CO2-induced improvements in both drought avoidance and drought tolerance.  In fact, during the driest part of this two-year study, CO2-enriched plants in the dry irrigation treatment exhibited leaf water potentials that were similar in value to those measured on ambiently-grown plants in the wet irrigation treatment.  Thus, elevated CO2 completely ameliorated the negative effects of water-stress in these plants, as inferred by leaf water potential data.

In a somewhat different type of study, Grant et al. (1999) used the ecosys crop growth model in an attempt to simulate wheat biomass production in response to elevated CO2 at low and high soil moisture contents.  Model predictions of wheat biomass were fairly consistent with observed values measured in the experiments conducted on spring wheat at the Arizona FACE facility.  At ambient CO2, for example, biomass (in g m^-2) was observed to be 1361 ± 97 and 1856 ± 145 under low and high soil moisture regimes, respectively, while corresponding simulated values were 1390 and 2050.  Likewise, at elevated CO2 observed biomass values were 1604 ± 151 and 2044 ± 167 under low and high soil moisture regimes, respectively, while corresponding simulated values were 1710 and 2240.  Consequently, the observed CO2-induced percentage biomass increases were 18% and 10% under low and high soil moisture regimes, respectively, while the corresponding simulated increases were 23% and 9%.  This good correspondence led the authors to conclude that, because of the basic nature of the plant physiological processes simulated in the model, the model could well be used to project the productivity responses of many additional crops to atmospheric CO2 enrichment under a wide range of soil, management and climate conditions, thus helping to better forecast agricultural responses to the ongoing rise in the air's CO2 content.

In conclusion, the results of these several FACE experiments clearly indicate that as the air's CO2 concentration continues to rise, wheat plants will likely fare considerably better under both normal and droughty conditions and high and low soil nitrogen conditions than they do currently.  Consequently, with the human population of the earth continuing to increase, we would be wise to think very carefully about limiting anthropogenic CO2 emissions that could do so much to alleviate hunger and its many negative consequences.  To spurn the hand of providence in providing this unsought blessing, which is also a boon to the natural environment, is a dangerous thing indeed, for it could well place us in the position of operating in opposition to both moral and logical mandates, the devastating consequences of which may be nigh unforgivable.

References
Brooks, T.J., Wall, G.W., Pinter Jr., P.J., Kimball, B.A., LaMorte, R.L., Leavitt, S.W., Matthias, A.D., Adamsen, F.J., Hunsaker, D.J. and Webber, A.N.  2000.  Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes.  3. Canopy architecture and gas exchange.  Photosynthesis Research 66: 97-108.

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.

Grant, R.F., Wall, G.W., Kimball, B.A., Frumau, K.F.A., Pinter Jr., P.J., Hunsaker, D.J. and Lamorte, R.L.  1999.  Crop water relations under different CO2 and irrigation: testing of ecosys with the free air CO2 enrichment (FACE) experiment.  Agricultural and Forest Meteorology 95: 27-51.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Kimball, B.A., LaMorte, R.L., Pinter Jr., P.J., Wall, G.W., Hunsaker, D.J., Adamsen, F.J., Leavitt, S.W., Thompson, T.L., Matthias, A.D. and Brooks, T.J.  1999.  Free-air CO2 enrichment and soil nitrogen effects on energy balance and evapotranspiration of wheat.  Water Resources Research 35: 1179-1190.

Li, A.-G., Hou, Y.-S., Wall, G.W., Trent, A., Kimball, B.A. and Pinter Jr., P.J.  2000.  Free-air CO2 enrichment and drought stress effects on grain filling rate and duration in spring wheat.  Crop Science 40: 1263-1270.

Osborne, C.P., LaRoche, J., Garcia, R.L., Kimball, B.A., Wall, G.W., Pinter, P.J., Jr., LaMorte, R.L., Hendrey, G.R. and Long, S.P.  1998.  Does leaf position within a canopy affect acclimation of photosynthesis to elevated CO2?  Plant Physiology 117: 1037-1045.

Wall, G.W.  2001.  Elevated atmospheric CO2 alleviates drought stress in wheat.  Agriculture, Ecosystems and Environment 87: 261-271.

Wechsung, G., Wechsung, F., Wall, G.W., Adamsen, F.J., Kimball, B.A., Pinter, P.J., Jr., LaMorte, R.L., Garcia, R.L. and Kartschall, T.  1999.  The effects of free-air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth.  Global Change Biology 5: 519-529.