As the CO2 content of the air continues to rise, nearly all of earth's plants will exhibit increases in photosynthesis and biomass production; but climate alarmists periodically claim that water stress will negate these benefits. In reviewing the scientific literature of the ten-year period 1983-1994, however, Idso and Idso (1994) found that water stress typically will not negate the CO2-induced stimulation of plant productivity. In fact, they found that the CO2-induced percentage increase in plant biomass production was often greater under water-stressed conditions than it was when plants were well-watered. We here review some of the recent scientific literature in this area to see if these conclusions still hold.
During times of water stress, atmospheric CO2 enrichment often stimulates plants to develop larger-than-usual and more robust root systems to probe greater volumes of soil for scarce and much-needed moisture. Wechsung et al. (1999), for example, observed a 70% increase in lateral root dry weights of water-stressed wheat grown at 550 ppm CO2, while De Luis et al. (1999) reported a 269% increase in root-to-shoot ratio of water-stressed alfalfa growing at 700 ppm CO2. Thus, elevated CO2 elicits stronger-than-usual positive root responses in agricultural species under conditions of water stress.
Elevated levels of atmospheric CO2 also tend to reduce the openness of stomatal pores on leaves, thus decreasing plant stomatal conductance. This phenomenon, in turn, reduces the amount of water lost to the atmosphere by transpiration and, consequently, lowers overall plant water use. Indeed, Serraj et al. (1999) report that water-stressed soybeans grown at 700 ppm CO2 reduced their total seasonal water loss by 10% relative to that of water-stressed control plants grown at 360 ppm CO2. In addition, Conley et al. (2001) noted that a 200-ppm increase in the air's CO2 concentration reduced cumulative evapotranspiration in water-stressed sorghum by approximately 4%.
Atmospheric CO2 enrichment thus increases plant water acquisition, by stimulating root growth, while it reduces plant water loss, by constricting stomatal apertures; and these dual effects typically enhance plant water-use efficiency, even under conditions of less-than-optimal soil water content. But these phenomena have other implications as well.
CO2-induced increases in root development together with CO2-induced reductions in stomatal conductance often contribute to the maintenance of a more favorable plant water status during times of drought. Sgherri et al. (1998), for example, have reported that leaf water potential, which is a good indicator of overall plant water status, was 30% higher (less negative and therefore more favorable) in water-stressed alfalfa grown at an atmospheric CO2 concentration of 600 ppm CO2 versus 340 ppm CO2. In addition, Wall (2001) reports that leaf water potentials were similar in CO2-enriched water-stressed plants and ambiently-grown well-watered control plants, which implies a complete CO2-induced amelioration of water stress in the CO2-enriched plants. Similarly, Lin and Wang (2002) demonstrated that elevated CO2 caused a several-day delay in the onset of the water stress-induced production of the highly reactive oxygenated compound H2O2 in spring wheat.
If atmospheric CO2 enrichment thus allows plants to maintain a better water status during times of water stress, it is only logical to expect that such plants should exhibit greater rates of photosynthesis than ambiently-grown plants. And so they do. With the onset of water stress in Brassica juncea, for example, photosynthetic rates dropped by 40% in plants growing in ambient air, while plants growing in air containing 600 ppm CO2 only experienced a 30% reduction in net photosynthesis (Rabha and Uprety, 1998). In another manifestation of this phenomenon, Ferris et al. (1998) reported that after imposing water-stress conditions on soybeans and allowing them to recover following complete rewetting of the soil, plants grown in air containing 700 ppm CO2 reached pre-stressed rates of photosynthesis after six days, while plants grown in ambient air never recovered to pre-stressed rates.
Reasoning analogously, it is also only to be expected that plant biomass production would be enhanced by elevated CO2 concentrations under drought conditions. In exploring this idea, Ferris et al. (1999) reported that water-stressed soybeans grown at 700 ppm CO2 attained seed yields that were 24% greater than those of similarly water-stressed plants grown at ambient CO2 concentrations, while Hudak et al. (1999) reported that water-stress had no effect on yield in CO2-enriched spring wheat.
In some cases, the CO2-induced percentage biomass increase is actually greater for water-stressed plants than it is for well-watered plants. Li et al. (2000), for example, reported that a 180-ppm increase in the air's CO2 content increased lower stem grain weights in water-stressed and well-watered spring wheat by 24 and 14%, respectively. Similarly, spring wheat grown in air containing an additional 280 ppm CO2 exhibited 57 and 40% increases in grain yield under water-stressed and well-watered conditions, respectively (Schutz and Fangmeier, 2001). Likewise, Ottman et al. (2001) noted that elevated CO2 increased plant biomass in water-stressed sorghum by 15%, while no biomass increase occurred in well-watered sorghum.
In summary, the conclusions of Idso and Idso (1994) are well supported by the recent peer-reviewed scientific literature, which indicates that the ongoing rise in the air's CO2 content will likely lead to substantial increases in plant photosynthetic rates and biomass production, even in the face of stressful conditions imposed by less-than-optimum soil moisture conditions. Indeed, in predicting maize and winter wheat yields in Bulgaria under increased air temperature and decreased precipitation scenarios, Alexandrov and Hoogenboom (2000) noted that yield losses were likely to occur if the atmospheric CO2 concentration remained unchanged. However, if the atmospheric CO2 concentration doubled, then maize and winter wheat yields would likely increase even under the stresses of elevated temperature and reduced rainfall. Thus, future increases in the air's CO2 content will likely lead to increased crop growth and yield production, even in areas where reduced soil moisture availability leads to plant water stress.
References
Alexandrov, V.A. and Hoogenboom, G. 2000. The impact of climate variability and change on crop yield in Bulgaria. Agricultural and Forest Meteorology 104: 315-327.
Conley, M.M., Kimball, B.A., Brooks, T.J., Pinter Jr., P.J., Hunsaker, D.J., Wall, G.W., Adams, N.R., LaMorte, R.L., Matthias, A.D., Thompson, T.L., Leavitt, S.W., Ottman, M.J., Cousins, A.B. and Triggs, J.M. 2001. CO2 enrichment increases water-use efficiency in sorghum. New Phytologist 151: 407-412.
De Luis, J., Irigoyen, J.J. and Sanchez-Diaz, M. 1999. Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water stress. Physiologia Plantarum 107: 84-89.
Ferris, R., Wheeler, T.R., Hadley, P. and Ellis, R.H. 1998. Recovery of photosynthesis after environmental stress in soybean grown under elevated CO2. Crop Science 38: 948-955.
Ferris, R., Wheeler, T.R., Ellis, R.H. and Hadley, P. 1999. Seed yield after environmental stress in soybean grown under elevated CO2. Crop Science 39: 710-718.
Hudak, C., Bender, J., Weigel, H.-J. and Miller, J. 1999. Interactive effects of elevated CO2, O3, and soil water deficit on spring wheat (Triticum aestivum L. cv. Nandu). Agronomie 19: 677-687.
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.
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.
Lin, J.-S and Wang, G.-X. 2002. Doubled CO2 could improve the drought tolerance better in sensitive cultivars than in tolerant cultivars in spring wheat. Plant Science 163: 627-637.
Ottman, M.J., Kimball, B.A., Pinter Jr., P.J., Wall, G.W., Vanderlip, R.L., Leavitt, S.W., LaMorte, R.L., Matthias, A.D. and Brooks, T.J. 2001. Elevated CO2 increases sorghum biomass under drought conditions. New Phytologist 150: 261-273.
Rabha, B.K. and Uprety, D.C. 1998. Effects of elevated CO2 and moisture stress on Brassica juncea. Photosynthetica 35: 597-602.
Serraj, R., Allen, L.H., Jr., Sinclair, T.R. 1999. Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment. Global Change Biology 5: 283-291.
Sgherri, C.L.M., Quartacci, M.F., Menconi, M., Raschi, A. and Navari-Izzo, F. 1998. Interactions between drought and elevated CO2 on alfalfa plants. Journal of Plant Physiology 152: 118-124.
Schutz, M. and Fangmeier, A. 2001. Growth and yield responses of spring wheat (Triticum aestivum L. cv. Minaret) to elevated CO2 and water limitation. Environmental Pollution 114: 187-194.
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 Jr., P.J., 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.