Atmospheric CO2 enrichment typically enhances photosynthesis and biomass production in wheat plants under normal growing conditions (see, for example, our Subject Index Summaries pertaining to Agriculture (Species - Wheat: Photosynthesis and Biomass). But what happens when environmental conditions are less than ideal? In this brief recapitulation of the results of studies for which we have produced Journal Reviews, we report on what has been learned when lack of water limits the growth of wheat.
In a study of the ecosys crop growth model, Grant et al. (1999) compared their model calculations of wheat biomass production in response to elevated CO2 at high and low soil moisture contents with observed values measured in a FACE experiment conducted on spring wheat grown at atmospheric CO2 concentrations of 350 and 500 ppm near Maricopa, Arizona, USA. They report that in that very realistic experimental study, the observed CO2-induced percentage increases in biomass were determined to be 10% in the high soil moisture regime but 18% in the low soil moisture regime.
In a subsequent FACE study conducted at the same location, Li et al. (2000) grew wheat plants at close to the same CO2 concentrations (370 and 550 ppm) under well-watered conditions and a water-stressed regime where the plants received only 50% as much irrigation water as the well-watered plants; and in this case, much as in the prior study, the CO2-enriched well-watered plants exhibited a grain weight increase of 14%, while the CO2-enriched water-stressed plants experienced a grain weight increase of 24%. At the same time, in an ancillary study of the same wheat crop, Wall (2001) observed that as the amount of moisture in the soil decreased, leaf water potentials of the CO2-enriched plants were always higher (less negative) than those of the ambiently-grown plants, as a consequence of CO2-induced improvements in both drought avoidance and drought tolerance. In fact, during the driest part of this two-year study, the CO2-enriched plants in the "dry" irrigation treatment exhibited leaf water potentials that were similar to those measured on ambiently-grown plants in the "wet" irrigation treatment. Thus, the extra 180 ppm CO2 of this study completely ameliorated the effects of water-stress in these plants under the driest conditions they encountered, as inferred by leaf water potential data. Also of interest in this regard, the study of Lin and Wang (2002) suggests that elevated levels of atmospheric CO2 may well have a greater impact on increasing drought resistance in inherently less-drought-tolerant species than they do in more-drought-tolerant species, which is to say that atmospheric CO2 enrichment may help those plants most that need help most.
Other researchers have obtained similar results. Schutz and Fangmeier (2001), for example, grew spring wheat for an entire season under well-watered and water-stressed conditions in pots located within open-top chambers maintained at atmospheric CO2 concentrations of 367 and 650 ppm, finding that the elevated CO2 stimulated yield by 40% in the well-watered treatment but by 57% in the water-stressed treatment. In like manner, Dong-Xiu et al. (2002) grew spring wheat in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm and soil moisture contents of 40 and 80% field capacity, finding that the extra CO2 of their study increased net photosynthesis by 48% in the high soil water treatment but by 97% in the low soil water treatment.
Based on results such as those described above that have been obtained from many real-world experimental studies, Reyenga et al. (2001) ran a cropping system model designed to reveal how predicted climate changes might impact wheat production in the southern part of Australia with an atmospheric CO2 concentration of 700 ppm in combination with several computer-generated scenarios of warmer temperature and reduced rainfall. They found that under most climate change scenarios, cropping range expanded northward due to the "carbon dioxide fertilization effect." In addition, the elevated CO2 increased yields by 13 to 52%, with the greater responses occurring in the drier climates.
In light of these several observations, it is clear that the ongoing rise in the air's CO2 content should have its greatest relative impact on the growth and development of wheat where a lack of sufficient soil moisture currently reduces grain yields below their genetic potential. As a result, people around the world who are forced to farm marginal lands beset by water shortages should most benefit, relatively speaking, from this phenomenon.
References
Dong-Xiu, W., Gen-Xuan, W., Yong-Fei, B., Jian-Xiong, L. and Hong-Xu, R. 2002. Response of growth and water use efficiency of spring wheat to whole season CO2 enrichment and drought. Acta Botanica Sinica 44: 1477-1483.
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.
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.
Reyenga, P.J., Howden, S.M., Meinke, H. and Hall, W.B. 2001. Global change impacts on wheat production along an environmental gradient in south Australia. Environmental International 27: 195-200.
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.