Atmospheric CO2 enrichment typically enhances photosynthesis and biomass production in wheat (Triticum aestivum L.) 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 summarization of the results of studies for which we have produced Journal Reviews, we report on what has been learned when the presence of air pollutants negatively impacts the growth and development of wheat.
Bender et al. (1999) analyzed the results of thirteen open-top chamber studies, wherein spring wheat was grown at ambient and twice ambient atmospheric CO2 concentrations in combination with ambient and elevated ozone (O3) concentrations. They found that the elevated O3 treatment had little effect on growth and yield, suggesting that either the O3 concentrations employed in the studies were not high enough to elicit a negative response in the specific cultivar tested (Minaret) or that the cultivar was highly tolerant of ozone. Consequently, elevated CO2 was the primary variable that influenced growth and yield; and it proved very effective in this regard, increasing aboveground biomass by an average of 37% (with a range of 11 to 128%) and grain yield by an average of 35% (with a range of 11 to 121%).
Tiedemann and Firsching (2000) grew spring wheat from germination to maturity in controlled environment chambers maintained at ambient (377 ppm) and enriched (612 ppm) atmospheric CO2 concentrations and ambient (20 ppb) and enriched (61 ppb) atmospheric O3 concentrations. The extra CO2 increased mean photosynthetic rates at both O3 concentrations, with the greatest absolute photosynthetic rates and the largest CO2-induced percentage increases in photosynthesis being observed in the elevated CO2/elevated O3 treatment. Total grain yield was also greatest in the high CO2/high O3 treatment, with the elevated CO2 increasing total grain yield at high O3 by 38% relative to that observed at ambient CO2 and elevated O3. Moreover, the absolute value of total grain yield in the high CO2/high O3 treatment was not significantly different from that produced at ambient O3, regardless of the atmospheric CO2 concentration. Thus, the deleterious effects of ozone on photosynthesis and yield were completely ameliorated by atmospheric CO2 enrichment in this study.
Pleijel et al. (2000) grew spring wheat in open-top chambers maintained at atmospheric CO2 concentrations of 340 and 680 ppm for three consecutive years. In addition, they exposed some plants in each CO2 treatment to ambient, 1.5 x ambient and 2 x ambient O3 concentrations. The elevated O3 concentrations negatively influenced wheat yield at both atmospheric CO2 concentrations. Nevertheless, grain yield was always higher for the plants grown in the CO2-enriched air, averaging 13% greater over the three years of the study and leading the scientists to conclude that "the positive effect of elevated CO2 could compensate for the yield losses due to O3."
Vilhena-Cardoso and Barnes (2001) grew spring wheat for two months in environmental chambers fumigated with air containing atmospheric CO2 concentrations of 350 and 700 ppm at ambient and elevated (75 ppb) O3 concentrations in soils of low, medium and high nitrogen content. They found that the elevated O3 treatment reduced photosynthetic rates in the ambient-CO2-grown plants, but it had no effect on the CO2-enriched plants, which maintained enhanced photosynthetic rates even in the high O3 treatments. With respect to biomass production, elevated CO2 increased total plant dry weight by 44, 29 and 12% at high, medium and low soil nitrogen supply, respectively; and although elevated O3 by itself reduced plant biomass, the simultaneous application of elevated CO2 completely ameliorated this detrimental effect at all soil nitrogen concentrations.
Fangmeier and Bender (2002) analyzed mean grain yields of spring wheat derived from the ESPACE-Wheat project of the European Stress Physiology and Climate Experiment - Project 1, which was conducted for three growing seasons at eight experimental field sites across Europe that employed atmospheric CO2 concentrations of 380, 540 and 680 ppm and O3 concentrations of 32.5 and 60.3 ppb for half-day periods (Jager et al., 1999). They found that the high O3 stress reduced wheat yields by an average of about 12% at the ambient CO2 concentration. However, as the air's CO2 concentration was increased to 540 and 680 ppm, there were no significant reductions in yield due to the high O3 stress. Hence, whereas wheat yield in ambient-O3 air increased by 34% over the entire CO2 enrichment range investigated (380 to 680 ppm), it increased by 46% in the high-O3 air, more than compensating for the O3-alone-induced yield losses.
In a final study of sulfur dioxide (SO2) pollution, Agrawal and Deepak (2003) grew two cultivars of wheat (Malviya 234 and HP1209) in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm in air of normal or increased (to 60 ppb) SO2 concentration, finding that fumigation with elevated SO2 did not significantly impact rates of photosynthesis in either cultivar. However, they determined that the extra SO2 decreased plant water use efficiency by 16% and reduced leaf protein levels by 13%; but when the air was simultaneously enriched with CO2, plant water use efficiency rose by twice as much (32%) as it had previously fallen, while leaf protein levels dropped by only 3% in HP1209 and actually increased by 4% in M234.
In considering the several results described above, it would appear that enriching the air with CO2 goes a long way, if not all the way, towards ameliorating a variety of negative influences of various types of air pollution (particularly ozone, which has been studied most extensively in this regard) on the well-being of wheat plants.
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.
Bender, J., Herstein, U. and Black, C.R. 1999. Growth and yield responses of spring wheat to increasing carbon dioxide, ozone and physiological stresses: a statistical analysis of 'ESPACE-wheat' results. European Journal of Agronomy 10: 185-195.
Fangmeier, A. and Bender, J. 2002. Air pollutant combinations - Significance for future impact assessments on vegetation. Phyton 42: 65-71.
Jager, H.-J., Hertstein, U. and Fangmeier, A. 1999. The European stress physiology and climate experiments - Project 1 - Wheat. European Journal of Agronomy 10: 153-260.
Pleijel, H., Gelang, J., Sild, E., Danielsson, H., Younis, S., Karlsson, P.-E., Wallin, G., Skarby, L. and Sellden, G. 2000. Effects of elevated carbon dioxide, ozone and water availability on spring wheat growth and yield. Physiologia Plantarum 108: 61-70.
Tiedemann, A.V. and Firsching, K.H. 2000. Interactive effects of elevated ozone and carbon dioxide on growth and yield of leaf rust-infected versus non-infected wheat. Environmental Pollution 108: 357-363.
Vilhena-Cardoso, J. and Barnes, J. 2001. Does nitrogen supply affect the response of wheat (Triticum aestivum cv. Hanno) to the combination of elevated CO2 and O3? Journal of Experimental Botany 52: 1901-1911.