Plants grown in CO2-enriched air nearly always exhibit increased photosynthetic rates and biomass production relative to plants grown at the current ambient CO2 concentration. In contrast, plants exposed to elevated ozone concentrations typically display reductions in photosynthesis and growth in comparison with plants grown at the current ambient ozone concentration. Thus, it is important to determine how major crops respond to concomitant increases in the abundances of these two trace gases of the atmosphere, as their concentrations will likely continue to increase for many years to come.
Several studies have used soybean as a model plant to study the effects of elevated CO2 and ozone on photosynthesis and growth. Reid et al. (1998), for example, grew soybeans for an entire season at different combinations of atmospheric CO2 and ozone, reporting that elevated CO2 enhanced rates of photosynthesis in the presence or absence of ozone and that it typically ameliorated the negative effects of elevated ozone on carbon assimilation. At the cellular level, Heagle et al. (1998a) reported that at twice the current ambient ozone concentration, soybeans simultaneously exposed to twice the current ambient atmospheric CO2 concentration exhibited far less foliar injury while maintaining significantly greater leaf chlorophyll contents than control plants exposed to elevated ozone and ambient CO2 concentrations. By harvest time, these same plants (grown in the elevated ozone/elevated CO2 treatment combination) had produced 53% more total biomass than their counterparts did at elevated ozone and ambient CO2 concentrations (Miller et al., 1998). Finally, in analyzing seed yield, it was determined that atmospheric CO2 enrichment enhanced this parameter by 20% at ambient ozone, while it increased it by 74% at twice the ambient ozone concentration (Heagle et al., 1998b). Thus, elevated CO2 completely ameliorated the negative effects of elevated ozone concentration on photosynthetic rate and yield production in soybean.
Such ameliorating responses of elevated CO2 to ozone pollution are not unique to soybeans; they have also been reported for various cultivars of spring and winter wheat. In the study of Tiedemann and Firsching (2000), for example, atmospheric CO2 enrichment not only overcame the detrimental effects of elevated ozone on photosynthesis and growth, it overcame the deleterious consequences resulting from inoculation with a biotic pathogen as well. Indeed, although infected plants displayed less absolute yield than non-infected plants at elevated ozone concentrations, atmospheric CO2 enrichment caused the greatest relative yield increase in infected plants (57% vs. 38%).
McKee et al. (2000) reported that O3-induced reductions in leaf rubisco contents in spring wheat were reversed when plants were simultaneously exposed to twice-ambient concentrations of atmospheric CO2. In the study of Vilhena-Cardoso and Barnes (2001), elevated ozone concentrations reduced photosynthetic rates in spring wheat grown at three different soil nitrogen levels. However, when concomitantly exposed to twice-ambient atmospheric CO2 concentrations, elevated ozone had absolutely no effect on rates of photosynthesis, regardless of soil nitrogen. Going a step further, Pleijel et al. (2000) observed that ozone-induced reductions in spring wheat yield were partially offset by concomitant exposure to elevated CO2 concentrations. Similar results have been reported in spring wheat by Hudak et al. (1999) and in winter wheat by Heagle et al. (2000).
Other studies have observed similar responses in cotton and potato. Cotton plants grown at elevated ozone concentrations exhibited 25 and 48% reductions in leaf mass per unit area and foliar starch concentration, respectively, relative to control plants grown in ambient air. When simultaneously exposed to twice-ambient CO2 concentrations, however, the reductions in these parameters were only 5 and 7%, respectively (Booker, 2000). With respect to potato, Wolf and van Oijen (2002) used a validated potato model to predict increases in European tuber production ranging from 1,000 to 3,000 kg of dry matter per hectare in spite of concomitant increases in ozone concentrations and air temperatures.
It is clear from these studies that elevated CO2 reduces, and nearly always completely overrides, the negative effects of ozone pollution on plant photosynthesis, growth and yield. When explaining the mechanisms behind such responses, most authors suggest that atmospheric CO2 enrichment tends to reduce stomatal conductance, which causes less indiscriminate uptake of ozone into internal plant air spaces and reduces subsequent conveyance to tissues where damage often results to photosynthetic pigments and proteins, ultimately reducing plant growth and biomass production.
References
Booker, F.L. 2000. Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition. Plant, Cell and Environment 23: 573-583.
Heagle, A.S., Miller, J.E. and Booker, F.L. 1998a. Influence of ozone stress on soybean response to carbon dioxide enrichment: I. Foliar properties. Crop Science 38: 113-121.
Heagle, A.S., Miller, J.E. and Pursley, W.A. 1998b. Influence of ozone stress on soybean response to carbon dioxide enrichment: III. Yield and seed quality. Crop Science 38: 128-134.
Heagle, A.S., Miller, J.E. and Pursley, W.A. 2000. Growth and yield responses of winter wheat to mixtures of ozone and carbon dioxide. Crop Science 40: 1656-1664.
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.
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.
Miller, J.E., Heagle, A.S. and Pursley, W.A. 1998. Influence of ozone stress on soybean response to carbon dioxide enrichment: II. Biomass and development. Crop Science 38: 122-128.
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.
Reid, C.D. and Fiscus, E.L. 1998. Effects of elevated [CO2] and/or ozone on limitations to CO2 assimilation in soybean (Glycine max). Journal of Experimental Botany 18: 885-895.
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.
Wolf, J. and van Oijen, M. 2002. Modelling the dependence of European potato yields on changes in climate and CO2. Agricultural and Forest Meteorology 112: 217-231.