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Growth Response to CO2 with Other Variables (Ozone) -- Summary
Plants grown in CO2-enriched atmospheres 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 understand how plants will respond to concomitant increases in atmospheric CO2 and ozone concentrations, as these trace gases may both continue to rise in the future.

Several studies have been conducted using 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 many other herbaceous plants (Volin et al. 1998), as Tiedemann and Firsching (2000) recently noted for spring wheat.  In their study, 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%).

Other studies have extended such findings to long-lived perennial woody species like trees.  In a study by Karnosky et al. (1999), for example, ozone-induced physical foliar damage on aspen clones was significantly alleviated by elevated CO2 concentrations.  Broadmeadow et al. (1999) made similar observations in sessile oak, sweet chestnut, and European beech.  In a separate study on European beech, elevated ozone did not cause any reductions in photosynthesis, as long as seedlings were simultaneously fumigated with elevated CO2 concentrations (Grams et al. 1999).  And in black cherry, green ash, and yellow-poplar seedlings, elevated CO2 not only ameliorated the negative effects of ozone pollution on photosynthesis and biomass production, it often caused the greatest CO2-induced responses in these parameters under such unfavorable growing conditions (Loats and Rebbeck, 1999).

Thus, it is clear that elevated CO2 reduces, and nearly always completely overrides, any 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 reduced subsequent conveyance to tissues where damage often results to photosynthetic pigments and proteins, ultimately reducing plant growth and biomass production.

Broadmeadow, M.S.J., Heath, J. and Randle, T.J.  1999.  Environmental limitations to O3 uptake - Some key results from young trees growing at elevated CO2 concentrations.  Water, Air, and Soil Pollution 116: 299-310.

Grams, T.E.E, Anegg, S., Haberle, K.-H., Langebartels, C. and Matyssek, R.  1999.  Interactions of chronic exposure to elevated CO2 and O3 levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica).  New Phytologist 144: 95-107.

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.

Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G.  1999.  Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment.  Water, Air, and Soil Pollution 116: 311-322.

Loats, K.V. and Rebbeck, J.  1999.  Interactive effects of ozone and elevated carbon dioxide on the growth and physiology of black cherry, green ash, and yellow-poplar seedlings.  Environmental Pollution 106: 237-248.

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.

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.

Volin, J.C., Reich, P.B. and Givnish, T.J.  1998.  Elevated carbon dioxide ameliorates the effects of ozone on photosynthesis and growth: species respond similarly regardless of photosynthetic pathway or plant functional group.  New Phytologist 138: 315-325.