Environmental stresses induced by exposure to pollutants, drought, intense solar radiation and high air or water temperatures generate highly-reactive oxygenated compounds that damage plants. Ameliorating these stresses typically involves the production of antioxidant enzymes that scavenge and detoxify the highly-reactive oxygenated compounds. Hence, when stresses are present, concentrations and/or activities of antioxidants in plants are generally observed to be high; and as Lesser (1997) has noted in corals, they tend to counter the effects of the stress. A good question to ask, therefore, is how atmospheric CO2 enrichment impacts this relationship and what the observed results imply.
In a study of two soybean genotypes, Pritchard et al. (2000) reported that three months' exposure to twice-ambient CO2 concentrations reduced the activities of superoxide dismutase and catalase by an average of 23 and 39%, respectively. Likewise, Polle et al. (1997) showed that two years of atmospheric CO2 enrichment reduced the activities of several key antioxidative enzymes, including catalase and superoxide dismutase, in beech seedlings. Moreover, Schwanz and Polle (1998) demonstrated that this phenomenon can persist indefinitely, as they discovered similar reductions in these same enzymes in mature oak trees that had been growing near natural CO2-emitting springs for 30 to 50 years.
The standard interpretation of these results is that the observed reductions in the activities of antioxidative enzymes under CO2-enriched conditions imply that plants exposed to higher-than-current atmospheric CO2 concentrations experience less oxidative stress and thus have a reduced need for antioxidant protection. This conclusion further suggests that "CO2-advantaged" plants will be able to funnel more of their limited resources into the production of other plant tissues or processes essential to their continued growth and development.
On the other hand, when oxidative stresses do occur under high CO2 conditions, the enhanced rates of photosynthesis and carbohydrate production resulting from atmospheric CO2 enrichment generally enable plants to better deal with such stresses by providing more of the raw materials needed for antioxidant enzyme synthesis. Thus, when CO2-enriched sugar maple seedlings were subjected to an additional 200 ppb of ozone, Niewiadomska et al. (1999) reported that ascorbate peroxidase, which is the first line of enzymatic defence against ozone, significantly increased. Likewise, Schwanz and Polle (2001) noted that poplar clones grown at 700 ppm CO2 exhibited a much greater increase in superoxide dismutase activity upon chilling induction than clones grown in ambient air. In addition, Lin and Wang (2002) observed that activities of superoxide dismutase and catalase were much higher in CO2-enriched wheat than in ambiently-grown wheat following the induction of water stress.
In some cases, the additional carbon fixed during CO2-enrichment is invested in antioxidative compounds, rather than enzymes. In the study of Idso et al. (2002), for example, a 75% increase in atmospheric CO2 concentration boosted the vitamin C concentration within the juice of sour oranges by 5% over a five-year period. In addition, Estiarte et al. (1999) reported that a 180-ppm increase in the air's CO2 content increased the foliar concentrations of flavonoids, which protect against UV-B radiation damage, in field-grown spring wheat by 11 to 14%. Finally, it is interesting to note that the increased consumption of such plant material - naturally enriched with antioxidative compounds as a consequence of the historical rise in the air's CO2 content - may have played a role in the observed decline in human mortality rates over the period 1950-1994 (Tuljapurkar et al., 2000).
In summary, as the CO2 content of the air rises, plants typically experience less oxidative stress; and since they thus need fewer antioxidants for protection, antioxidant levels in their leaves decline, which enables them to use more of their valuable resources for other important purposes. However, when oxidative stresses are present, elevated CO2 helps to increase the synthesis and activities of antioxidants that tend to alleviate the problems the stresses cause. Moreover, the possibility exists that the greater concentrations of antioxidative compounds in plant tissues caused by the historical rise in the air's CO2 content may have contributed to the human life span extension experienced over the course of the Industrial Revolution.
References
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Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D. 2002. The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice. Agriculture, Ecosystems and Environment 90: 1-7.
Lesser, M.P. 1997. Oxidative stress causes coral bleaching during exposure to elevated temperatures. Coral Reefs 16: 187-192.
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.
Niewiadomska, E., Gaucher-Veilleux, C., Chevrier, N., Mauffette, Y. and Dizengremel, P. 1999. Elevated CO2 does not provide protection against ozone considering the activity of several antioxidant enzymes in the leaves of sugar maple. Journal of Plant Physiology 155: 70-77.
Polle, A., Eiblmeier, M., Sheppard, L. and Murray, M. 1997. Responses of antioxidative enzymes to elevated CO2 in leaves of beech (Fagus sylvatica L.) seedlings grown under a range of nutrient regimes. Plant, Cell and Environment 20: 1317-1321.
Pritchard, S.G., Ju, Z., van Santen, E., Qiu, J., Weaver, D.B., Prior, S.A. and Rogers, H.H. 2000. The influence of elevated CO2 on the activities of antioxidative enzymes in two soybean genotypes. Australian Journal of Plant Physiology 27: 1061-1068.
Schwanz, P. and Polle, A. 2001. Growth under elevated CO2 ameliorates defenses against photo-oxidative stress in poplar (Populus alba x tremula). Environmental and Experimental Botany 45: 43-53.
Schwanz, P. and Polle, A. 1998. Antioxidative systems, pigment and protein contents in leaves of adult mediterranean oak species (Quercus pubescens and Q. ilex) with lifetime exposure to elevated CO2. New Phytologist 140: 411-423.
Tuljapurkar, S., Li, N. and Boe, C. 2000. A universal pattern of mortality decline in the G7 countries. Nature 405: 789-792.