Background
In introducing their study, the authors write that "as well as damaging numerous physiological functions, abiotic stress [such as drought] also leads to higher concentrations of reactive oxygen species, which are present in nature in all plants, but which may damage cell components and disturb metabolic processes when present in larger quantities," citing Omran (1980), Larson (1988) and Dat et al. (2000). On the other hand, they say that "many authors have demonstrated that the [atmosphere's] CO2 concentration has a substantial influence on the stress sensitivity of plants via changes in antioxidant enzyme activity," citing Fernandez-Trujillo et al. (2007), Ali et al. (2008) and Varga and Bencze (2009), such that increases in the air's CO2 concentration may increase various antioxidant enzyme activities and reduce the negative effects of abiotic stress.

What was done
In an experiment designed to further explore this subject, Varga et al. grew two varieties of winter wheat within phytotrons maintained at either 380 or 750 ppm CO2, where the potted plants were watered daily and supplied with nutrient solution twice a week until the start of drought treatments, when drought was induced in three phases - at first node appearance, heading and grain filling - by completely withholding water for seven days, which dropped the volumetric soil water content in the pots from 20-25% to 3-5%.

What was learned
The four researchers - all of whom are associated with the Agricultural Research Institute of the Hungarian Academy of Sciences - report that they observed "changes in enzyme activity" that "indicated that enhanced CO2 concentration delayed the development of drought stress up to first node appearance, and stimulated antioxidant enzyme activity when drought occurred during ripening, thus reducing the unfavorable effects of [drought] stress."

What it means
In the words of Varga et al., the increases in the antioxidant enzymes they analyzed "may help to neutralize the reactive oxygen species induced by stress during various parts of the vegetation period." And this phenomenon may help mankind's crops to better cope with whatever extremes of moisture insufficiency might be lurking in our future.

References
Ali, M.B., Dewir, Y.H., Hahn, E. and Peak, K. 2008. Effect of carbon dioxide on antioxidant enzymes and ginsenoside production in root suspension cultures of Panax ginseng. Environmental and Experimental Botany 63: 297-304.

Dat, J., Vandenabeele, S., Vranova, A., Van Montagu, M., Inze, D., Van Breusegem, F. 2000. Dual action of the active oxygen species during plant stress responses. Cellular and Molecular Life Sciences 57: 779-995.

Frenandez-Trujillo, J.P., Nock, J.F. and Watkins, C.B. 2007. Antioxidant enzyme activities in strawberry fruit exposed to high carbon dioxide atmospheres during cold storage. Food Chemistry 104: 1425-1429.

Larson, R.A. 1988. The antioxidants of higher plants. Phytochemistry 27: 969-978.

Omran, R.G. 1980. Peroxide levels and the activities of catalase, peroxidase and indoleacetic acid oxidase during and after chilling cucumber seedlings. Plant Physiology 65: 407-408.

Varga, B. and Bencze, Sz. 2009. Comparative study of drought stress resistance in two winter wheat varieties raised at ambient and elevated CO2 concentration. Cereal Research Communications 37: 209-212.