In the presence of elevated concentrations of atmospheric CO2, many of earth's plants experience an enhanced rate of photosynthetic carbon uptake, which leads to increased production of plant secondary carbon compounds, including phenolics. Consequently, and since the resulting increases in foliar phenolic concentrations often enhance plant resistance to herbivore and pathogen attack, we here review the results of some studies that have dealt with this important subject in herbaceous plants.
Hoorens et al. (2002) grew two plants common to dune grasslands of the Netherlands (Calamagrostis epigejos and Vicia lathyroides) along with two species common to Dutch peatlands (Carex rostrata and Sphagnum recurvum) in greenhouses fumigated with air containing either 390 or 700 ppm CO2 for a period of five months. Then, after senescence had occurred, they collected the resulting leaf litter and analyzed it for the presence of various substances. Among other things, they found that elevated CO2 had little impact on the presence of phenolics in Calamagrostis and Sphagnum litter, but that it increased phenolic concentrations in Vicia and Carex litter by 20 and 32%, respectively.
Castells et al. (2002) grew fourteen genotypes of two perennial grasses (Dactylis glomerata and Bromus erectus) common to the Mediterranean area in glasshouses maintained at atmospheric CO2 concentrations of 350 and 700 ppm to determine if elevated CO2 impacts phenolic production in a genotypic-dependent manner. Their research indicated that elevated CO2 increased total phenolic concentrations in Dactylis and Bromus by 15 and 87%, respectively, and that there were no significant CO2 x genotype interactions in either species. As the atmosphere's CO2 concentration rises ever higher, therefore, these two perennial grasses will likely exhibit greater resistance to herbivory without having to sacrifice their genotypic diversity.
In the watery world of aquatic plants, Wetzel and Tuchman (2005) grew cattails for a period of three years in open-bottom root boxes out-of-doors within clear-plastic-wall open-top chambers maintained at either ambient (360 ppm) or elevated (720 ppm) atmospheric CO2 concentrations from early spring through leaf senescence. During this period, green and naturally-senesced leaves were collected and analyzed for the fraction of leaf mass composed of total phenolics. This program revealed that green leaf material contained 27.6% more total phenolics when the plants were grown in CO2-enriched as opposed to ambient air, while it demonstrated that senesced leaf material grown in CO2-enrihced air contained 40.6% more total phenolics than similar leaves produced in ambient air.
Finally, in yet a different type of study with human health implications, Wang et al. (2003) grew strawberry plants in six clear-acrylic open-top chambers, two of which were maintained at the ambient atmospheric CO2 concentration, two of which were maintained at ambient + 300 ppm CO2, and two of which were maintained at ambient + 600 ppm CO2 from early spring of 1998 through June of 2000. The researchers harvested the strawberry fruit, in their words, "at the commercially ripe stage" in both 1999 and 2000, after which they analyzed them for the presence of a number of different health-promoting substances with "potent antioxidant properties." This work revealed that CO2 enrichment increased fruit ascorbic acid, glutathione, phenolic acid, flavonol, and anthocyanin concentrations, and that plants grown under CO2 enrichment conditions also had higher oxygen radical absorbance activity against many types of harmful oxygen radicals in the fruit.
In conclusion, it is clear that many, but not all, herbaceous plants experience increases in foliar and fruit phenolic concentrations in response to atmospheric CO2 enrichment, which responses appear to have a number of positive implications for both man and the biosphere.
References
Castells, E., Roumet, C., Penuelas, J. and Roy, J. 2002. Intraspecific variability of phenolic concentrations and their responses to elevated CO2 in two mediterranean perennial grasses. Environmental and Experimental Botany 47: 205-216.
Hoorens, B., Aerts, R. and Stroetenga, M. 2002. Litter quality and interactive effects in litter mixtures: more negative interactions under elevated CO2? Journal of Ecology 90: 1009-1016.
Wang, S.Y., Bunce, J.A. and Maas, J.L. 2003. Elevated carbon dioxide increases contents of antioxidant compounds in field-grown strawberries. Journal of Agricultural and Food Chemistry 51: 4315-4320.
Wetzel, R.G. and Tuchman, N.C. 2005. Effects of atmospheric CO2 enrichment and sunlight on degradation of plant particulate and dissolved organic matter and microbial utilization. Archiv fur Hydrobiologie 162: 287-308.
Last updated 28 September 2005