Wayne et al. (2002) grew common ragweed plants from seed in controlled-environment glasshouses maintained at ambient (350 ppm) and enriched (700 ppm) atmospheric CO2 concentrations for 84 days, after which they sampled the pollen from the central plants of each stand, assessed its characteristics, and then harvested all mature seeds and above-ground shoot material. This work revealed, in their words, that "stand-level pollen production was 61% higher in elevated versus ambient CO2 environments," and that "CO2-induced growth stimulation of stand shoot biomass was similar to that of total pollen production." Although the researchers admitted it would be "challenging to accurately predict the future threat to public health caused by CO2-stimulated pollen production," since "it is likely that plant pollen production will also be influenced by factors expected to change in concert with CO2, including temperature, precipitation, and atmospheric pollutants," they nevertheless suggested that "the incidence of hay fever and related respiratory diseases may increase in the future."

Interestingly, in a guest editorial in the Annals of Allergy, Asthma & Immunology, Weber (2002) discusses the study of Wayne et al., which was published in the same issue of the journal. He begins by saying "one can always wonder whether such manipulations [i.e., those employed in Wayne et al.'s study] have any relationship to present reality, or indeed, conditions that one can expect in the near future," whereupon he proceeds methodically to his conclusion that "it would be premature to assume that increased pollen grain numbers necessarily leads to an increased aeroallergen exposure."

Elucidating some of the reasons for his assessment of the issue, Weber notes that "allergenic activity of short ragweed will vary from year to year, even from the same source and supplier (Maasch et al., 1987)," and he cites Lee et al. (1979) as having found "varying potency in plants at the same site from year to year, which [were] attributed to seasonal climatic differences, primarily of rainfall." In fact, the latter researchers found a four-fold range in the allergenic potency of ragweed pollen within a single county in Illinois, USA. Consequently, Weber concludes that "a constant relationship between pollen mass and allergenic protein content is not a given" and will remain speculative until it is determined whether "the increased pollen grains seen with the increased ambient CO2 levels maintain the same ratio of allergenic proteins."

A further demonstration of the tenuousness of the suggestion of Wayne et al. - i.e., that "the incidence of hay fever and related respiratory diseases may increase in the future," due to the near-universal growth-promoting effects of atmospheric CO2 enrichment - is provided by Rogers et al. (2006), who collected and vernalized ragweed seeds by sowing them in containers kept in a refrigerator maintained at 4°C, after which they transferred one third of the seeded containers at 15-day intervals to glasshouse modules maintained at atmospheric CO2 concentrations of either 380 or 700 ppm, where the seeds were allowed to germinate (also at 15-day intervals, with the middle germination date approximating that of plants currently growing naturally in the vicinity of where the seeds were collected), and where they remained under well-watered and fertilized conditions until they senesced and were harvested, at which time assessments of plant and allergenic pollen biomass were made.

As best we can determine from the graphical representations of Rogers et al.'s data, the end-of-season CO2-induced increase in aboveground plant biomass was about 16% for the date of emergence typical of the present, while the corresponding increase in pollen production was about 32%. However, for the 15-day earlier date of emergence, which was chosen to represent "anticipated advances of spring several decades into the future," based upon projected rates of future global warming, the end-of-season CO2-induced change in aboveground plant biomass was only about +3%, while the end-of-season CO2-induced change in pollen production was actually a negative 3%. The most meaningful way of viewing the results, therefore, is to determine the change in pollen production that would occur in going from today's atmospheric CO2 concentration and date-of-onset of spring (380 ppm, middle date of germination) to the elevated CO2 concentration and earlier date-of-onset of biological spring (700 ppm, 15-day earlier date of germination); and when this is done, the production of allergenic pollen is seen to rise by a less-than-whopping 1-2%, which is obviously totally insignificant.

Turning our attention to some other noxious plants, Caporn et al. (1999) studied bracken, a weed that poses a potential threat to human health in the United Kingdom and other regions. Specimens of this plant were grown for 19 months in controlled environment chambers maintained at atmospheric CO2 concentrations of 370 and 570 ppm and normal and high levels of fertilization; and by so doing, it was learned that the elevated CO2 consistently increased rates of net photosynthesis in bracken by some 30 to 70%, depending upon soil fertility and time of year. However, the elevated CO2 did not increase total plant dry mass nor the dry mass of any plant organs, including rhizomes, roots, and fronds. In fact, the only significant effect of the elevated CO2 on bracken growth was observed in the normal nutrient regime, where elevated CO2 actually reduced the area of bracken fronds.

Matros et al. (2006) grew tobacco plants in pots filled with quartz sand placed in controlled-climate chambers maintained at either 350 or 1000 ppm CO2 for a period of eight weeks, where they were irrigated daily with a complete nutrient solution containing either 5 or 8 mM NH4NO3. In addition, some of the plants in each treatment were mechanically infected with potato virus Y (PVY) when they were six weeks old. At the end of the study, the researchers reported that the plants grown at elevated CO2 and 5 mM NH4NO3 "showed a marked and significant decrease in content of nicotine in leaves as well as in roots," while at 8 mM NH4NO3 the same was found to be true of upper leaves but not of lower leaves and roots. With respect to the PVY part of the study, they found that the plants grown at high CO2 "showed a markedly decreased spread of virus."

Keeping the story simple, Matros et al. thus reported that "tobacco plants grown under elevated CO2 show a slight decrease of nicotine contents," and that "elevated CO2 resulted in reduced spread of PVY." Both of these impacts would likely be considered beneficial by most people, as potato virus Y is an economically important virus that infects many crops and ornamental plants throughout the world, while nicotine is nearly universally acknowledged to have significant negative impacts on human health (Topliss et al., 2002).

Last of all, to the date of this writing, we have the study of Mohan et al. (2006), who investigated the effects of an extra 200 ppm of atmospheric CO2 on the growth and development of poison ivy, as well as its effect on the plant's toxicity, over a period of six years at the Duke Forest FACE facility, where the noxious vine grew naturally in a loblolly pine plantation's understory, where clumps of the plant were surrounded by 4-cm plastic-mesh exclosures to protect them from damage by indigenous white-tailed deer. This long and detailed study revealed that CO2 enrichment increased poison ivy photosynthesis by 77%, while boosting its water use efficiency by 51%. The researchers also note that at the end of the study's sixth year, the aboveground biomass of poison ivy plants in the CO2-enriched plots was 62% greater than that of plants in the ambient-treatment plots. In addition, they report that the high-CO2-grown plants produced "a more allergenic form of urushiol," which is the substance that produces the plant's allergic reaction in humans.

The seven scientists involved in the research say their findings indicate that under future levels of atmospheric CO2, poison ivy "may grow larger and become more noxious than it is today." And so it may. At the same time, however, a multitude of studies have indicated that the quantities and qualities of health-promoting substances found in many food and medicinal plants will likewise be enhanced by the ongoing rise in the air's CO2 concentration (see Health Effects (CO2 - Plant Production of Health-Promoting Substances) in our Subject Index). Hence, it is our feeling - and we believe it will be yours as well after perusing these materials - that the many beneficial impacts of elevated atmospheric CO2 concentrations on food and medicinal plants will far outweigh the negative health implications arising from CO2-induced increases in the growth of plants that may induce human allergies (such as poison ivy and ragweed). Indeed, it is a fact of life that "the wheat and the tares" - the good and the bad - will continue to grow together; but it would appear, from the preponderance of experimental studies conducted to date, that the ongoing rise in the air's CO2 content is gradually shifting the balance in favor of the good.

References
Caporn, S.J.M., Brooks, A.L., Press, M.C. and Lee, J.A. 1999. Effects of long-term exposure to elevated CO2 and increased nutrient supply on bracken (Pteridium aquilinum). Functional Ecology 13: 107-115.

Lee, Y.S., Dickinson, D.B., Schlager, D. and Velu, J.G. 1979. Antigen E content of pollen from individual plants of short ragweed (Ambrosia artemisiifolia). Journal of Allergy and Clinical Immunology 63: 336-339.

Maasch, H.J., Hauck, P.R., Oliver, J.D. et al. 1987. Allergenic activity of short ragweed pollen (Ambrosia elatior) from different years and/or suppliers: criteria for the selection of an in-house allergen reference preparation. Annals of Allergy 58: 429-434.

Matros, A., Amme, S., Kettig, B., Buck-Sorlin, G.H., Sonnewald, U. and Mock, H.-P. 2006. Growth at elevated CO2 concentrations leads to modified profiles of secondary metabolites in tobacco cv. SamsunNN and to increased resistance against infection with potato virus Y. Plant, Cell and Environment 29: 126-137.

Mohan, J.E., Ziska, L.H., Schlesinger, W.H., Thomas, R.B., Sicher, R.C., George, K. and Clark, J.S. 2006. Biomass and toxicity responses of poison ivy (Toxicodendron radicans) to elevated atmospheric CO2. Proceedings of the National Academy of Sciences, USA: 103: 9086-9089.

Rogers, C.A., Wayne, P.M., Macklin, E.A., Muilenberg, M.L., Wagner, C.J., Epstein, P.R. and Bazzaz, F.A. 2006. Interaction of the onset of spring and elevated atmospheric CO2 on ragweed (Ambrosia artemisiifolia L.) pollen production. Environmental Health Perspectives 114: 665-669.

Topliss, J.G., Clark, A.M., Ernst, E. et al. 2002. Natural and synthetic substances related to human health. Pure and Applied Chemistry 74: 1957-1985.

Wayne, P., Foster, S., Connolly, J., Bazzaz, F. and Epstein, P. 2002. Production of allergenic pollen by ragweed (Ambrosia artemisiifolia L.) is increased in CO2-enriched atmospheres. Annals of Allergy, Asthma, and Immunology 88: 279-282.

Weber, R.W. 2002. Mother Nature strikes back: global warming, homeostasis, and implications for allergy. Annals of Allergy, Asthma & Immunology 88: 251-252.