Reading these words literally sent chills up our spines, for one of the best ways we know to improve plant resistance to environmental stresses is to enrich the atmosphere with carbon dioxide.  Is it possible that this phenomenon also improves the nutritional quality of the food we eat?  We have long felt that it may indeed do so; and there is now another reason for believing that that hypothesis has merit.

The first of the foundational elements of this CO2-induced, plant-stress-relieving, human-health-promoting hypothesis was established by Idso and Idso (1994), who demonstrated that environmental stresses due to air pollution, high temperature and lack of water are all significantly mitigated by atmospheric CO2 enrichment.  In some experiments involving high temperatures, in fact, they noted that supplying plants with extra CO2 actually made the difference between the plants living or dying (Idso et al.,1989, 1995).

The second foundational element of the hypothesis is elucidated by Demmig-Adams and Adams, who make a good case for its validity too.  They begin by noting that many plant products or phytochemicals that play vital roles in a variety of plant protective processes are also capable of manipulating human cellular signaling and gene expression.  As but one example, they report that carotenoid pigments such as zeaxanthin and lutein, which protect photosynthesis "under a plethora of environmental stresses," have been identified "as possible protective agents in human vision and immune function and in the prevention of cancer and heart disease."  Hence - and after considering mountains of additional evidence - they conclude that "improvements in amounts or functions of these and other phytochemicals may well aid both plant productivity and human health."

The point that remains to be resolved in establishing the ultimate validity of this CO2 health-promoting hypothesis is whether atmospheric CO2 enrichment, as a specific agent of plant stress reduction, accomplishes its work by means of improving the amounts and/or functions of various phytochemicals - of which many hundreds have been identified - or if it supplies its benefits by some other means.  In broaching this question, we feel there is both reason and evidence to indicate there are many opportunities for elevated levels of atmospheric CO2 to elicit both types of responses, i.e., to improve both the production and functioning of many of these important health-promoting substances.

With respect to reason, we note that with the aerial fertilization effect of atmospheric CO2 enrichment promoting the production of more plant biomass - which is constructed primarily from carbon (C), hydrogen (H) and oxygen (O) - there definitely will be more of these basic elements available for the creation of greater quantities of carbon-based secondary compounds, many of which are composed exclusively of C, H and O, such as vitamins A, C, D and K.  Hence, one would expect that at least some of the hundreds of phytochemicals that protect humans from degenerative diseases would indeed be found in greater concentrations in plants grown in CO2-enriched air.  Also, the work of Rao et al. (1995) suggests that atmospheric CO2 enrichment leads to the maintenance of higher activities of antioxidant enzymes such as glutathione reductase; and since this enzyme is primarily responsible for the high redox states of both glutathione and ascorbate (Foyer et al., 1994), this phenomenon may well be operative among a host of antioxidants, and especially among vitamins.

Given these reasons to expect CO2-induced increases in the concentrations of various phytochemicals that may be beneficial to human health, what evidences are there that such concentration increases really occur?

In the case of ascorbate or vitamin C, Barbale (1970) and Madsen (1971, 1975) observed CO2-induced increases in the concentration of this important phytochemical in both the leaves and fruit of tomato plants; while Tajiri (1985) observed elevated concentrations in bean sprouts and Schwanz et al. (1996) and Idso et al. (2002) measured elevated levels in both the foliage and fruit of sour orange trees.  Kimball and Mitchell (1981) also found that atmospheric CO2 enrichment stimulated the production of vitamin A in tomato plants.  In addition, Stuhlfauth et al. (1987) and Stuhlfauth and Fock (1990) demonstrated that elevated levels of atmospheric CO2 stimulate the production of the heart-helping cardiac glycoside digoxin in the wooly foxglove plant; while Idso et al. (2000) determined that atmospheric CO2 enrichment enhances the production of several chemical constituents of the bulb of a tropical spider lily that have been proven to be effective against lymphocytic leukemia and ovary sarcoma (Pettit et al., 1986), melanoma, brain, colon, lung and renal cancers (Pettit et al., 1993) and Japanese encephalitis and yellow, dengue, Punta Tora and Rift Valley fevers (Gabrielsen et al., 1992a,b).

Clearly, there are many good reasons and much experimental evidence to justify serious study of the hypothesis that increases in the air's CO2 content promote the production of phytochemicals that are beneficial to human health; for even with all that is known about the subject, the fertile surface of this most important field of research has barely been scratched.  We must delve deeper and more extensively into this vitally important area of concern that touches the lives of every inhabitant of the planet.  Indeed, we owe it to ourselves, in a very personal way, to evaluate all of the evidence related to the question of whether the ongoing rise in the air's CO2 content is good or bad for the biosphere; and the potential impact of this phenomenon on the nutritive value of the food we eat ranks right up there with whatever else is near the top of the list of prime concerns.

References
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Demmig-Adams, B. and Adams III, W.H.  2002.  Antioxidants in photosynthesis and human nutrition.  Science 298: 2149-2153.

Foyer, C.H., Descourvieres, P. and Kunert, K.J.  1994.  Protection against oxygen radicals: an important defense mechanism studied in transgenic plants.  Plant, Cell and Environment 17: 507-523.

Gabrielsen, B., Monath, T.P., Huggins, J.W., Kefauver, D.F., Pettit, G.R., Groszek, G., Hollingshead, M., Kirsi, J.J., Shannon, W.F., Schubert, E.M., Dare, J., Ugarkar, B., Ussery, M.A. and Phelan, M.J.  1992a.  Antiviral (RNA) activity of selected Amaryllidaceae isoquinoline constituents and synthesis of related substances.  Journal of Natural Products 55: 1569-1581.

Gabrielsen, B., Monath, T.P., Huggins, J.W., Kirsi, J.J., Hollingshead, M., Shannon, W.M. and Pettit, G.R.  1992b.  Activity of selected Amaryllidaceae constituents and related synthetic substances against medically important RNA viruses.  In: Chu, C.K. and Cutler, H.G. (Eds.), Natural Products as Antiviral Agents.  Plenum Press, New York, NY, pp. 121-135.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Idso, S.B., Allen, S.G., Anderson, M.G. and Kimball, B.A.  1989.  Atmospheric CO2 enrichment enhances survival of Azolla at high temperatures.  Environmental and Experimental Botany 29: 337-341.

Idso, S.B., Idso, K.E., Garcia, R.L., Kimball, B.A. and Hoober, J.K.  1995.  Effects of atmospheric CO2 enrichment and foliar methanol application on net photosynthesis of sour orange tree (Citrus aurantium; Rutaceae) leaves.  American Journal of Botany 82: 26-30.

Idso, S.B., Kimball, B.A., Pettit III, G.R., Garner, L.C., Pettit, G.R. and Backhaus, R.A.  2000.  Effects of atmospheric CO2 enrichment on the growth and development of Hymenocallis littoralis (Amaryllidaceae) and the concentrations of several antineoplastic and antiviral constituents of its bulbs.  American Journal of Botany 87: 769-773.

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.

Kimball, B.A. and Mitchell, S.T.  1981.  Effects of CO2 enrichment, ventilation, and nutrient concentration on the flavor and vitamin C content of tomato fruit.  HortScience 16: 665-666.

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Madsen, E.  1975.  Effect of CO2 environment on growth, development, fruit production and fruit quality of tomato from a physiological viewpoint.  In: Chouard, P. and de Bilderling, N. (Eds.), Phytotronics in Agricultural and Horticultural Research.  Bordas, Paris, pp. 318-330.

Pettit, G.R., Gaddamidi, V., Herald, D.L., Singh, S.B., Cragg, G.M., Schmidt, J.M., Boettner, F.E., Williams, M. and Sagawa, Y.  1986.  Antineoplastic agents, 120.  Pancratium littorale.  Journal of Natural Products 49: 995-1002.

Pettit, G.R., Pettit III, G.R., Backhaus, R.A., Boyd, M.R. and Meerow, A.W.  1993.  Antineoplastic agents, 256.  Cell growth inhibitory isocarbostyrils from Hymenocallis.  Journal of Natural Products 56: 1682-1687.

Rao, M.V., Hale, B.A. and Ormrod, D.P.  1995.  Amelioration of ozone-induced oxidative damage in wheat plants grown under high carbon dioxide: role of antioxidant enzymes.  Plant Physiology 109: 421-432.

Schwanz, P., Kimball, B.A., Idso, S.B., Hendrix, D.L. and Polle, A.  1996.  Antioxidants in sun and shade leaves of sour orange trees (Citrus aurantium) after long-term acclimation to elevated CO2.  Journal of Experimental Botany 47: 1941-1950.

Stuhlfauth, T. and Fock, H.P.  1990.  Effect of whole season CO2 enrichment on the cultivation of a medicinal plant, Digitalis lanata.  Journal of Agronomy and Crop Science 164: 168-173.

Stuhlfauth, T., Klug, K. and Fock, H.P.  1987.  The production of secondary metabolites by Digitalis lanata during CO2 enrichment and water stress.  Phytochemistry 26: 2735-2739.

Tajiri, T.  1985.  Improvement of bean sprouts production by intermittent treatment with carbon dioxide.  Nippon Shokuhin Kogyo Gakkaishi 32 (3): 159-169.