When the air's CO2 concentration is experimentally increased, plants commonly reduce their foliar concentrations of the nitrogen-rich photosynthetic enzyme rubisco, which is typically present in excess amounts in ambient air (see Acclimation in our Subject Index).  Consequently, exposure to elevated CO2 concentrations frequently results in lowered foliar nitrogen concentrations, which adjustment allows excess nitrogen to be mobilized away from the photosynthetic process and into processes more limiting to growth.  Schappi and Korner (1997), for example, noted that two species common to the Swiss Alps displayed reduced foliar nitrogen concentrations in response to atmospheric CO2 enrichment.  However, Goverde et al. (1999) found no change in the leaf nitrogen contents of Lotus corniculatus plants exposed to twice-ambient atmospheric CO2 concentrations, nor did David et al. (2001) in leaves of Medicago and Trifolium species.  In the latter study, in fact, the authors actually reported a CO2-induced increase in leaf nitrogen content for a certain Galactites species.  Nevertheless, in a review of 67 published experimental observations made on several dozen species, Norby et al. (2001) concluded that a doubling of the atmospheric CO2 concentration reduced leaf nitrogen content, on average, by approximately 7%.

At the other end of the spectrum of leaf responses to atmospheric CO2 enrichment, excess carbohydrates resulting from CO2-induced increases in photosynthetic rates are often used to enhance the biosynthesis of secondary carbon compounds.  Norby et al. (2001), for example, analyzed 46 published experimental observations and determined that a doubling of the atmospheric CO2 content increased leaf lignin concentrations in a number of species by an average of 6.5%.  Leaf chlorophyll contents have also been reported to increase in young orchid plantlets exposed to elevated CO2 concentrations (Gouk et al., 1999).  Also, Goverde et al. (1999) observed elevated CO2 concentrations to increase leaf tannin concentrations in Lotus corniculatas, although Kerslake et al. (1998) could not discern any CO2 effect on foliar phenolic concentrations in Calluna vulgaris.  Finally, in another review of the literature, Idso and Idso (2001) noted that a near-tripling of the air's CO2 content enhanced the concentration of the heart-helping compound digoxin in the woolly foxglove by about 12%, and that a 75% increase in the air's CO2 content produced 6 to 28% increases in the concentrations of five different substances produced by spider lilies that have proven effective in treating a number of human cancers and viral diseases.

In summary, the ongoing rise in the air's CO2 concentration will definitely alter many physical and physiological leaf characteristics of earth's plants.  Furthermore, the available data suggest the resulting changes will lead to more efficient plant growth, while increasing the production of secondary carbon compounds, some of which have high potential for combating a number of human diseases.  Thus, the increasing CO2 content of the air may significantly improve the quality of human life.

References
David, J.-F., Malet, N., Couteaux, M.-M. and Roy, J.  2001.  Feeding rates of the woodlouse Armadillidium vulgare on herb litters produced at two levels of atmospheric CO2.  Oecologia 127: 343-349.

Gouk, S.S., He, J. and Hew, C.S.  1999.  Changes in photosynthetic capability and carbohydrate production in an epiphytic CAM orchid plantlet exposed to super-elevated CO2.  Environmental and Experimental Botany 41: 219-230.

Goverde, M., Bazin, A., Shykoff, J.A. and Erhardt, A.  1999.  Influence of leaf chemistry of Lotus corniculatus (Fabaceae) on larval development of Polyommatus icarus (Lepidoptera, Lycaenidae): effects of elevated CO2 and plant genotype.  Functional Ecology 13: 801-810.

Idso, S.B. and Idso, K.E.  2001.  Effects of atmospheric CO2 enrichment on plant constituents related to animal and human health.  Environmental and Experimental Botany 45: 179-199.

Kerslake, J.E., Woodin, S.J. and Hartley, S.E.  1998.  Effects of carbon dioxide and nitrogen enrichment on a plant-insect interaction: the quality of Calluna vulgaris as a host for Operophtera brumata.  New Phytologist 140: 43-53.

Norby, R.J., Cotrufo, M.F., Ineson, P., O'Neill, E.G. and Canadell, J.G.  2001.  Elevated CO2, litter chemistry, and decomposition: a synthesis.  Oecologia 127: 153-165.

Schappi, B. and Korner, C.  1997.  In situ effects of elevated CO2 on the carbon and nitrogen status of alpine plants.  Functional Ecology 11: 290-299.