As the air's CO2 concentration rises, plants commonly reduce the concentration of the nitrogen-rich photosynthetic enzyme rubisco in their leaves, which is normally present in excess amounts at ambient atmospheric CO2 concentrations, as noted in our Summary on Acclimation.  Consequently, elevated CO2 exposure frequently results in reduced foliar nitrogen concentrations, which allows excess nitrogen in leaves to be mobilized away from the photosynthetic apparatus and devoted to processes more limiting to growth.  In the study of Monje and Bugbee (1998), for example, a near-900-ppm increase in the air's CO2 concentration reduced leaf nitrogen contents in wheat by 28%.  Similar results were obtained for soybeans grown at twice-ambient CO2 concentrations (Sims et al., 1998).  It is thus likely that many agricultural species will exhibit reductions in foliar nitrogen content in response to atmospheric CO2 enrichment.

Besides nitrogen, elevated CO2 can mobilize additional limiting resources away from the photosynthetic machinery and direct them into other plant parts important to growth and development.  In the study of Watling et al. (2000), for example, a doubling of the atmospheric CO2 concentration reduced the thickness of specialized bundle sheath cells in sorghum by approximately 50%, which surely freed up important resources that were sent to other parts of the plant for utilization, as indicated by a 36% enhancement in total plant biomass.

Elevated CO2 may also impact leaf concentrations of chlorophylls, which are important light absorbing pigments involved in the photosynthetic process.  In the study of Sgherri et al. (1998), for example, water-stressed alfalfa displayed a 30% reduction in leaf chlorophyll content, but water-stressed plants exposed to 600 ppm CO2 only exhibited a 6% reduction.  In potato, on the other hand, Sicher and Bunce (1999) found no change in leaf chlorophyll content when exposing plants to twice-ambient levels of atmospheric CO2, nor did Monje and Bugbee (1998) in their previously-mentioned study on wheat.  Thus, leaf chlorophyll contents of agricultural species may or may not maintain their current concentrations under CO2-enriched conditions.

Excess carbohydrates resulting from enhanced photosynthetic rates can be used to increase leaf growth.  Reddy et al. (1998) reported that cotton plants grown at 700 ppm CO2 displayed individual leaf areas that were 20% greater than those of leaves of ambiently-grown control plants.  Similarly, Masle (2000) noted that a 600-ppm increase in atmospheric CO2 concentration increased individual leaf size in wheat by increasing the number of photosynthetic mesophyll cell layers, as well as overall leaf thickness.  In a related study on sunflower, Sims et al. (1999) did not report an increase in leaf size or thickness, but they documented a CO2-induced shift in the distribution of leaf area that concentrated 30 to 40% more of it in the upper layer of the plant canopy, where the CO2-induced photosynthetic stimulation was greatest.

Sometimes, excess carbohydrates are used to enhance the biosynthesis of secondary carbon compounds in leaves.  In the study of Estiarte et al. (1999), for example, leaves of spring wheat grown at 550 ppm CO2 displayed 14% higher total flavanoid concentrations than leaves of plants grown at 370 ppm CO2.  This observation is important, for flavanoids are generally characterized as having anti-herbivory properties.  Thus, less pest-induced yield losses in this important grain crop may occur under future climates characterized by elevated atmospheric CO2 concentrations.

In conclusion, increasing atmospheric CO2 concentrations will likely affect many leaf characteristics of agricultural plants.  Fortunately, the available data suggest the majority of these changes will likely lead to greater rates and higher efficiencies of photosynthesis and growth.  As a result, agricultural yields will likely rise ever higher in the future, due in part to changes in foliar properties mediated by the on-going rise in the air's CO2 content.

References
Estiarte, M., Penuelas, J., Kimball, B.A., Hendrix, D.L., Pinter Jr., P.J., Wall, G.W., LaMorte, R.L. and Hunsaker, D.J.  1999.  Free-air CO2 enrichment of wheat: leaf flavonoid concentration throughout the growth cycle.  Physiologia Plantarum 105: 423-433.

Masle, J.  2000.  The effects of elevated CO2 concentrations on cell division rates, growth patterns, and blade anatomy in young wheat plants are modulated by factors related to leaf position, vernalization, and genotype.  Plant Physiology 122: 1399-1415.

Monje, O. and Bugbee, B.  1998.  Adaptation to high CO2 concentration in an optimal environment: radiation capture, canopy quantum yield and carbon use efficiency.  Plant, Cell and Environment 21: 315-324.

Reddy, K.R., Robana, R.R., Hodges, H.F., Liu, X.J. and McKinion, J.M.  1998.  Interactions of CO2 enrichment and temperature on cotton growth and leaf characteristics.  Environmental and Experimental Botany 39: 117-129.

Sgherri, C.L.M., Quartacci, M.F., Menconi, M., Raschi, A. and Navari-Izzo, F.  1998.  Interactions between drought and elevated CO2 on alfalfa plants.  Journal of Plant Physiology 152: 118-124.

Sicher, R.C. and Bunce, J.A.  1999.  Photosynthetic enhancement and conductance to water vapor of field-grown Solanum tuberosum (L.) in response to CO2 enrichment.  Photosynthesis Research 62: 155-163.

Sims, D.A., Cheng, W., Luo, Y. and Seeman, J.R.  1999.  Photosynthetic acclimation to elevated CO2 in a sunflower canopy.  Journal of Experimental Botany 50: 645-653.

Sims, D.A., Luo, Y. and Seeman, J.R.  1998.  Comparison of photosynthetic acclimation to elevated CO2 and limited nitrogen supply in soybean.  Plant, Cell and Environment 21: 945-952.

Watling, J.R., Press, M.C. and Quick, W.P.  2000.  Elevated CO2 induces biochemical and ultrastructural changes in leaves of the C4 cereal sorghum.  Plant Physiology 123: 1143-1152.