Plants grown in elevated CO2 environments often exhibit some degree of physiological adaptation. The most common of these adjustments is the phenomenon of photosynthetic acclimation or down regulation, which is characterized by reductions in the activity and/or amount of the primary plant carboxylating enzyme rubisco (see Acclimation). In addition, plant acclimation to elevated atmospheric CO2 may lead to reductions in leaf nitrogen and chlorophyll concentrations.
In this regard, it is important to note that acclimation to elevated CO2 is not a detrimental phenomenon. In fact, it is a positive response, which facilitates the redistribution of excess nitrogen and other limiting resources away from the photosynthetic apparatus towards other growth-limiting processes. In an open-top chamber experiment on spruce saplings that lasted three years, for example, CO2-enriched trees exhibited photosynthetic rates that were 62% greater than those of their ambiently-grown counterparts, in spite of their lower leaf chlorophyll contents (Centritto and Jarvis, 1999). Similar studies on herbaceous (Ong et al., 1998; Pritchard et al., 2000) and woody (Grams et al., 1999; Ormrod et al., 1999) species have yielded analogous results, indicating that current leaf chlorophyll concentrations in these species are much greater than what are needed in CO2-enriched atmospheres.
Nevertheless, atmospheric CO2 enrichment does not always result in decreased leaf chlorophyll contents. In an open-top chamber experiment with alfalfa, for example, plants grown at an atmospheric CO2 concentration of 600 ppm actually displayed greater leaf chlorophyll concentrations than those observed in plants grown at 340 ppm (Sgherri et al., 1998). Likewise, exposure of an orchid to a super-elevated CO2 concentration of 10,000 ppm resulted in a 64% increase in its leaf chlorophyll concentration relative to that measured in leaves of plants grown at ambient CO2 (Gouk et al., 1999).
In between these two responses, increases in the air's CO2 content have sometimes been demonstrated to have no significant effect on leaf chlorophyll concentration. Sicher and Bunce (1999), for example, reported that twice-ambient CO2 concentrations elicited no change in leaf chlorophyll contents of potato plants during a three-year study. Even with higher CO2 enrichment levels (870 ppm above ambient concentrations), Monje and Bugbee (1998) failed to detect any CO2-induced changes in leaf chlorophyll content of wheat. Similar results have also been reported in woody plants, where a doubling of the atmospheric CO2 concentration had no significant impact on leaf chlorophyll concentrations within sugar maple (Li et al., 2000) and oak species (Carter et al., 2000; Stylinski et al., 2000).
In summary, these several studies demonstrate that atmospheric CO2 enrichment may either increase, decrease or have no effect on leaf chlorophyll concentrations; and even when concentrations are decreased, the reallocation of nitrogen away from chlorophyll and other photosynthetic components typically occurs without any adverse consequences, as most plants displaying this response almost always continue to exhibit significant increases in photosynthesis and biomass production.
References
Carter, G.A., Bahadur, R. and Norby, R.J. 2000. Effects of elevated atmospheric CO2 and temperature on leaf optical properties in Acer saccharum. Environmental and Experimental Botany 43: 267-273.
Centritto, M. and Jarvis, P.G. 1999. Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis). II. Photosynthetic capacity and nitrogen use efficiency. Tree Physiology 19: 807-814.
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.
Grams, T.E.E, Anegg, S., Haberle, K.-H., Langebartels, C. and Matyssek, R. 1999. Interactions of chronic exposure to elevated CO2 and O3 levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica). New Phytologist 144: 95-107.
Li, J.-H., Dijkstra, P., Hymus, G.J., Wheeler, R.M., Piastuchi, W.C., Hinkle, C.R. and Drake, B.G. 2000. Leaf senescence of Quercus myrtifolia as affected by long-term CO2 enrichment in its native environment. Global Change Biology 6: 727-733.
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.
Ong, B.-L., Koh, C.K-K. and Wee, Y.-C. 1998. Effects of CO2 on growth and photosynthesis of Pyrrosia piloselloides (L.) Price gametophytes. Photosynthetica 35: 21-27.
Ormrod, D.P., Lesser, V.M., Olszyk, D.M. and Tingey, D.T. 1999. Elevated temperature and carbon dioxide affect chlorophylls and carotenoids in Douglas-fir seedlings. International Journal of Plant Science 160: 529-534.
Pritchard, S.G., Ju, Z., van Santen, E., Qiu, J., Weaver, D.B., Prior, S.A. and Rogers, H.H. 2000. The influence of elevated CO2 on the activities of antioxidative enzymes in two soybean genotypes. Australian Journal of Plant Physiology 27: 1061-1068.
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.
Stylinski, C.D., Oechel, W.C., Gamon, J.A., Tissue, D.T., Miglietta, F. and Raschi, A. 2000. Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange, biochemistry and optical techniques. Plant, Cell and Environment 23: 1353-1362.