As the CO2 content of the air increases, nearly all of earth's plants will exhibit increases in their rates of photosynthesis. Additional photosynthetic sugars will then be available to increase plant growth and development. These sugars can also be transferred to the belowground environment to enhance the growth and development of symbiotic fungal organisms that live in close association with plant roots. This latter phenomenon is extremely important, for fungi form intricate mycelial networks that connect the root systems of different plants to each other and the soils that support them. They also increase nutrient and water acquisition from the soil environment and the subsequent transfer of these vital resources to, and among, host plants. In this summary, we review how elevated CO2 impacts these relationships in herbaceous plants.
As documented for both grasses and woody plants, elevated CO2 also increases the percentage root colonization by fungi in herbaceous species. In the study of Rouhier and Read (1998), for example, the authors reported a 75% increase in this parameter for Plantago lanceolata plants inoculated with an arbuscular mycorrhizal fungi and exposed to an atmospheric CO2 concentration of 540 ppm for three months. This increase in fungal root colonization was found to support increased root uptake of phosphorus and greater total phosphorus incorporation into the plant tissues. In a similar study, Staddon et al. (1999a) also reported that atmospheric CO2 enrichment enhanced fungal root colonization and total phosphorus incorporation into Plantago lanceolata tissues. In the study of Insam et al. (1999), however, elevated CO2 did not increase the percentage root colonization of certain herbaceous tropical plants; but it allowed them to support greater numbers of mycorrhizal fungi due to CO2-induced increases in root length.
Sometimes atmospheric CO2 enrichment results in the phenomenon of photosynthetic acclimation, which reduces CO2-enhanced plant photosynthetic rates due to the accumulation of photosynthetic sugars within plant leaves. However, plants involved in symbiotic relationships with fungal species often avoid photosynthetic acclimation by transporting photosynthetic sugars belowground and into the soil rhizosphere, where they can be utilized by fungi to enhance their growth and the construction of mycelial networks. In the study of Staddon et al. (1999b), for example, CO2-enriched Plantago lanceolata plants exhibited no photosynthetic acclimation, nor did CO2-enriched potato plantlets in the study of Louche-Tessandier et al. (1999), which also displayed enhanced mycelial growth.
In summary, it is clear that the increasing atmospheric CO2 concentration will likely increase the photosynthetic rates of earth's herbaceous plants. Such plants can then utilize these greater sugar supplies to enhance their own growth, or they can transport them belowground to stimulate the growth of symbiotic fungal organisms associated with their roots. If the latter phenomenon occurs, it is likely that the fungal organisms will enhance their colonization of plant roots and construct larger mycelial networks for seeking out and obtaining more soil minerals and water to sustain the enhanced growth of their vegetative hosts. In addition, the mobilization of carbohydrates out of the plants and into the rhizosphere will allow them to avoid photosynthetic acclimation, thus enabling them to continue extracting carbon from the atmosphere at accelerated rates.
References
Insam, H., Baath, E., Berreck, M., Frostegard, A., Gerzabek, M.H., Kraft, A., Schinner, F., Schweiger, P. and Tschuggnall, G. 1999. Responses of the soil microbiota to elevated CO2 in an artificial tropical ecosystem. Journal of Microbiological Methods 36: 45-54.
Louche-Tessandier, D., Samson, G., Hernandez-Sebastia, C., Chagvardieff, P. and Desjardins, Y. 1999. Importance of light and CO2 on the effects of endomycorrhizal colonization on growth and photosynthesis of potato plantlets (Solanum tuberosum) in an in vitro tripartite system. New Phytologist 142: 539-550.
Rouhier, H. and Read, D.J. 1998. The role of mycorrhiza in determining the response of Plantago lanceolata to CO2 enrichment. New Phytologist 139: 367-373.
Staddon, P.L., Fitter, A.H. and Graves, J.D. 1999a. Effect of elevated atmospheric CO2 on mycorrhizal colonization, external mycorrhizal hyphal production and phosphorus inflow in Plantago lanceolata and Trifolium repens in association with the arbuscular mycorrhizal fungus Glomus mosseae. Global Change Biology 5: 347-358.
Staddon, P.L., Fitter, A.H. and Robinson, D. 1999b. Effects of mycorrhizal colonization and elevated atmospheric carbon dioxide on carbon fixation and below-ground carbon partitioning in Plantago lanceolata. Journal of Experimental Botany 50: 853-860.