Grassland species grown in elevated CO2 environments often, but not always, exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by long-term rates of photosynthesis that are somewhat lower than what would be expected on the basis of measurements made during short-term exposure to CO2-enriched conditions. These downward adjustments result from modest long-term decreases in the activities and/or amounts of the primary plant carboxylating enzyme rubisco (Davey et al., 1999; Bryant et al., 1998; Rogers et al., 1998).
Acclimation is said to be present when the photosynthetic rates of long-term CO2-enriched plants are found to be lower than those of long-term non-CO2-enriched plants when the normally CO2-enriched plants are measured during brief exposures to ambient CO2 concentrations. Nonetheless, in nearly every reported case of photosynthetic acclimation in CO2-enriched plants, rates of photosynthesis displayed by grassland species grown and measured at elevated CO2 concentrations are typically greater than those exhibited by control plants grown and measured at ambient CO2 concentrations (Davey et al., 1999; Bryant et al., 1998).
CO2-induced photosynthetic acclimation often results from insufficient plant sink strength, which can lead to carbohydrate accumulation in source leaves and the triggering of photosynthetic end-product feedback inhibition, which reduces rubisco activity and rates of net photosynthesis (Roumet et al., 2000). As one example of this phenomenon, Rogers et al. (1998) reported that perennial ryegrass grown at an atmospheric CO2 concentration of 600 ppm and low soil nitrogen exhibited leaf carbohydrate contents and rubisco activities that were 100% greater and 25% less, respectively, than those observed in control plants grown at 360 ppm CO2, prior to a cutting event. Following the cutting, which effectively reduced the source:sink ratio of the plants, leaf carbohydrate contents in CO2-enriched plants decreased and rubisco activities increased, completely ameliorating the photosynthetic acclimation in this species. However, at high soil nitrogen, photosynthetic acclimation to elevated CO2 did not occur. Thus, photosynthetic acclimation appears to result from the inability of plants to develop adequate sinks at low soil nitrogen, and is not necessarily induced directly by atmospheric CO2 enrichment.
In some cases, plants can effectively increase their sink strength and thus reduce the magnitude of CO2-induced acclimation by forming symbiotic relationships with certain species of soil fungi. Under such conditions, photosynthetic down regulation is not triggered as rapidly, or as frequently, by end-product feedback inhibition, as excess carbohydrates are mobilized out of source leaves and sent belowground to symbiotic fungi. Indeed, Staddon et al. (1999) reported that photosynthetic acclimation was not induced in CO2-enriched Plantago lanceolata plants that were inoculated with a mycorrhizal fungus, while it was induced in control plants that were not inoculated with the fungus. Thus, CO2-induced acclimation appears to be closely related to the source:sink balance that exists within plants, and is triggered when sink strength falls below, and source strength rises above, certain critical thresholds in a species-dependent manner.
As the CO2 content of the air slowly but steadily rises, these peer-reviewed studies suggest that grassland species may not exhibit photosynthetic acclimation if they can maintain a balance between their sources and sinks for carbohydrates at the whole-plant level. But in the event this balancing act is not initially possible, acclimation represents a beneficial secondary mechanism for ultimately achieving that balance by redistributing limiting resources away from the plant's photosynthetic machinery to strengthen its sink development and/or nutrient-gathering activities.
References
Bryant, J., Taylor, G. and Frehner, M. 1998. Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment. Plant, Cell and Environment 21: 159-168.
Davey, P.A., Parsons, A.J., Atkinson, L., Wadge, K. and Long, S.P. 1999. Does photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland species in open-top chambers. Functional Ecology 13: 21-28.
Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P. 1998. Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization. Perennial ryegrass under free-air CO2 enrichment. Plant Physiology 118: 683-689.
Roumet, C., Garnier, E., Suzor, H., Salager, J.-L. and Roy, J. 2000. Short and long-term responses of whole-plant gas exchange to elevated CO2 in four herbaceous species. Environmental and Experimental Botany 43: 155-169.
Staddon, P.L., Fitter, A.H. and Robinson, D. 1999. 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.