Rubisco is the primary carboxylating enzyme used by C3 plants during photosynthesis to incorporate CO2 into sugars needed for growth and development. Even C4 and CAM plants, which use PEP-carboxylase as their primary carboxylating enzyme, utilize rubisco during subsequent secondary CO2 assimilation events. Thus, rubisco is universally present in all of earth's vegetation and is, in fact, the most abundant plant enzyme on the planet, comprising up to 40 to 50% of total foliage protein. Hence, rubisco represents an enormous sink for nitrogen and other valuable resources within plants.
Rubisco, however, is a bifunctional enzyme that also possesses oxygenation activity; and when oxygenation reactions occur, photorespiration is enhanced, resulting in an increased loss of carbon from plant tissues. Thus, CO2 and O2 compete for active sites on rubisco to drive photosynthesis and photorespiration, respectively. How, then, might these biochemical processes be affected by the rising CO2 content of the air? And what are the implications of any potential changes in plant content and/or activity of rubisco?
Voluminous experimental data demonstrate that atmospheric CO2 enrichment favors carboxylation over oxygenation, thereby increasing photosynthetic rates with concomitant reductions in photorespiratory rates. The rising CO2 content of the air thus invariably leads to greater rates of net photosynthesis and a more efficient process of carbon fixation. Hence, less rubisco is needed to obtain the carbon required for plant growth and development under CO2-enriched conditions.
As a consequence of these facts, plants grown in elevated CO2 environments often, but not always (Ziska et al., 1999), exhibit some degree of photosynthetic acclimation or downregulation, which is typically characterized by reduced amounts of rubisco and/or decreases in its activation state. However, in nearly every reported case of CO2-induced photosynthetic acclimation, net photosynthetic rates displayed by CO2-enriched plants were still significantly greater than those exhibited by plants growing at ambient CO2 concentrations. In this summary, we thus review the results of some recent studies of the photosynthetic acclimation induced by elevated CO2 concentrations in grassland species.
In the study of Cheng et al. (1998), the authors grew the herbaceous plant Arabidopsis thaliana at an atmospheric CO2 concentration of 1,000 ppm for 40 days and reported that elevated CO2 reduced foliar rubisco contents by 34%. Nonetheless, foliar contents of glucose and fructose were enhanced more than 2-fold by elevated CO2, while starch concentrations were increased more than 3.5-fold. Thus, although elevated CO2 reduced the amount of rubisco in leaves, photosynthetically-derived sugars and starch still accumulated to tremendous values.
When Midgley et al. (1999) grew four Leucadendron species from South Africa in twice-ambient CO2 concentrations, they observed a 30% reduction in the activity of rubisco. However, rates of net photosynthesis in the CO2-enriched plants were still about 40% greater than rates measured in ambiently-grown plants. Bryant et al. (1998) reported similar results for chalk grassland species exposed to an atmospheric CO2 concentration of 600 ppm for 14 months. In their study, the authors noted how elevated CO2 caused an average reduction in rubisco activity of 32% in two forbs and one C3 grass, while still maintaining photosynthetic rates that were about 28% greater than those observed in ambiently-growing plants. Likewise, after growing three grasslands species from the United Kingdom for two full years at 700 ppm CO2, Davey et al. (1999) reported that elevated CO2 reduced rubisco activity by an average of 27% and increased photosynthetic rates from 12 to 74% in a nutrient-dependent manner.
Finally, in the interesting study of Rogers et al. (1998), swards of perennial ryegrass grown in air containing an extra 240 ppm of CO2 did not exhibit any reductions in rubisco content as long as they were supplied with high levels of soil nitrogen. In contrast, at low soil nitrogen contents, CO2-enriched plants displayed a 25% reduction in rubisco levels prior to mechanical cutting. After cutting, however, which removed a large portion of leaf area, CO2-enriched plants in low nitrogen completely reversed their acclimation response and increased their levels of rubisco to facilitate greater carbon uptake to repair the damage.
The bottom line of these several observations is that the reduced need for nitrogen investment in leaf rubisco in plants growing in CO2-enriched environments gives them the opportunity to reallocate some of this "surplus" nitrogen to other limiting processes required for optimal growth and development without compromising enhanced carbon gains via photosynthesis.
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.
Cheng, S.-H., Moore, B.D. and Seemann, J.R. 1998. Effects of short- and long-term elevated CO2 on the expression of ribulose-1,5-bisphosphate carboxylase/oxygenase genes and carbohydrate accumulation in leaves of Arabidopsis thaliana (L.) Heynh. Plant Physiology 116: 715-723.
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.
Midgley, G.F., Wand, S.J.E. and Pammenter, N.W. 1999. Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment. Journal of Experimental Botany 50: 533-542.
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.
Ziska, L.H., Sicher, R.C. and Bunce, J.A. 1999. The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates. Physiologia Plantarum 105: 74-80.