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 might these biochemical processes be affected by the rising CO2 content of the air? And what are the implications of any potential changes in leaf content and/or activity of rubisco?
Voluminous experimental data demonstrate that atmospheric CO2 enrichment favors carboxylation over oxygenation, thereby increasing photosynthetic rates while concomitantly reducing photorespiratory rates. The rising CO2 content of the air thus 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 (Stylinski et al., 2000; Beerling et al., 1998), exhibit some degree of photosynthetic acclimation or downregulation, which is typically characterized by reduced amounts of rubisco (Gleadow et al., 1998) and/or decreases in its activation state (Hamerlynck et al., 2002; Kubiske et al., 2002). In nearly every reported case of CO2-induced photosynthetic acclimation, however, net photosynthetic rates displayed by CO2-enriched plants have always been significantly greater than those exhibited by plants growing at ambient CO2 concentrations (Murray et al., 2000).
In the study of Tjoelker et al. (1998), for example, a 210-ppm increase in the air's CO2 content increased the average rate of net photosynthesis in aspen and birch seedlings by 57% in spite of inducing a 24% reduction in foliar rubisco content. Similarly, Takeuchi et al. (2001) reported that aspen seedlings grown at an atmospheric CO2 concentration of 560 ppm exhibited photosynthetic rates in upper-canopy leaves that were 26% greater than those displayed by upper-canopy leaves of control plants grown in ambient air in spite of a 28% decrease in foliar rubisco concentrations. Other reports of CO2-induced reductions in foliar rubisco contents that did not offset CO2-induced photosynthetic enhancements have been reported in oak (Blaschke et al., 2001) and pine (Turnbull et al., 1998) trees.
In another manifestation of photosynthetic acclimation, Centritto et al., 1999 observed elevated CO2 to reduce the activity of rubisco without negating CO2-induced increases in photosynthesis; while Centritto and Jarvis (1999) reported that twice-ambient atmospheric CO2 concentrations reduced rubisco activity in Sitka spruce needles by 36% while simultaneously enhancing photosynthetic rates by 62%. Similarly, Turnbull et al., (1998) reported that pine seedlings grown at 650 ppm CO2 displayed 40% reductions in rubisco activity while exhibiting photosynthetic rates that were 31% greater than those observed in ambiently-grown control seedlings. Rey and Jarvis (1998) have also documented this phenomenon in young silver birch trees.
These observations demonstrate the reduced need for nitrogen investment in rubisco in woody species growing in CO2-enriched environments, which phenomenon gives them the opportunity to reallocate some of the resultant "surplus" nitrogen to other processes required for optimal growth and development without compromising enhanced carbon gains via photosynthesis.
References
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Blaschke, L., Schulte, M., Raschi, A., Slee, N., Rennenberg, H. and Polle, A. 2001. Photosynthesis, soluble and structural carbon compounds in two Mediterranean oak species (Quercus pubescens and Q. ilex) after lifetime growth at naturally elevated CO2 concentrations. Plant Biology 3: 288-297.
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