Other chapters in this volume have explored carbon cycles within and among ecosystems, especially their response to the global changes that are occurring on earth today. In this chapter, the focus shifts from factors that influence carbon flux dynamics to the ways in which the composition of the atmosphere and thermal environment influence the type of photosynthetic system that predominates within a terrestrial ecosystem. In turn, the kind of photosynthetic system present has significant impacts on the distribution of the grazing animals that are dependent on primary productivity generated across the landscape, both in the short-term and over evolutionary time periods.
Three photosynthetic pathways exist in terrestrial plants: C3, C4, and CAM photosynthesis (Ehleringer and Monson, 1993). C3 photosynthesis is the ancestral pathway for carbon fixation and occurs in all taxonomic plant groups. C4 photosynthesis occurs in the more advanced plant taxa and is especially common among monocots, such as grasses and sedges, but not very common among dicots (Ehleringer et ai, 1997; Sage and Monson, 1999). CAM photosynthesis occurs in many epiphytes and succulents from very arid regions, but is sufficiently limited in distribution so that CAM plants are not an appreciable component of the global carbon cycle. Therefore, this chapter will focus on the factors influencing the dynamics of C3- and Q-dominated ecosystems.
Photosynthesis is a multistep process in which the C from CO, is fixed into stable organic products. In the first step, RuBP car-boxylase-oxygenase (Rubisco) combines RuBP (a 5C molecule) with CO, to form two molecules of phosphoglycerate (PGA, 3C molecule). However, Rubisco is an enzyme capable of catalyzing two distinct reactions: one leading to the formation of two molecules of PGA when CO, is the substrate and the other resulting in one molecule each of PGA and phosphoglycolate (PG, 2C molecule) when 02 is the substrate (Lorimer, 1981). The latter oxygenase reaction results in less net carbon fixation and eventually leads to the production of C02 in a process known as photorespiration:
The proportion of the time for which Rubisco catalyzes C02 versus 02 is dependent on the [C02]/ ratio; the reaction is also temperature-dependent, with oxygenase activity increasing with temperature. This dependence of Rubisco on the [C02]/ ratio establishes a firm link between current atmospheric conditions and photosynthetic activity. As a consequence of Rubisco sensitivity to 02, the efficiency of the C3 pathway decreases as atmospheric C02 decreases.
C4 photosynthesis represents a biochemical and morphological modification of C3 photosynthesis to reduce Rubisco oxygenase activity and thereby increase the photosynthetic rate in low-C02 environments such as we have today (Ehleringer et al, 1991; Sage and Monson, 1999). In C4 plants, the C3 cycle of the photosynthetic pathway is restricted to interior cells within the leaf (usually the bundle-sheath cells). Surrounding the bundle-sheath cells are mesophyll cells in which a much more active enzyme, PEP carboxylase, fixes C02 (but as HCO,~) into oxaloacetate, a C4 acid. The C4 acid diffuses to the bundle-sheath cell, where it is decar-boxylated and refixed in the normal C3 pathway. As a result of the higher activity of PEP carboxylase, C02 is effectively concentrated in the regions where Rubisco is located and this results in a high C02/02 ratio and limited photorespiratory activity.
When the focus is on ecosystem processes, an appropriate question to ask might be, "Why be concerned about the fact that different photosynthetic pathways exist?" There are several important global biogeogh emigal cycles in the climate system
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and clear answers to this question. First, C3 and C4 species are capable of giving quite different photosynthetic rates and primary productivity rates, even when grown under similar environmental conditions (Sage and Monson, 1999). Second, morphological and possibly defensive-compound differences between C3 and C4 species lead to differences in feeding preferences among herbivores (Caswell et al., 1973; Ehleringer and Monson, 1993; Sage and Monson, 1999). Third, photosynthetic pathways among intensively managed ecosystems, such as pastures and agricultural crops, differ in both productivity and water-use efficiency, exhibiting strong geographical tendencies that reflect climatic variations. Last, the natural distributions of both C3 and C4 species exhibit strong relationships with both atmospheric C02 and climate, suggesting that future plant distributions need not be similar to today's distributions.
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