Demand for CO2the CO2Response Curve

The response of photosynthetic rate to CO2 concentration is the principal tool to analyze the demand for CO2 and partition the limitations imposed by demand and supply (Warren 2007, Flexas et al. 2008) (Fig. 6). The graph giving net CO2 assimilation (An) as a function of CO2 concentration at the site of Rubisco in the chloroplast (Cc) is referred to as the An-Cc curve. With rising CO2, there is no net CO2 assimilation, until the production of CO2 in respiration (mainly photorespiration, but also some dark respiration occurring in the light) is fully compensated by the fixation of CO2 in photosynthesis. The CO2 concentration at which this is reached is the CO2-com-pensation point (r). In C3 plants this is largely determined by the kinetic properties of Rubisco,

Figure 5. Reactions and organelles involved in photorespiration. In C3 plants, at 20% O2,0.035% CO2, and 20C, two out of ten RuBP molecules are oxygenated, rather than carboxylated. The oxygenation reaction produces phos-phoglycolate (GLL-P), which is dephosphorylated to glyco-late (GLL). Glycolate is subsequently metabolized in peroxisomes and mitochondria, in which glyoxylate

Figure 5. Reactions and organelles involved in photorespiration. In C3 plants, at 20% O2,0.035% CO2, and 20C, two out of ten RuBP molecules are oxygenated, rather than carboxylated. The oxygenation reaction produces phos-phoglycolate (GLL-P), which is dephosphorylated to glyco-late (GLL). Glycolate is subsequently metabolized in peroxisomes and mitochondria, in which glyoxylate

(GLX) and the amino acids glycine (GLY) and serine (SER) play a role. Serine is exported from the mitochondria and converted to hydroxypyruvate (OH-PYR) and then glyce-rate (GLR) in the peroxisomes, after which it returns to the chloroplast (after Ogren 1984). Reprinted with kind permission from the Annual Review of Plant Physiology, Vol. 35, copyright 1984, by Annual Reviews Inc.

Figure 6. The relationship between the rate of net CO2 assimilation (An) and the CO2 concentration at the site of Rubisco in the chloroplasts (Cc) for a C3 leaf: the ''demand function''. The concentration at which An = 0 is the CO2-compensation point (r). The rate of diffusion of CO2 from the atmosphere to the intercellular spaces and to Rubisco in the chloroplast is given by the ''supply functions'' (the red and blue lines). The slopes of these lines are the leaf conductance (gL) and mesophyll

Figure 6. The relationship between the rate of net CO2 assimilation (An) and the CO2 concentration at the site of Rubisco in the chloroplasts (Cc) for a C3 leaf: the ''demand function''. The concentration at which An = 0 is the CO2-compensation point (r). The rate of diffusion of CO2 from the atmosphere to the intercellular spaces and to Rubisco in the chloroplast is given by the ''supply functions'' (the red and blue lines). The slopes of these lines are the leaf conductance (gL) and mesophyll conductance (gm), respectively. The intersection of the ''supply functions'' with the ''demand function'' is the actual rate of net CO2 assimilation at a value of Ci and Cc that occurs in the leaf intercellular spaces (Ci) and at the site of Rubisco (Cc) for Ca in normal air (indicated by the vertical line). The difference in An described by the demand function and the two horizontal lines depicts the degree of limitation imposed by the mesophyll resistance and leaf resistance.

with values for r in the range 40-50 prnol (CO2) mol-1 (air) (at 25°C and atmospheric pressure).

Two regions of the CO2-response curve above the compensation point can be distinguished. At low Cc, that is below values normally found in leaves (approximately 165 mmol mol-1), photosynthesis increases steeply with increasing CO2 concentration. This is the region where CO2 limits the rate of functioning of Rubisco, whereas RuBP is present in saturating quantities (RuBP-saturated or CO2-limited region). This part of the An-Cc relationship is also referred to as the initial slope or the carbox-ylation efficiency. At light saturation and with a fully activated enzyme (Sect. 3.4.2 for details on "activation"), the initial slope governs the carboxy-lation capacity of the leaf which in turn depends on the amount of active Rubisco.

In the region at high Cc, the increase in An with increasing Cc levels off. CO2 no longer restricts the carboxylation reaction, but now the rate at which RuBP becomes available limits the activity of Rubisco (RuBP-limited region). This rate, in turn, depends on the activity of the Calvin cycle, which ultimately depends on the rate at which ATP and

NADPH are produced in the light reactions; in this region, photosynthetic rates are limited by the rate of electron transport. This may be due to limitation by light or, at light saturation, by a limited capacity of electron transport (Box 2A.1). Even at a high Cc, in the region where the rate of electron transport, J, no longer increases with increasing Cc, the rate of net CO2 assimilation continues to increase slightly, because the oxygenation reaction of Rubisco is increasingly suppressed with increasing CO2 concentration, in favor of the carboxylation reaction. At a normal atmospheric concentration of CO2 (Ca) and O2 (ca. 380 and 210000 prnol mol-1, respectively) and at a temperature of 20°C, the ratio between the car-boxylation and oxygenation reaction is about 4:1. How exactly this ratio and various other parameters of the An-Cc curve can be assessed is further explained in Box 2A.1. Typically, plants operate at a Cc where CO2 and electron transport co-limit the rate of CO2 assimilation (i.e., the point where the Rubisco-limited/RuBP-saturated and the RuBP-limited part of the CO2-response curve intersect). This allows effective utilization of all components of the light and dark reactions.

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