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Source. Henderson et al. 1992.

On the path from intercellular spaces to Rubisco a number of additional steps take place, where some isotope fractionation can occur. Taken together, the isotope effect in C3 plants is approximated by the empirical equation (Farquhar et al. 1982):

where Ra and Rp are the molar abundance ratios of the atmospheric CO2 and of the C fixed by the plant, respectively; the symbols Ca and Ci are the atmospheric and the intercellular partial pressure of CO2, respectively. The value 1.027 is an empirical value, incorporating the major fractio-nation by Rubisco, as well as accounting for the internal diffusion resistance for CO2 (gm).

Since values for Ra/Rp appear rather "clumsy," data are commonly expressed as fractionation values, A ("capital delta''), defined as (Ra/Rp - 1) x 1000, or:

A = [(1.0044 Ca - 1.0044 Ci + 1.027 Q)/Ca]- 1 = [(1.0044 Ca + 0.0226 Ci)/Ca]- 1 (3)

The isotope composition is described as fi13C ("lower case delta''):

where Ssource ffi -8% if the source is air (Sair) (to be entered as -0.008 in Equation (5); a S value of -27%, therefore, converts to a A value of 19.5%). The standard is a cretaceous limestone consisting mostly of the fossil carbonate skeletons of Belemnitella americana (referred to as PDB-belemnite). By definition, it has a S13C value equal to 0%. Plant S13C values are negative, because they are depleted in 13C compared with the fossil standard. Diffusion and carboxylation discriminate against 13CO2; S-values for C3 plants are approx. -27%, showing that Rubisco is the predominant factor accounting for the observed values and that diffusion is less important.

For C4 plants, the following empirical equation has been derived:

where f refers to the leakage of CO2 from the bundle sheath to the mesophyll.

Where do these equations lead us? Within C3 plants the fi13C of whole-plant biomass gives a better indication of Ci over a longer time interval than can readily be obtained from gas-exchange measurements. The value of Ci in itself is a reflection of stomatal conductance (gs), relative to photosynthetic activity (A). As such, fi13C provides information on a plant's water-use efficiency (WUE) (Sect. 5.2). How do we arrive there? As can be derived from Equation (3), the extent of the fractionation of carbon isotopes depends on the intercellular partial pressures of CO2, relative to that in the atmosphere. If Ci is high, gs is large relative to A, and much of the 13CO2 discriminated against by Rubisco diffuses back to the atmosphere; hence the fractiona-tion is large. If Ci is low, then relatively more of the accumulated 13CO2 is fixed by Rubisco, and therefore the fractionation of the overall photosynthesis process is less. Comparison of WUE calculated on the basis of fi13C is only valid at constant vapor pressure difference (Aw) and is called intrinsic WUE (A/gs).

Box 2A.2 Continued

Under many situations fi13C is a good proxy for WUE and it can be used for, e.g., paleoclimatic studies and genetic screening for drought-tolerant varieties. However, under conditions where Aw varies or gs and gm are not strongly correlated, fi13C may not be a good predictor of WUE.

Carbon-isotope fractionation values differ between C3, C4 and CAM species (Sects. 9 and 10). In C4 plants, little of the 13CO2 that is discriminated against by Rubisco diffuses back to the atmosphere. This is prevented, first, by the diffusion barrier between the vascular bundle sheath and the mesophyll cells. Second, the mesophyll cells contain PEP carboxylase, which scavenges most of the CO2 that escapes from the bundle sheath (Table 1). Fractionation during photosynthesis in C4 plants is therefore dominated by fractionation during diffusion (4.4%). There is also little fractionation in CAM plants, where the heavy isotopes discriminated against cannot readily diffuse out of the leaves because the sto-mata are closed for most of the day. The actual fi13C of CAM plant biomass depends on the fractions of the carbon fixed by CAM and C3 photosynthesis.

Aquatic plants show relatively little fractiona-tion, due to unstirred layers surrounding the leaf, rather than to a different photosynthetic pathway (Sect. 11.6). The unstirred boundary layers cause diffusion to be a major limitation for their photosynthesis, so that fractionation in these plants tends toward the value found for the diffusion process (Fig. 1).

Figure 1. The relationship between the ratio of the internal and the atmospheric CO2 concentration, at a constant Ca of 340 mmol mol-1. Data for both C3 and C4 species are presented; the lines are drawn on the basis of a number of assumptions, relating to the extent of leakage of CO2 from the bundle sheath back to the mesophyll (Evans et al. 1986, Australian Journal of Plant Physiology 13: 281-292). Copyright CSIRO, Australia.

Figure 1. The relationship between the ratio of the internal and the atmospheric CO2 concentration, at a constant Ca of 340 mmol mol-1. Data for both C3 and C4 species are presented; the lines are drawn on the basis of a number of assumptions, relating to the extent of leakage of CO2 from the bundle sheath back to the mesophyll (Evans et al. 1986, Australian Journal of Plant Physiology 13: 281-292). Copyright CSIRO, Australia.

substantially lower than Ci(the CO2 concentration in the intercellular spaces); a difference of about 80 mmol mol-1is common, as compared with Ca-Ci of about 100 mmol mol-1. The mesophyll conductance varies widely among species and correlates with the photosynthetic capacity (Amax) of the leaf (Fig. 7). Interestingly, the relationship between mesophyll conductance and photosynthesis is rather similar for scleromorphic and mesophytic leaves, but scler-omorphs tend to have a somewhat larger draw-down of CO2 between intercellular space and chloroplast (Ci-Cc) (Warren & Adams 2006).

The mesophyll conductance is a complicated trait, involving diffusion of CO2 in the intercellular spaces in the gas phase, dissolving of CO2 in the liquid phase, conversion of CO2 into HCO3~ catalyzed by carbonic anhydrase, and diffusion in the liquid phase and across membranes. The resistance in the gas phase is low and is considered as normally not a limiting factor (Bernacchi et al. 2002). Diffusion in the liquid phase is much slower (104 times less), and the path length is minimized by chloroplast position against the cell wall opposite intercellular spaces (Fig. 1). This component likely represents a large fraction of total rm, and carbonic anhydrase is important for minimizing it (Gillon & Yakir 2000). Evidence for an important role for the area of chlor-oplasts bordering intercellular spaces as a determinant of gm stems from a positive relationship with this parameter per unit leaf area (Evans & Loreto 2000). Data about a similar parameter, chloroplast area per leaf area, are more widely available and vary by an order of magnitude among species (Table 1) which is likely associated with gm. There

Figure 7. The relationship between the rate of photosynthesis (An) and maximum mesophyll conductance (gm), determined for a wide range of species. Values for scleromorphic leaves are at most 0.21 mol m-2 s-1 bar-1 (gm) and 22.9 mmol m-2 s-1 (An), whereas those for mesomorphic leaves span the entire range shown here. The units of conductance as used in this graph differ from those used elsewhere in this text. The reason is

Figure 7. The relationship between the rate of photosynthesis (An) and maximum mesophyll conductance (gm), determined for a wide range of species. Values for scleromorphic leaves are at most 0.21 mol m-2 s-1 bar-1 (gm) and 22.9 mmol m-2 s-1 (An), whereas those for mesomorphic leaves span the entire range shown here. The units of conductance as used in this graph differ from those used elsewhere in this text. The reason is that when CO2 is dissolving to reach the sites of carbox-ylation, the amount depends on the partial pressure of CO2 and conductance has the units used in this graph. For air space conductance the units could be the same as used elsewhere: mol m-2 s-1, if CO2 is given as a mole fraction (based on data compiled in Flexas et al. 2008). Courtesy, J. Flexas, Universitat de les Illes Balears, Palma de Mallorca, Balears, Spain.

is evidence that specific aquaporins facilitate transport of CO2 across membranes. Their role in the transport of CO2 might account for rapid modulation of gm in response to environmental factors such as temperature, CO2, and desiccation (Flexas et al. 2006a). The mesophyll conductance is proportional to chloroplast surface area within a given functional group. The difference in gm between functional groups is associated with mesophyll cell wall thickness, which varies from 0.1 mm in annuals, 0.2-0.3 mm in deciduous, broad-leaved species, and 0.3-0.5 mm in evergreen, broad-leaved species (Terashima et al. 2006).

When stomatal and mesophyll conductance are considered in conjunction with the assimilation of CO2, the ''supply function'' (Equation 1) tends to intersect the ''demand function'' in the region where carbox-ylation and electron transport are co-limiting (Fig. 6).

Table 1. The area of the chloroplast in palisade (P) and spongy (S) mesophyll (Areachior) expressed per unit leaf area (Arealeaf) for species from the mountain range of the East Pamirs, Tadjikistan (3500-4500 m).*

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