C3 and C4 plants

Where CO2 in the free atmosphere has a 5 C value of-7%o, C3 and C4 plants are anticipated to have 5 C values of about -26.5%o and -12.5%o respectively (van der Merwe 1989) archaeological maize, however, typically averages -9.5%o(Schwarcz et al. 1985). The isotopic values of modern maize and C3 plant foods in Mesoamerica (Wright 1994 203-206), after correction for the Industrial Effect, average -9.6%o and -26.4%o respectively. Since herbivore collagen is typically enriched by +5%o relative to the diet (van der Merwe 1989), animals from this region with a pure C3 plant diet should... 

When plants are consumed as food by herbivorous animals, the isotopic signatures in the plants are passed on to the consumers. Therefore, provided the isotopic signatures of C3 and C4 plants are known, determining the isotopie signatures in the tissues of herbivorous animals enables one to determine the relative amounts of C3 and C4 plants that the animals consumed as food, and to reconstruct their diets. Moreover, since carnivorous and omnivorous animals, including humans, feed on herbivorous animals as well as on plants, determining the isotopic signatures of the isotopes of carbon in tissues of ancient animals and humans makes it possible to elucidate the components of their diets. 

An alternative approach is to combine models with field measurements to assist in developing carbon budgets (Huggins et al. 1998). Clay et al. (2005) used first-order models to calculate the amount of residue returned to the soil from C3 and C4 plants over an 8-year period. Based on the mineralization rates and when the C3 and C4 residues were returned, the 813C signature of non-harvested biomass was determined. Based on the rates, carbon turnover, the amount of SOC mineralized, and the amount of fresh biomass incorporated into the SOC over an 8-year period were determined. 

Fig. 2.10 Histogram of 5 C-values of C3 and C4 plants (after Ceding and Harris, 1999)...

The reasons for isotope discrimination are isotope effects which are caused by both kinetic and thermodynamic factors. Especially the kinetic isotope effect during primary C02-fixation in photosynthesis is relevant for the source-specific discrimination of compounds from C3 and C4 plants. 

Naturally occurring stable isotopes of C, N, and S have been used extensively for over a decade as direct tracers of element cycling in marine and terrestrial food webs (34-39). Carbon and sulfur isotopes fractionate very little between food and consumer thus their measurement indicates which primary producers or detrital pools are sources of C and S for consumers. For example, a study of plants and animals in Texas sand dunes showed that insect species had 813C values either like those of C3 plants or like those of C4 plants (-27 and -13%o, respectively). Rodent species had intermediate values near -20%o that indicated mixed diets of both C3 and C4 plants (40). The 13C measurements, used in simple linear mixing models, proved to be quick and reliable indicators of which plant sources provided the carbon assimilated by higher trophic levels. 

Matsuoka, M. Hata, S. (1987). Comparative studies of phosphoeno/pyruvate carboxylase from C3 and C4 plants. Plant Physiology 85, 947-51. 

Stable carbon isotopes have been commonly used to distinguish between allochthonous versus autochthonous organic carbon inputs to estuaries. One of the most important pieces of information that can be gathered from this information is the delineation between C3 and C4 plant inputs. 

Leaney, F.W., Osmond, C.B., Allison, G.B., and Ziegler, H. (1985) Hydrogen-isotope composition of leaf water in C3 and C4 plants its relationship to the hydrogen-isotope composition of dry matter. Planta 164, 215-220. 

Differences between C3 and C4 Plants The plant genus Atriplex includes some C3 and some C4 species. From the data in the plots below (species 1, upper curve species 2, lower curve), identify which is a C3 plant and which is a C4 plant. Justify your answer in molecular terms that account for the data in all three plots. 

Watkins, N. K., Fitter, A. H., Graves, J. D. Robinson, D. (1996). Quantification using stable carbon isotopes of carbon transfer between C3 and C4 plants linked by a common mycorrhizal network. Soil Biology and Biochemistry, 28, 471-7. 

In Equation 8.27, Vmax and, to some extent, Kcch depend on the photosynthetic photon flux (PPF), temperature, and nutrient status. For instance, Vmax is zero in the dark because photosynthesis ceases then, and it is directly proportional to PPF up to about 50 jimol m-2 s-1. If we continually increase the PPF, Fmax can reach an upper limit, its value for light saturation. This usually occurs at about 600 junol m-2 s-1 for most C3 plants, whereas photosynthesis for C4 plants is generally not light saturated even at full sunlight, 2000 pmol m-2 s-1 (see Chapter 6, Section 6.3D for comments on C3 and C4 plants also see Fig. 8-20 for responses of leaves of C3 plants and a C4 plant to PPF). Photosynthesis is maximal at certain temperatures, often from 30°C to 40° C. We note that Vmax increases as the leaf temperature is raised to the optimum and then decreases with a further increase in temperature. 

Figure 8-15. Carboxylase reactions and locations for the three photosynthetic pathways (a) C3, (b) C4, and (c) Crassulacean acid metabolism (CAM). The reactions for C3 and C4 plants occur during the daytime. The indicated decarboxylations of C4 acids occur in the cytosol of bundle sheath cells for C4 plants and the cytosol of mesophyll cells for CAM plants.

Figure 8-18. Dependence of net CO2 uptake on external C02 level for leaves of representative C3 and C4 plants. C3 plants require a higher Noo2at tlie 02 compensation point (Jco2= 0) and for C02 saturation than C4 plants. We note that because photosynthesis for C4 plants is already nearly saturated at current atmospheric CO2 levels, higher C02 levels generally will not substantially enhance their photosynthetic rates, whereas the increasing atmospheric C02 levels will progressively increase net C02 uptake for C3 plants.

If we reduce the amount of light incident on a leaf from the value for direct sunlight, we eventually reach a PPF for which there is no net CO2 uptake (Fig. 8-19). This PPF for which Jcch is zero is known as the light compensation point for photosynthesis. Because photorespiration depends on photosynthetic products, both photorespiration and gross photosynthesis decrease as the PPF is lowered. Hence, the light compensation point for leaves is approximately the same for C3 and C4 plants—at 20° C and 380 pmol CO2 mol-1 in the turbulent air near a leaf, light... 

We will next recapitulate some of the previously introduced characteristics of C3 and C4 plants. After examining the influence of stomata on maximal photosynthetic rates under optimal conditions, we will predict effects on WUE for elevated levels of atmospheric CO2, as have occurred in the past and are also currently occurring. 

At 30°C and for an absorbed PPF up to about 100 jimol m-2 s-1, leaves of C3 and C4 plants can have a similar quantum yield (approximately 0.053 mol CCh/mol photons at an of 325 pmol mol-1 Fig. 8-20 Ehleringer and Bjorkman, 1977 see Chapter 4, Section 4.4B for a definition of quantum yield). As the temperature is raised, however, photorespiration increases relative to photosynthesis, so the quantum yield declines for C3 plants but is essentially unchanged for C4 plants. On the other hand, lowering the ambient O2 level raises the quantum yield for C3 (photorespiring) plants because the oxygenase activity of Rubisco (see Fig. 8-13) is then suppressed such changes have little effect on C4 plants until the 02 level falls below about 2%, where mitochondrial respiration is affected. 

Figure 8-24. Relation between stomatal C02 conductance ( co2 anc net COz uptake for various categories of C3 and C4 plants under optimal conditions and a jV of 360 [xmol C02 mol-1. Ignoring boundary layer effects as a first approximation, note that each line can be represented by Jco2= co2 co2 so tlie slope equals the drop in 0O2 mole fraction across the stomata, which differs for the two photosynthetic types.

Graph B illustrates how the photosynthetic activity of C3 and C4 plants varies with CO2 concentration when temperature (30°C) and light intensity (high) are constant. 

Flanagan L. B., Bain J. F., and Ehleringer J. R. (1991) Stable oxygen and hydrogen isotope composition of leaf water in C3 and C4 plant-species under field conditions. Oecologia 88(3), 394-400. 

Lin G. H. and Ehleringer J. R. (1997) Carbon isotopic fractionation does not occur during dark respiration in C3 and C4. Plant Physiol. 114(1), 391-394. 

Another complication results from unknown year-to-year variations in the photosynthesis of C3 and C4 plants (because these two types of plants discriminate differently against the heavier isotope). C4 plants discriminate less than C3 plants and leave a signal that looks oceanic, thus confounding the separation of land and ocean exchanges. These uncertainties of the approach are most troublesome over long periods (Battle et al., 2000) the approach is more reliable for reconstructing interannual variations in sources and sinks of carbon. 

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