C4-plants

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

FIGURE 22.30 Essential features of the coinpartinenCation and biochemistry of die Hatch-Slack padiway of carbon dioxide uptake in C4 plants. Carbon dioxide is fixed into organic linkage by PEP carboxylase of meso-phyll cells, forming OAA. Eidier malate (die reduced form of OAA) or aspartate (the ami-iiated form) serves as die carrier transpordiig CO9 to the bundle slieadi cells. Within die bundle slieadi cells, CO9 is liberated by decar-boxyladon of malate or aspartate die C-3 product is returned to die mesophyll cell. 

Collatz, G. J., Ribas-Carbo, M. and Berry, J. A. (1992). Coupled photosynthesis-stomatal conductance model for leaves of C4 plants, Aust.. Plant Physiol. 19,519-538. 

The major sources of variation in 8 C are the two C4 plants, maize and sugar cane. Most other foods are C3 and animals used for food were mainly feeding on C3 plants. Maize and sugar cane (in the form of molasses) are found... 

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... 

Table 2.2. Carbon isolope values in collagen of modern and ancicni fauna wiih C3 plant diets. The isotope values for ancient armadillo were not included in the average since they dearly consumed some C4 plants.

The following differences have been observed in modern mammals from cold and temperate areas with no C4 plants a slight enrichment in C in collagen between herbivores and carnivores a clear difference in carbonate hydroxylapatite 8 C values between herbivores and carnivores (including bears), and thus a difference between A Cvalues of herbivores and carnivores a clear enrichment in N between herbivore and carnivore bone collagen. 

Tieszen 1991). Water stress, for instance, lowers photo synthetic discrimination against C, resulting in isotopic enrichment. C4 plants are not affected isotopically by environmental parameters although their distributions are (Ehleringer et al. 1997). Under conditions of increasing aridity, the climate variable of most likely importance in African sites, C3 plants should become slightly more enriched on average. Therefore one should be cautious about use... 

Marino, B.D. and McElroy, M.B. 1991 Isotopic composition of atmospheric COj inferred from carbon in C4 plant cellulose. Nature 349 127-131. 

Some paleodiet studies, particularly of consumers of C4 plants such as maize, provide an interesting contrast with the expectations of this model. For... 

Elevated CO C4 plant compared with C, plant Bermuda grass (Cynodon dactylon) Perennial ryegrass. Wheat (Triiicuni aesti-viim). 40. 41... 

More than 95% of the plants of the world follow the C3path, whereby the carbon dioxide is initially incorporated, by the plants, into intermediate compounds made up of three atoms of carbon - which is the reason these plants are known as C3 plants. Only about 1% of all plants, including maize, millet, sorghum, and sugarcane, follow the C4 path, incorporating carbon dioxide into intermediate compounds made up of four atoms of carbon - which is the reason these plants are known as C4 plants. The remaining 4% of plants, mostly succulents (cacti), follow the third, CAM, path, which has no bearing on the discussion that follows. 

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. 

C4 plants Plants as, for example, maize, in which during the photosynthesis process, a stable compound consisting of four bonded carbon atoms is first formed during the fixation of carbon derived from carbon dioxide, in the plant less than 1% of the earth s plant species can be characterized as C4 plants see C3 and CAM plants. cable Cordage over 2.5 cm in diameter. 

In contrast to the exterior localization of cutin, suberin can be deposited in both external and internal tissues. External deposition occurs in the periderm of secondary roots and stems and on cotton fibers, whereas internal deposition occurs in the root endodermis and the bundle sheath of monocots. The Casparian strip of the root en-dodermis contains suberin, which produces a barrier isolating the apoplast of the root cortex from the central vascular cylinder. Suberin also produces a gas-impermeable barrier between the bundle sheath and mesophyll cells in C4 plants. The bark of trees contains periderm-derived cork cells that have a high suberin content. 

An important benefit of the isotopic approach is that below-ground biomass values are not required. The 13C natural abundance isotopic carbon budget approach is based on C3 plants having lower 13C value than C4 plants (Ehleringer 1991 Clay et al. 2006) and that the signatures can be tracked by placing C3 plant residue into a soil derived from C4 plants or vice versa. In these calculations, several definitions are needed. These definitions include... 

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