Isotopic fractionation of carbon during photosynthesis
The carbon atom in CO2 has two stable isotope forms, the common lighter 12C and rare heavier 13C. Plants assimilate atmospheric CO2 into tissues during the photosynthetic process and selectively incorporate CO2 with 12C. The amount of carbon isotopic fractionation is characteristic of the type of photosynthesis process used by the plant.
There are three photosynthetic pathways: C3 (Calvin), C4 (Hatch-Slack), or CAM (Crassulacean Acid Metabolism). The C3
photosynthetic process is most common, used by approximately 85% of
plants worldwide. Many major crops (e.g., cotton, rye, and wheat) are C3 plants. In general, the carbon isotope ratios of C3 plants are -30 to -22‰ (see diagram, below). On the other hand C4 plants have higher carbon isotope ratios than C3 plants: -18 to -10‰. Common C4
plants include corn, sugar cane, tropical grasses, and desert plants.
Plants using the CAM photosynthetic pathway also have generally higher
carbon isotope ratios than C3 plants.
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The analysis of a plant sample for its carbon isotope ratio allows
investigators to determine which photosynthetic pathway the plant
employs. Because herbivores incorporate C atoms from diet, the analysis
of an animal tissue likewise allows investigators to determine which
type of plants the animal consumed. Herbivores are in turn consumed by
other animals and carbon isotope analysis can thus be used to trace the
flow of organic matter through a food web.
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