Maize for the Gods by Blake Michael;

Author:Blake, Michael;
Language: eng
Format: epub
Publisher: University of California Press
Published: 2017-11-19T05:00:00+00:00

STABLE CARBON ISOTOPES AND PLANT METABOLISM

Most of us don’t realize that when we eat maize, or anything made out of maize—such as corn tortilla chips, soda drinks made with high fructose corn syrup, and beef raised on maize—we are loading our bodies with more of a slightly heavier “variety,” or isotope, of carbon than we would normally take in if we were eating other common foods made from wheat, rice, or potatoes. We don’t notice because this heavier carbon isotope, 13C, acts the same (and tastes the same—that is, it is completely tasteless) as the much more common 12C. As a result of the particular way that maize and other tropical grasses in the Poaceae family metabolize carbon, they absorb slightly more of the rare heavier 13C isotope, relative to 12C, than is taken up during photosynthesis by the majority of species in the plant kingdom.

The more common type of plant metabolism—whereby plants take in nutrients and convert them into energy using photosynthesis, also called fixation—is called the C3 cycle or Calvin-Benson cycle, discovered in the 1940s.3 About 99 percent of plant species on earth (and about 95 percent of all the plant biomass) use this metabolism process—which is thought to be the ancestral type. In the 1960s biochemists discovered another kind of plant metabolism, the C4 cycle or Hatch-Slack cycle, wherein carbon is processed slightly differently in hot, arid environments in order to conserve water during transpiration.4 C4 cycle plants include many tropical grasses—for example, sugarcane, millet, sorghum, and some amaranths. Compared with C3 cycle plants, C4 cycle plants have a slightly higher ratio of 13C to 12C. Another group of plants—cacti, succulents, and their cousins—have, like C4 plants, evolved yet another metabolic process to survive and thrive in hot, arid conditions. They are collectively known as crassulacean acid metabolism plants—and their method of metabolism is called the CAM cycle. Like C4 plants they have, under certain conditions, higher ratios of 13C to 12C.5

In all three carbon cycles—C3, C4, and CAM—plants take in the proportions of the stable carbon isotopes that occur in their environments. But because of differences in the biochemical pathways for each cycle, they retain different proportions of these isotopes. In C3 cycle plants, the heavier 13C isotope is more rapidly depleted during the carbon fixation process. C4 cycle plants retain more of the 13C carbon that they take in, as do many CAM plants. As we will see below, when stable carbon ratios are measured using mass spectrometry, C3 and C4 plants can be distinguished from one another because the ratios of 13C to 12C (usually expressed as δ13C ‰) are distinctly different. C3 cycle plants have δ13C values in the -29 to -25 ‰ range, while C4 cycle plants have δ13C values in the -16 to -10 ‰ range.6 The nonoverlapping and distinct difference between these ratios extends to the consumers of these plants. People who eat strictly C3 cycle plants have body tissues that closely mirror the stable carbon values of those plants, and, likewise, people who eat only C4 plants have stable carbon values reflecting those plants.

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