pierre laszlo

C3 & C4 plants

Plant of the month    (©Pierre Laszlo, all rights reserved)
C3 & C4 plants: distinct groups, what we can learn from the distinction.

In addition to absorbing nutrients from their roots, plants draw on sunlight. Their leaves harvest it. Chlorophyll is the photon-absorbing pigment. That is the familiar part of the story — although only the beginning.
The follow-up is even more interesting, but is rather involved, even complicated. Suffice to say leaves come in two types, each processing carbon dioxide absorbed from air differently.
CO2 has a single carbon atom, the C in the formula. However, the end result of the chemistry performed by leaves — indeed they amount to a chemical plant — is an assembly of six carbon atoms, namely a molecule from the family of sugars. Leaves manufacture the complex (six carbons) from the simple (a single carbon).
What does the complex product consist of? It may be glucose. It may be fructose. Those two may in turn combine into a sucrose molecule (ordinary sugar). Or glucose might self-assemble into a chain, made of identical links, a polymer. Both cellulose and starch polymers form in this manner.
In-between the one-carbon stage  and the six-carbon stage, some plant leaves — in so-called C3 plants — go through a three-carbon intermediate. Other plants — C4 plants — go instead through a four-carbon intermediate.
This momentous difference is as significant as between invertebrate and vertebrate animals. C4 plants originated in subtropical areas, while C3 plants populated a far broader range of climates. The former group includes sugarcane, sedges (such as papyrus),
corn, sorghum, millet, … The latter group includes rice, wheat, rye, potatoes, yams, barley, cassava, spinach, algae, …
Under dry, hot conditions, C4 plants outperform C3 plants in photosynthetic prowess. Sugarcane is a world record holder, with 7 % efficiency converting sunlight into carbohydrates.
However, if there is sufficient water and sun, C3 plants outperform C4 plants. There are about 260,000 known species of plants, but only 0.4 % are C4 plants. There also exist a few genera with a hybrid C3-C4 photosynthetic system.
Why should we care about the C3/C4 distinction? For one thing, it allows us to determine the nutritional habits of our distant ancestors: the Iceman found in the Alps - Indians of the American Plains - Neanderthals - Australopithecus. Such knowledge is obtained from stable isotope ratios, which distinguish C3 from C4 plants, and thus predominant C3 eaters from predominant C4 eaters. The two stable isotopes of carbon involved are the abundant 12C and the less abundant 13C. The ratio of the latter to the former is in the range 22-34 ‰ for the C3s, it is in the range 8-16 ‰ for C4s. Hence, human skeletons serve as a treasure trove of information. Bone collagen isotopic data reflects the diet of the last decade of life. Conversely, tooth enamel and dentin preserve isotopic values from the first years of life.
What strikes me is how resourceful mankind has been from the very beginnings. Australopithecus africanus were highly opportunistic and adaptable in their feeding habits. They lived ca. 2.5 to 2.0 million years ago. This hominin was intensively engaged with the savanna foodweb. But the dietary variation between individuals was more pronounced than for any other early hominin or non-human primate species on record.
Likewise, we can say confidently that Late Neanderthals, in the Jonzac area of southwestern France, from approximately 55,000 to 40,000 BP (before the present), obtained their main protein sources from large herbivores, in particular bovids and horses.
In the highland Andes during the first millennium BCE, maize was only a minor component of the diet. It became prominent during later periods. In coastal Ecuador,  maize is not noticeable in diets until the end of the Valdivia period (phases 7–8, about 2000 BCE), long after the appearance of settled villages with ceramics and ceremonial architecture. It became a dietary staple only in the second half of the 1st millennium BCE.
In the southern highlands of Peru, members of the cosmopolitian yana and aclla servant classes immigrated to the site from different regions to serve the royal Inca estate at Machu Picchu. Their diets were highly varied as children:  many got a majority of dietary energy from C4  sources such as maize or kiwicha (amaranth, a C4 grain) ; others consumed mostly C3  energy sources such as legumes, quinoa, tubers and other vegetables; some had mixed C3 /C4  energy sources.
Late Prehistoric cave dwellers in present-day Virginia had a diet of primarily C4 plant proteins (maize) and some terrestrial animals (rabbit and deer).   
Isotope profiles differentiate North American Plains Indians, the Lower Brule reservation Sioux of 1892, and the Blackfoot reservation in 1892 and 1935. The resultant dietary profiles indicate a higher consumption of meat by the Blackfoot and a higher consumption of maize (or of animals that had fed on maize or other C4 plants) by the Lower Brule. The two Blackfoot groups show similar isotopic profiles despite the passage of four decades, pointing to the importance of cultural preferences, even as food sources change.
Why do we want to know what our long-gone ancestors ate? Our interest in human history and prehistory is a quest for our roots. We have a need to ascertain both the continuities — recourse to maize, for instance, as an important foodstuff in the Americas; and the discontinuities — whether cultural, such as cannibalism, or natural, such as those brought about by changes in climate, such as the Ice Ages.
And what about the current age, so-called Anthropogene, that is causing drastic reduction of biodiversity and global warming? Which plants, C3 or C4, will manage to adapt to increased temperatures? That is just one among many questions.