C3, C4, and CAM Plants: Researching Adaptations to Climate Change

Why Climate Researchers Investigate Plant Photosynthesis Pathways

Pineapple Plantation
Can We Learn to Live on a C4 Diet of Pineapple?. Daisuke Kishi / Getty Images

C3, C4, and CAM are designations used by botanists to classify the earth's plants into categories, designations which reference the way each category absorbs and processes carbon dioxide during photosynthesis.

All plants ingest atmospheric carbon dioxide and convert it into sugars and starches through photosynthesis, but they do it in different manners: C3, C4, and CAM. The specific photosynthesis method (or pathway) used by the plant classes are variations of a set of chemical reactions called the Calvin Cycle.

Those reactions take place within each plant, affecting the number and type of carbon molecules the plant creates, the places where those molecules are stored in the plant, and, most importantly to us today, the plant's ability to withstand low carbon atmospheres, higher temperatures and reduced water and nitrogen.

These processes are directly relevant to global climate change studies because C3 and C4 plants respond differently to changes in atmospheric carbon dioxide concentration and changes in temperature and water availability. Humans are currently relying on the type of plant that does not do well under warmer, dryer, and erratic conditions: we are going to have to find some way to adapt, and changing photosynthesis processes may be one way to do that. 

Photosynthesis and Climate Change

Global climate change is resulting in increases in daily, seasonal and annual mean temperatures, and increases in the intensity, frequency, and duration of abnormally low and high temperatures.

Temperature limits plant growth and is a major determining factor in the plant distribution across different environments: since plants themselves can't move, and since we rely on plants to feed us, it would be very useful indeed if our plants were able to withstand and/or acclimate to the new environmental order.

That's what the study of C3, C4, and CAM pathways may give us.

C3 Plants

  • Plants: grain cereals rice, wheat, soybeans, rye, barley; vegetables such as cassava, potatoes, spinach, tomatoes, and yams; trees such as apple, peach, and eucalyptus
  • Enzyme: ribulose bisphosphate (RuBP or Rubisco) carboxylase oxygenase (Rubisco)
  • Process: convert CO2 into a 3 carbon compound 3-phosphoglyceric acid (or PGA)
  • Where carbon it fixed: all leaf mesophyll cells
  • Biomass rates: -22% to -35%, with a mean of -26.5%

The vast majority of land plants that we rely on for human food and energy today use the C3 pathway, and no wonder: the C3 photosynthesis process is the oldest of the pathways for carbon fixation, and it is found in plants of all taxonomies. But the C3 pathway is also inefficient. Rubisco reacts not only with CO2 but also O2, leading to photorespiration, which wastes assimilated carbon. Under current atmospheric conditions, potential photosynthesis in C3 plants is suppressed by oxygen as much as 40%. The extent of that suppression increases under stress conditions such as drought, high light and high temperatures.

Almost all of the food we humans eat is C3, and that includes almost all extant nonhuman primates across all body sizes, including prosimians, new and old world monkeys, and all the apes, even those who live in regions with C4 and CAM plants.

As global temperatures rise, the C3 plants will struggle to survive and since we are reliant on them, so will we.

C4 Plants

  • Plants: common in forage grasses of lower latitudes, maize, sorghum, sugarcane, fonio, tef, and papyrus
  • Enzyme: phosphoenolpyruvate (PEP) carboxylase
  • Process: convert CO2 into 4-carbon intermediate
  • Where carbon it fixed: the mesophyll cells (MC) and the bundle sheath cells (BSC). C4s have a ring of BSCs surrounding each vein and an outer ring of MCs surrounding the bundle sheath, known as the Kranz anatomy.
  • Biomass rates: -9 to -16%, with a mean of -12.5%.

Only about 3% of all land plant species use the C4 pathway, but they dominate nearly all grasslands in the tropics, subtropics, and warm temperate zones. They also include highly productive crops like maize, sorghum, and sugar cane: these crops lead the field for bioenergy use but are not really suitable for human consumption.

Maize is the exception, but it is not truly digestible unless it is ground into a powder; maize and the others are also used as food for animals, converting the energy to meat, which is another inefficient use of plants.

C4 photosynthesis is a biochemical modification of the C3 photosynthesis process. In C4 plants, the C3 style cycle only occurs in the interior cells within the leaf; surrounding them are mesophyll cells which have a much more active enzyme, called phosphoenolpyruvate (PEP) carboxylase. Because of this, C4 plants are those that thrive on long growing seasons with lots of access to sunlight. Some are even saline-tolerant, allowing researchers to consider whether areas which have experienced salinization resulting from past irrigation efforts can be restored by planting salt-tolerant C4 species.

CAM Plants

  • Plants: cactuses and other succulents, clusia, tequila agave, pineapple,
  • Enzyme: phosphoenolpyruvate (PEP) carboxylase
  • Process: four phases that are tied to available sunlight, CAM plants collect CO2 during the day and then fix CO2 at night as a 4 carbon intermediate
  • Where carbon it fixed: vacuoles
  • Biomass rates: can fall into either C3 or C4 ranges

CAM photosynthesis was named in honor of the plant family in which is was first documented, Crassulacean, the stonecrop family or the orpine family. CAM photosynthesis is an adaptation to low water availability, and it occurs in orchids and succulents from very arid regions. The process of chemical change can be that followed by either C3 or C4, in fact, there's even a plant called Agave augustifolia which switches back and forth between modes as the local system requires.

In terms of human use for food and energy, CAM plants are relatively unexploited, with the exceptions of pineapple and a few agave species, such as the tequila agave. CAM plants exhibit the highest water-use efficiencies in plants which enable them to do well in water-limited environments, such as semi-arid deserts.

Evolution and Possible Engineering

Global food insecurity is already an extremely acute problem, and continued reliance on inefficient food and energy sources is dangerous, especially because we do not know what might happen to those plant cycles as our atmosphere becomes more carbon rich.

The reduction in atmospheric CO2 and the drying of the Earth's climate are thought to have promoted C4 and CAM evolution, which raises the alarming possibility that elevated CO2 may reverse the conditions that favored these alternatives to C3 photosynthesis.

Evidence from our ancestors shows that hominids can adapt their diet to climate change. Ardipithecus ramidus and Ar anamensis were both C3-focused consumers. But when a climate change altered eastern Africa from wooded regions to savannah about 4 million years ago (mya), the species which survived were mixed C3/C4 consumers (Australopithecus afarensis and Kenyanthropus platyops). By 2.5 mya, two new species evolved, Paranthropus who shifted to a C4/CAM specialist, and early Homo, which used both C3/C4 foods.

Expecting H. sapiens to evolve within the next fifty years is not practical: maybe we can change the plants. Many climate scientists are trying to find ways to move C4 and CAM traits (process efficiency, tolerance of high temperatures, higher yields, and resistance to drought and salinity) into C3 plants. Hybrids of C3 and C4 have been pursued for 50 years or more, but they have yet to succeed because of chromosome mismatching and hybrid sterility: some scientists hope for success by using enhanced genomics.

Why is That Even Possible?

Some modifications to C3 plants are thought possible because comparative studies have shown that C3 plants already have some rudimentary genes that are similar in function to C4 plants. The evolutionary process that created C4 out of C3 plants occurred not once but at least 66 times in the past 35 million years. That evolutionary step achieved high photosynthetic performance and high water- and nitrogen- use efficiencies; C4 plants have twice as high photosynthetic capacity, particularly at higher temperatures as those of C3 plants, and can cope with less water and available nitrogen. For this reason, biochemists have been attempting to move C4 traits to C3 plants as a way to offset environmental changes faced by global warming.

The potential to enhance food and energy security has led to marked increases in research on photosynthesis. Photosynthesis provides our food and fiber supply, but it also provides most of our sources of energy. Even the bank of hydrocarbons that reside in earth's crust was originally created by photosynthesis. As those fossil fuels are depleted or if humans limit fossil fuel to forestall global warming, people will face the challenge of replacing the energy supply with renewable resources. Food and energy: two things humans cannot live without.

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