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Untangling C3 and C4 plants

What is the difference between these two plant types, and how does it affect the Prairies?

Recent research articles have reported that scientists are making progress converting C3 plants to C4 plants. Other articles say that C3 plants will thrive with more CO2 in the air. What’s the difference between C3 and C4 plants? What does this mean for Western Canadian agriculture?

C3 and C4 plants

Plants in Western Canada — crops and weeds — can be divided into two main categories (C3 and C4) based on the way they use carbon dioxide (CO2). The difference is complicated, but interesting. It explains why kochia does better in warm weather, and why scientists are trying to find a way to make rice a C4 plant instead of a C3 plant.

Explained in the most basic way and ignoring many complicated details, C4 plants process CO2 in a more complex way, and can process more CO2 when it’s hot and dry than C3 plants. C3 plants are more common, and are referred to as cool environment plants. These include wheat, canola, flax and soybeans. About 85 per cent of plant species are C3 plants. “C4 plants have an additional metabolic process over and above what C3 plants have that allows them to utilize carbon dioxide to a lower concentration,” says Clark Brenzil, Saskatchewan’s provincial weed specialist.

Researchers think C4 plants probably evolved after C3 plants. C4 plants include corn, kochia, green foxtail, pigweed, lamb’s quarters and barnyard grass. These plants thrive under hot, dry conditions but are not as competitive in wet years.

When plants are under stress due to dry conditions, they typically close up their stomata — the pores in the leaves that allow gas exchange and moisture to exit. The stomata drive the process where the plant sucks up water from the ground and moves it up through the plant to the top of the leaves (transpiration). Water vapour evaporates through stomata at the top of the leaves, pulling more water up through the plant to keeping fueling the process.

“When plants are under moisture stress, they will close down those stomata as a way to prevent moisture loss…” says Brenzil. “As a result, they restrict the amount of carbon dioxide that can enter the leaf as well. Typically, the C3 plant will draw down the CO2 to a particular level and then stop photosynthesizing, so it’s kind of stuck, it doesn’t have any more raw material — which is the CO2 — to run the process.”

C4 plants have a process that allows them to use CO2 down to a much lower level than C3 plants. “That means that C4 plants are typically better adapted to hot weather and drought just because of that simple process of closing off the stomata and restricting gas exchange,” says Brenzil.

Adapting to climate change

With increasing levels of CO2 in the atmosphere, C4 plants are certainly not going to be at a disadvantage under warmer, potentially moisture-limiting conditions.

In a carbon rich environment, however, the C4 mechanism is a drag on the plants’ metabolism — it takes more energy to run the more complicated process. Given adequate growing conditions and cooler or moist conditions, a C3 plant is more competitive than a C4 plant. “We find that if we have nice, lush growing conditions and moderate temperatures, so it’s not too hot, the C3 plants are just cranking along like crazy,” says Brenzil. “Everything being equal, if you raise the CO2 levels the C3 plants will theoretically benefit from increased CO2 more than C4 plants.”

Most of the crops grown on the Prairies, including wheat, barley, canola, flax and soybeans are C3 plants. Corn and sorghum are C4 plants, which is why heat units are important to these crops. “Corn and sorghum are tropical grasses essentially, so they will do better under really hot conditions and produce more, so more heat, the better the production,” says Brenzil.

Glyphosate resistance

A recent research review suggests that, because, to put it simply, C4 plans have a more complicated photosynthesis process than C3 plans, it would be harder for C4 plants to develop resistance to glyphosate than C3 plants. While there may be some merit to this, Brenzil is not completely convinced.

“Remember that plants have a few tricks up their sleeve when it comes to developing glyphosate resistance. There are several different ways that plants can evolve with glyphosate that are multi-system and multi-genetic processes that don’t necessarily relate to that specific photosynthetic pathway in the plant,” says Brenzil. “They get around it in other ways. Mother Nature is pretty inventive about how she gets around barriers.”

There are many other pathways that plants use to develop resistance to glyphosate and other herbicides, including creating barriers to uptake or translocation, or isolating the glyphosate molecule by pumping it into the vacuole — basically the garbage can of the cell for unwanted materials. “One of the adaptations that has been documented is that that cell becomes adept at moving glyphosate across the membrane into the vacuole. There’s a protein system on the outer membrane that is referred to as a pump, and it allows the herbicide to go across the membrane, but it won’t go back out of the cell, so it’s a one-way gate,” says Brenzil. “That gate gets transferred to the vacuole as well, so no sooner does the glyphosate get into the cell, it goes into the vacuole and is tied up.”

Converting C3 plants to C4 plants

The C4 Rice Consortium, led by Oxford University in the United Kingdom, has been trying to make a C4 version of rice, which is a C3 plant. The project goal is to find the genes responsible for the C4 photosynthesis process and incorporate them into a rice plant, to make it more productive under drier conditions.

“It is an interesting idea and it might be one of the adaptive mechanisms that we use to deal with the potential for more variable weather as a result of climate change,” says Brenzil. “In rice, that’s important because rice is a plant that, especially in Asia, tends to grow in wet areas.”

The C4 advantage under dry conditions could make C4 weeds more of a problem in dry, hot conditions, but generally on the Prairies, says Brenzil, they aren’t that competitive.

“Unless you’re in those conditions where hot temperatures or dry climate are something you regularly experience, there’s no real advantage to the C4 mechanism,” says Brenzil. “We find that when things are lush here that C4 weeds tend not to be as big a problem. As a general rule, if you have a good competitive crop, things like kochia, green foxtail, pigweed, lamb’s quarters and barnyard grass, which are all C4s, tend not to be very competitive. If you can get that cool season crop up and ahead of C4 weeds, they tend to not to be big problems for yield loss.”

About the author

Contributor

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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