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Tale of two grasses

Woody Lane, Ph.D., for Progressive Forage Published on 31 August 2021
photosynthesis

Photosynthesis is a little like Ben & Jerry’s ice cream – it comes in a variety of flavors. Some folks can probably recite all the ice cream flavors from memory, but how many folks know the types of photosynthesis?

So in this month’s article, rather than concentrate on ice cream, let’s concentrate on photosynthesis. Both flavors. Because the two types of photosynthesis give us two very different types of grasses: the cool-season grasses and the warm-season grasses.

Everyone is familiar with the cool-season grasses. They’re as common as vanilla and chocolate. These are species like orchardgrass, tall fescue, perennial ryegrass, wheat and oats. They all use the same type of photosynthesis. Photosynthesis is what plant leaves do for a living. Plant leaves capture the energy from sunlight and transform it into carbohydrates. Although photosynthesis is a complex chemical process, we can still list the general equation for photosynthesis:

CO2 + H2O + sunlight à carbohydrates + oxygen

We’re interested in the type of carbohydrate formed by this equation. In the leaves of cool-season grasses, photosynthesis produces a carbohydrate called “three-phosphoglyceric acid,” which contains three carbons. This three-carbon unit is then used to form six-carbon sugars which are then used for all sorts of metabolic processes. Therefore, cool-season grasses are also called C3 grasses because the first metabolic product of their photosynthesis contains three carbons.

But there is a second type of photosynthesis – a second “flavor” – which creates a different primary product. Instead of producing three-phosphoglyceric acid, this second flavor of photosynthesis combines carbon dioxide with other compounds in a slightly different pathway to form the carbohydrate “oxaloacetate” – which contains four carbons. Grasses that use this type of photosynthesis are called … drumroll here … C4 grasses.

C3 vs. C4

Here is the important point: The C4 grasses are the warm-season grasses. These grass species don’t grow well in the spring or late fall, but they thrive during the summer heat. Warm-season grasses are species like bermudagrass, sudangrass, sorghum and corn. Not only do they flourish in warm weather, but they are often capable of explosive growth under conditions of relatively low moisture and low nitrogen. This is due to the special nature of their C4 photosynthesis.

Actually, the leaves of warm-season grasses contain both the C4 and C3 types of photosynthesis. Huh? Warm-season grasses contain two types of photosynthetic cells. The C4 process in one type of cell results in a four-carbon unit (oxaloacetate), which is transported to the cells that perform the C3 process. In those C3 cells, the carbon dioxide is metabolically removed from the oxaloacetate and funneled directly into the standard C3 process.

Thus, warm-season grasses really carry out photosynthesis in two stages: The first stage (in C4 cells) feeds into the second stage (in C3 cells). Why did these grasses evolve such a convoluted two-stage system? Because of the pressures of plant competition combined with a peculiar idiosyncrasy of the C3 photosynthesis process. A key enzyme in this process (“ribulose diphosphate carboxylase”) has an unusual characteristic. This enzyme produces the standard three-carbon units of photosynthesis, but if photosynthesis occurs under low levels of carbon dioxide, this enzyme also sustains a reaction called photorespiration, which is a wasteful process that reduces the yield of carbohydrate products. Kind of the yin and yang of metabolism.

Unfortunately, the interior region of grass blades always contains low levels of carbon dioxide, and therefore cool-season grasses always suffer from some inefficiency of their photosynthesis. You may ask: If low levels of cellular carbon dioxide are normal in plant leaves, what’s the problem? The problem is that the yield reduction due to photorespiration gets worse at higher temperatures.

Efficiency improvement

The C4 process is really a clever solution to this problem. C4 photosynthesis delivers carbon dioxide into the C3 cell by ferrying it in the form of oxaloacetate (recall that I mentioned this molecule in the fourth paragraph.) This increases the cellular levels of carbon dioxide and forces that key enzyme to devote 100% of its efforts to producing carbohydrates. Although this system uses extra energy, sunlight is cheap in grass leaves. In effect, the C4 process acts as a turbocharger, and the response can be spectacular. Under good warm conditions, C4 grasses will perform photosynthesis at twice the rates of C3 grasses, generating lots of carbohydrates and lots of growth. Think of corn.

But so what? Well, in warm temperatures, C4 grasses are more efficient than C3 grasses at fixing carbon dioxide into carbohydrates and recycling nitrogen in the soil. This can give them a competitive advantage, especially in regions of hot summers and some moisture stress. Hence their common name, “warm-season grasses.”

Specifically, these grasses grow best at temperatures of 90ºF to 95ºF, and they won’t even consider growing until the soil temperatures reach at least 65ºF. Cool-season grasses, on the other hand, grow best at temperatures of 65ºF to 75ºF, and they will even show some growth when soil temperatures drop as low as 40ºF. Also, the roots of C4 grasses tend to be longer than C3 grasses, so they can extract water and nutrients from deeper layers of the soil. And their fiber structure (proportions of cellulose, hemicellulose and lignin) also differs from cool-season grasses, so their digestibility characteristics can be slightly different.

The styles of C4

Not all C4 grasses are tall-growing crops like corn and sudangrass. C4 grasses come in many styles: annuals, perennials, bunch grasses, sod-forming grasses. As you’d expect, the conditions of hot summers and dry weather favor them. In fact, the magnificent natural grasslands of the American Great Plains – the tall-grass prairies and short-grass prairies – were originally vast stands of C4 grasses: switchgrass, indiangrass, gamagrass, the bluestems, etc. Plowing and overgrazing often destroyed these grasslands, but today hundreds of prairie restoration projects are trying to bring them back. Also, quite a few graziers across the country are now looking at these species with fresh eyes. And some forage seed companies are following suit by developing new C4 varieties with better traits for grazing or harvesting.

Why? Because in many climates across the continent, warm-season grasses can produce good feed during midsummer when the cool-season grasses go dormant. And these grasses can also provide convenient stockpiled growth for fall and winter feed.

What to do

So let me ask a question about my place: What grass should I plant for summer feed? There is one interesting C4 species that may surprise you: crabgrass (Digitaria ciliaris). Yes, the crabgrass. Before you laugh too hard, think of how often you’ve had to mow a crabgrass lawn during the summer. Well, we now have genetically improved varieties of crabgrass on the market – high-yielding varieties that are self-seeding C4 annuals with high-value summer growth. Sounds great, but hmmm … what about my neighbors? A little prudence here may be a good idea. Before I plant 20 acres of seed-producing crabgrass, maybe I should ask my neighbors what they think.  end mark

Woody Lane is a certified forage and grassland professional with AFGC and teaches forage/grazing and nutrition courses in Oregon, with an affiliate appointment with the crop and soil science department at Oregon State University. His new book, Capturing Sunlight, Book 1: Skills & Ideas for Intensive Grazing, Sustainable Pastures, Healthy Soils, & Grassfed Livestock, is available on Amazon and through Woody Lane.

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