Grasses: Green Machines and Slow-pokes

Why are some grasses (like the lawn) greening up quickly, while others (like the prairie) are slower?

You may have heard of warm season versus cool season grasses. Cool season grasses are greening quickly right now, while the warm season grasses wait for higher temperatures. Cool season grasses grow well when moisture is abundant in the early spring, even though the lower temperatures reduce the rate of metabolism (the process of turning CO₂ and sunlight into energy). 

Sandberg’s bluegrass with the C4 grasses
little bluestem and sideoats grama behind.

Warm season grasses, on the other hand, can tolerate reduced water availability to maximize growth when amounts of sunshine and temperatures are high. How do they tolerate the drought? They have developed a method to capture CO₂ for use in photosynthesis that does not require their stomata (small openings in leaves that allow gas exchange) to remain open all day. 

For photosynthesis to occur, plants must take in CO₂ through their stomata. The CO₂ is captured, or “fixed,” by an enzyme that binds the CO₂ to an organic molecule, and ultimately the CO₂ is incorporated into sugars. Cool season grasses, and most other plants, attach the CO₂ to make a molecule with three carbon atoms—this form of photosynthesis is called C3. Warm season grasses fix CO₂into a molecule with four carbons. These are called—you guessed it!—C4 plants. Only about 6% of all plant species use the C4 method, but it is quite common in grasses.  

The disadvantage of C4 photosynthesis is that it requires additional steps, and therefore the plant must invest more energy. However, it is much more efficient in fixing CO₂than the C3 method, so photosynthesis is more rapid. This high efficiency means that the plant doesn’t need to keep its stomata open as much of the time. Since water is lost from leaves through the stomata whenever they’re open, being able to keep them closed allows the plant to use water much more efficiently, and thereby tolerate dry environments much better than C3 plants. 

On the Berry Prairie, one of the first grasses to show signs of growth this spring was Sandberg’s bluegrass (Poa secunda, shown top right), which is closely related to Kentucky bluegrass (Poa pratensis), the most common lawn grass. As cool season grasses, they green early, but, as any lawn-keeper knows, require lots of water to stay that way through the heat of summer.