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How Does Grass Scream?

Updated: Aug 19

“Yelp”, “shriek”, “holler”, “cry” and “howl”. What do all these words have in common? They express pain. What do plants do when they’re in pain? While humans scream physically, grass scream… chemically. You know that fresh grass smell that you love so much after you mow the lawn? Well, that’s grass’ screams, also known as green leaf volatiles. Don’t you feel evil now?



What Are Green Leaf Volatiles (GLVs)?

When grass is cut, it releases green leaf volatiles (GLVs), a type of volatile organic compound found in plants that act as a chemical warning against danger. Volatile organic compounds (VOCs) are carbon-based chemicals that have a high vapor pressure at room temperature and low water solubility, meaning they are usually found in a gaseous form. Plants usually release GLVs when they are threatened, such as being stepped on and eaten by an animal, cut, or mowed. According to a study conducted at the Max Planck Institute for Chemical Ecology, different types of GLVs are released by different plants, or even when the same plant is threatened differently. Specific insects, such as Geocoris, are often attracted to the aroma of GLVs. If they detect GLVs from their favorite snack, they will rush to the site of the injured plant and begin feasting on it.



What Actually Happens When Grass Is Cut?

The node, or a bulge at the bottom of the grass, is where new grass grows. When a blade of grass is cut, an electrical signal is sent to the node through the plant’s nervous system, jumpstarting a series of repair processes. An enzyme, called lipoxygenase, breaks down fat from the wound site to form linoleic and linolenic acid. These acids break down further to form those GLVs and other organic compounds. The grass smell comes from (Z)-3-Hexenal, a GLV, because it has a low odor threshold of 0.25 parts per billion, meaning very little is needed for humans to detect the smell. (Z)-3-Hexenal is unstable and reduced into (Z)-3-Hexenol (leaf alcohol)—a colorless, oily liquid with a strong “grass” smell—and (E)-2-Hexenal (leaf aldehyde)—a similar compound found in large quantities in ripe tomatoes.



What are GLVs Used For?

Green leaf volatiles serve primarily to protect the plants using a variety of different methods. GLVs can induce defense responses in other plants or in the original plant itself. For example, jasmonic acid—a plant hormone found in jasmine plants—can change the biomolecular composition of the plant to make themselves less nutritious and appealing to predators. Another method of becoming less nutritious is translocation, transporting sugar towards the roots, which will also help the grass grow back from the node faster. The sugar is formed at the source, cells in the leaf, and transport to the sink, the root.

Additionally, certain GLVs can attract the offending bug’s predator, such as Geocoris—also known as big-eyed bugs—are attracted to plants attacked by tobacco hornworms—their prey.


We have blood platelets to stop wounds and use antibiotics to prevent infections, similarly, GLVs stimulate the formation of new cells at the wound site and act as antibiotics. A study suggested leaf aldehyde has anti-microbial properties against Botrytis, a fungus plaguing wine grapes. Thus, GLVs are important in keeping plants healthy.



Why Should We Care?

The foundation of the ecosystems which we rely so heavily upon are plants. On a more relatable note, humans detect GLVs through our olfactory system to determine when a fruit or vegetable is ripe. Ultimately, the natural defense systems of plants in the form of GLVs preserve their life and prevent crops from failing by stopping bug attacks or fungus plagues. That’s how you could pick the ripe apple in the grocery store or make sure you don’t eat sour berries on a hiking trip. After all, smells are just chemicals wafting through the air.



Definitions (for reference/fun):

Green Leaf Volatiles (GLVs) - a type of volatile organic compound found in plants

Volatile organic compounds (VOCs) - carbon-based chemicals that have a high vapor pressure at room temperature and low water solubility

Lipoxygenase - a family of iron-containing enzymes that break down polyunsaturated fatty acids

Linoleic acid - a colorless or white oil that is insoluble in water and an essential fatty acid for humans

Linolenic acid - an essential fatty acid found in seeds and oils

(Z)-3-Hexenal - a GLV commonly found in low quantities in unharmed plants

(Z)-3-Hexenol (leaf alcohol) - a colorless, oily liquid with a strong “grass” smell; also found in low quantities in unharmed plants

(E)-2-Hexenal (leaf aldehyde) - a colorless compound with a “grass” smell found in small quantities in unharmed plants and large quantities in ripe tomatoes

Jasmonic acid - a plant hormone found in jasmine plants



Sources:

Breyer, M. "What Fresh Cut Grass Is Saying With Its Scent." Treehugger, 2019, www.treehugger.com/what-fresh-cut-grass-saying-its-scent-4855746. Accessed 24 July 2020.

Compound Chemistry. "What Causes The Smell of Fresh-Cut Grass?." Compound Chemistry, 2014, www.compoundchem.com/2014/04/25/what-causes-the-smell-of-fresh-cut-grass/. Accessed 21 July 2020.

Demirdöven, A. Fig 1, "Lipoxygenase in fruits and vegetables: A review." ResearchGate, 2007.

Escu, A. Unsplash, 2019.

Fentress, S. "Why Mowing the Lawn Doesn’t Kill the Grass." Moment of Science, 2015, indianapublicmedia.org/amomentofscience/why-mowing-the-lawn-doesnt-kill-the-grass.php. Accessed 24 July 2020.

Grunbaum, M. "Why Does Freshly Cut Grass Smell So Nice?." Live Science, 2019, www.livescience.com/65400-why-freshly-cut-grass-smells-good.html. Accessed 21 July 2020.

Kerr S. "Sugar Transport in Plants: Phloem." Biology 1520, 2020, bio1520.biology.gatech.edu/nutrition-transport-and-homeostasis/plant-transport-processes-ii/.

Loboda, B. Geocoris Sp. Adult on Leaf, "Big‐Eyed Bug: A MVP of Generalist Natural Enemies." Creative Commons, n.d.

Soniak, M. "That "Fresh Cut Grass" Smell Is a Distress Signal." Mental Floss, 2012, www.mentalfloss.com/article/30573/what-causes-fresh-cut-grass-smell. Accessed 21 July 2020.

Talashuk, V. Unsplash, 2020.

Wechschmied, J. Unsplash, 2016.


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