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The smell of freshly cut grass on a warm summer day might make you think of lazy days in a hammock, sipping lemonade. But to the mangled grass producing that scent, it is the pungent perfume of pure terror...

Hosted by: Michael Aranda
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Sources:
http://mentalfloss.com/article/30573/what-causes-fresh-cut-grass-smell
http://science.sciencemag.org/content/329/5995/1075.abstract
http://today.agrilife.org/2014/09/22/mown-grass-smell-sends-sos-for-help-in-resisting-insect-attacks-researchers-say/
https://www.sciencedaily.com/releases/2014/09/140922145805.htm
http://research.tamu.edu/2014/09/23/that-fresh-mown-lawn-smell-is-actually-a-distress-call-to-insects/
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.544.4066&rep=rep1&type=pdf
http://www.ncbi.nlm.nih.gov/pubmed/19206090
https://books.google.com/books?id=4tETDAAAQBAJ&pg=PA455&lpg=PA455
http://www.nap.edu/read/9650/chapter/7#111
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3794753/
http://learn.genetics.utah.edu/content/selection/corn/
[SciShow intro plays]

Michael: You know that grassy smell right after you mow the lawn, it's kind of fresh, or maybe reminds you of  spring and warm weather. Grass normally smell like that though, so what's the deal when it gets cut?

Well when their leaves are injured, grass and other leafy plants make organic compounds for protection. Some of these chemicals, called Green Leaf Volatiles or GLVs, evaporate into the air and produce that signature scent. And we think these GLVs are important signals that attract the predators of grass-munching insects.

When a leafy plant is damaged it makes lots of chemicals to protect itself. Some of these chemicals are signaling molecules inside the plant. Jasmonic acid, and salicylic acid, for example, help the plant synthesize compounds that make it less appetizing or defend against fungal and bacterial infections. Other signalling molecules, like traumatic acid, tell the plant to make more cells to close up the wound.

Green leaf volatiles act a little differently, though. They're volatile organic compounds, which means they easily become gases and are released into the air, basically acting as a call for help. See, before lawn mowers existed, leafy plants were hurt by insect like caterpillars eating them. The leafy plant releases a bunch of GOVs which include chemicals like aldehydes, alcohols, and esters. Some of these chemicals are the culprit behind that fresh grassy smell, but more importantly for the grass, these chemicals act as a dinner signal to other insects like parasitic wasps, which can lay their eggs in the caterpillars and eventually kill them.

To figure out how important these smelly chemicals are to plants, a researcher from Texas A&M University was studying a mutant strain of corn that couldn't make GLVs. And corn is, botanically, a grass. Over time, this mutant corn had more insect damage than its GLV-producing counterparts, in a lab setting, and in the field. Predatory insects just don't show up as often to eat their herbivorous prey without a chemical signal to tip them off. So, next time you smell that fresh cut grass, remember that it's just adding insect to injury.

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