CALL 911! THE GRASS IS WOUNDED!

Do you love the smell of freshly-cut grass? Does it remind you of happy memories being a child playing outside with your friends? If yes,you’re a sadistic bastard because that scent is actually a sign of the plant in agony, it releases airborne chemical compounds when attacked. It’s the plant’s way of “communicating” to the environment that it needs help.
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What Chemical Compounds Create This Smell?

This smell is caused by the release of enzymes, particularly the "cis-3-hexenal". It's a colorless liquid aromatic compound, a carbonyl group at the end of the chain, making it an aldehyde, and there is a double bond three carbons up from the end. It is one of the volatile organic compounds released by plants (when attacked) called 'green leaf volatiles' (GLVs). Among the GLVs released by damaged grass are a group of eight related oxygenated hydrocarbons, including aldehydes and alcohols.

Trivia: Studies about the behavior of monkeys involving PET (positron emission tomography) have shown that the ‘green odor’ mixture of trans-2-hexenal and cis-3-hexen-1-ol has a healing effect on the psychological damage caused by stress. image source
The mixture of the chemical compounds activates the regional blood flow in the primary olfactory cortex (like other smelly molecules) and also in regions that are not normally reached by aromatic molecular compounds, which may explain the healing effect. It has also been found that ‘green odor’ mixture reduces stress in rats and has likewise been found to reduce pain sensation in humans. So the green grass smell really does seem to be good for us humans.

Why Does Grass Do This?

We know that it’s the plant’s way of “communicating”, or a distress signal to the environment that it needs help, but to whom? To inform other patches of grass that it needs to be rescued? According to research, the signal is not actually for plants, it is a distress signal to parasitic wasps to go and lay eggs in the plant-eating insects (caterpillars) to keep them away. One of the many other compounds released by grass when in distress is the jasmonic acid. This substance acts as an insecticide, signaling plant-eating insects to keep their distance at bay.

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Another compound is salicylic acid, which has the potential to reduce pH of the water, and consequently the proliferation of bacteria is reduced. Other signaling molecules, like Traumatic acid, tell the plants to make more cells to heal immediately. Overall, distress-signal chemical compounds have a dual function; the first is to keep plant-eating insects away and the second is to attract parasitic wasps to destroy those plant-eating insects. All’s fair in love and war – and nature, apparently.

Environmental Effect

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Apparently, the effect of that wonderful smell is costly. According to Austrian researchers, these compounds are precursors to ozone formation and can contribute to the formation of photo-chemical smog in urban areas. Photochemical smog is produced due to the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds (produced by freshly-cut grass) in the atmosphere, which leaves airborne particles and ground-level ozone.

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References:
http://mentalfloss.com/article/30573/what-causes-fresh-cut-grass-smell
http://research.tamu.edu/2014/09/23/that-fresh-mown-lawn-smell-is-actually-a-distress-call-to-insects/
https://en.wikipedia.org/wiki/Cis-3-Hexenal
https://www.chemistryworld.com/podcast/hexenal/6836.article
https://curiosity.com/topics/the-reasons-behind-that-pleasant-fresh-cut-grass-smell-arent-so-pleasant-curiosity/