botany - Why does freshly cut grass smell like a watermelon?

I found that, when I smell watermelon aroma outdoors, someone nearby has mowed the grass. Is there an underlying basis?

Answer

cis-3-hexenal is probably the main reason. It is one of the main chemicals in fresh-cut grass smell (lots of references besides wiki if you google it). This fairly comprehensive paper on watermelon odorants also says that they find a lot of cis-3-hexenal in fresh cut watermelon. See tables 3.1, 3.2 and 3.3.