Why Some Flowers Only Smell at Night

This article was originally posted on RealClearScience.

“Mary, Mary, quite contrary, how does your garden grow? With silver bells and cockle shells, and one measly petunia!”  –Curly

Curly Howard didn’t think highly of petunias, but had the Three Stooges spent more time gardening, they would have known that petunias are most fragrant at nighttime. Now, scientists have figured out the reason why.

Just like animals, plants have circadian rhythms (internal “biological clocks”). Some plant behaviors occur during the day, while others occur during the evening. In the case of flowering, plants prefer to send out scented signals to attract pollinators during the times at which they are most active. For Petunia hybrida cv. Mitchell, which produces white flowers and was the subject of a new PNAS study, this occurs at night, most likely in order to attract moths.

The authors, who were based at the University of Washington, were interested in elucidating the genetic mechanism behind this phenomenon. Their research homed in on a particular regulatory protein, called LHY, whose expression varied throughout the day. Specifically, it was most highly expressed during the morning (when the flowers were unscented) and rarely expressed during the night (when the flowers were most scented).

The figure above depicts the timing of the release of two scented molecules by P. hybrida, methyl benzoate(which has a fruity odor) and benzyl benzoate (which has a balsamic odor). Because LHY expression was lowest during these times, the authors hypothesized that LHY was suppressing genes involved in the production of scent.

And that’s exactly what their genetic analysis found. Furthermore, they found that this particular LHY-based circadian mechanism was only present in flower tissue, not in leaves, suggesting that the circadian clock operates differently in different parts of the plant.

Finally, the researchers propose that understanding the genetic mechanism of the circadian rhythm will allow the engineering of plants that exhibit desirable behaviors at particular times of the day. A florist may prefer only flowers that are scented during the day, not during the evening. Similarly, crops could also be engineered to express certain traits throughout the day. Of course, as intriguing as these possibilities are, they require that the vast majority of people somehow overcome their irrational fear of GMOs.

Source: Myles P. Fenske, Kristen D. Hewett Hazelton, Andrew K. Hempton, Jae Sung Shim, Breanne M. Yamamoto, Jeffrey A. Riffell, and Takato Imaizumi. “Circadian clock gene LATE ELONGATED HYPOCOTYL directly regulates the timing of floral scent emission in Petunia.” PNAS. Published online before print: 29-June-2015. doi: 10.1073/pnas.1422875112