Q&A: Nyctinasty

Question: I’ve enjoyed having flowers in my garden for years. And I’ve always known that some flowers close their petals at night and some don’t. How does this happen, and is there a reason for it? — EE, Algoma, WI

Answer: The action of opening petals at sunrise, and closing them at sunset, is called nyctinasty. It’s a common behavior for many flowering species. How it happens is well understood. Why it happens is not.

When the Sun goes down the most plant growth comes to a halt. Parts of the flower petals closest to the stem have first access to whatever nutrients remain to be circulated, so they continue to grow while the rest of the plant shuts down. This continued growth at the base of the petal forces the flower to curl and close. At sunrise, when the nutrient flow resumes, the growth resumes and the flower reopens.

This is a very simple process not to be confused with tropisms, which involve much more complex mechanisms like sunflowers following the Sun — an example of phototropism.

The video is a 64X time lapse of a group of Mexican Poppies, one of the first wildflowers to appear every spring in the Sonoran Desert. While playing around with my old Galaxy S9 I discovered a video function called “Hyperlapse” — Samsung’s name for “time lapse,” and decided to try it out for this post. It worked quite well, as you can see.

All my previous time lapse videos were done with my Canon 20D and an intervalometer. Hyperlapse was quick and easy, requiring only a tripod and phone adapter. There’s some “noise” from the wind blowing the flowers around, and clouds passing in front of the Sun, but it nicely shows the flowers opening as the shadow of the house retreats.

Why plants exhibit this behavior is less well understood. Botanists have several theories that make sense, all of which would confer an evolutionary advantage. Closing the petals at night would reduce:

  • evaporation of moisture from the plant by decreasing its exposed surface area.
  • evaporation of pheromones that attract daytime pollinators.
  • ground cover and make nocturnal herbivores more vulnerable to their own predators.
  • dew formation on pollen that makes it sticky and less easily dispersed.
  • disruptions to circadian rhythms from bright moonlight.

The taxonomical kingdom Plantae exhibits many surprising types of motion. From Venus Fly Traps (Dionaea muscipula) to Sensitivity Plants (Mimosa pudica), there’s a wide range range of motile behaviors. Each has evolved over millennia to provide some type of survival advantage. Sometimes the benefit is obvious, as in the case of Venus Fly Traps. Nyctinasty in flowers, however, remains a puzzle.

Next Week in Sky Lights ⇒ Timekeeping on the Moon

Q&A: How Astronauts Sleep
Timekeeping on the Moon