From our perspective as Osteospermum plants, the closing of our flowers at night, a behavior known as nyctinasty, is not a matter of sleep as animals experience it. It is a precise, rhythmic movement driven by environmental cues, primarily light and temperature. This opening and closing is a type of nastic movement called photonasty, specifically triggered by changes in light intensity. As the sun sets and light levels diminish, a signal travels through our cells, instructing the petals to move. This daily cycle is an innate and crucial survival strategy honed by evolution to protect our most valuable assets and ensure successful reproduction.
The primary reason for this behavior is the protection of our reproductive structures. Our open flowers expose the delicate pistil and stamen—the female and male organs—to the elements. The cool, damp night air, and especially dew or frost, poses a significant threat. Moisture can damage pollen, making it ineffective for fertilization, or promote the growth of harmful fungi and molds on the vital organs. By closing our petals tightly at dusk, we create a protective barrier that shields these critical parts from nighttime moisture and drastic temperature drops, ensuring they remain functional and viable for the next day's pollinators.
Another vital reason for closing at night is resource conservation. Producing and maintaining large, showy flowers to attract pollinators requires a tremendous amount of energy. At night, when our primary pollinators—bees, butterflies, and other diurnal insects—are inactive, there is no benefit to keeping our flowers open and expending energy. By closing, we reduce the surface area exposed to the cool night air, minimizing respiratory water loss. This allows us to conserve precious nectar, pollen, and metabolic energy, redirecting those resources toward growth and preparation for the following day when pollinators are active again.
This movement is not merely a passive reaction. We possess an internal circadian rhythm, an innate biological clock that anticipates the daily cycle of day and night. Even if placed in constant light or darkness for a short period, we may continue to open and close on our roughly 24-hour schedule. However, this internal clock is fine-tuned by external environmental triggers. The most powerful cue is the change in light quality and intensity at dusk. A drop in temperature often accompanies this dimming light, which further reinforces the signal for our petals to close. The mechanism itself is governed by changes in turgor pressure within special cells at the base of each petal, called the pulvinus, which act like tiny hydraulics to enact the movement.
