Yes, many Osteospermum cultivars exhibit a fascinating plant behavior known as nyctinasty, where their flowers close at night or during periods of low light and overcast weather. This rhythmic movement is not a sign of the flower wilting or dying but is a highly regulated, turgor-driven process. From the plant's perspective, this is a vital survival strategy honed by evolution to protect its most precious assets and conserve valuable resources.
The primary reason for this nightly closure is the protection of the flower's reproductive structures. The center of an Osteospermum bloom contains the pistil and stamens, which produce pollen and await pollination. At night, temperatures drop, and humidity rises, creating conditions where dew can form. By closing its petals, the flower creates a protective barrier that shields this delicate core from moisture and potential fungal damage. Furthermore, most pollinators, such as bees and butterflies, are diurnal. Since their services are not required at night, there is no evolutionary advantage to keeping the flower open and exposed to nocturnal insects or potential cold damage.
Flower opening is an energetically expensive process. Pigment production, petal expansion, and the secretion of nectar and scent volatiles to attract pollinators all require a significant investment of the plant's energy and resources. By closing at night, the plant enters a state of rest, effectively halting these processes. This allows it to conserve its energy reserves, such as sugars and nutrients, for the following day when pollinators are active again. It is a highly efficient cycle: the flower opens its "shop" only during peak business hours, ensuring maximum return on its energetic investment.
The opening and closing are controlled by changes in turgor pressure within specific cells at the base of the petals, known as the pulvinus. During the day, stimulated by blue light wavelengths from the sun, these cells pump potassium ions (K+) into their tissues. Water follows the ions via osmosis, causing the cells to swell and become rigid, which forces the petals open. As light fades, the cells reverse the process, expelling the ions and losing water. This loss of turgor pressure causes the cells to become flaccid, and the petals gently fold inward and downward, closing the flower for the night.
It is important to note that not every Osteospermum variety closes with the same regularity. Through selective breeding, some newer cultivars have been developed to remain open for longer periods, even at night, to enhance their ornamental appeal in gardens. However, the fundamental biological impulse for nyctinasty remains a defining characteristic of the genus, a direct response to the plant's environmental conditions and its innate drive to reproduce successfully and efficiently.
