Fun facts : Sleeping Flowers
Unlike leaves, which, like children, go to sleep with the last rays of the sun and wake up at the first light of dawn, flowers display a much wider variety of behaviors. Some flowers close and “sleep” at night, while others do so during the day. Even the exact hours of closing and opening differ from species to species. Certain flowers are very diligent and open very early in the morning; others remain closed, with their curtain of petals drawn, until midday. Some close their corolla as early as 3 in the afternoon, others at 4, 6 or even 9 o’clock, following their own internal biological clock.
Regardless of these curious daily schedules, the main cause of the “sleep” state in flowers is, as in leaves, the variation in light intensity over time. Changes in light trigger differences in the rate of cell growth on the two sides of the petals, depending on the presence or absence of solar radiation. Unlike leaves, where temperature plays only a minor role in the movements of wakefulness and sleep, in flowers heat acts together with light, producing similar internal physiological phenomena that cause the corollas to close or open and regulate these nyctinastic movements.
Under the action of light, changes occur in the pH (acidity) of the cells. In the evening, as the pH increases, the cells on the outer surface of the petals grow more intensely, which makes the petals curve inward and cover the core of the flower. In the morning, as the pH decreases due to the reduction in accumulated carbohydrates, the cells on the inner surface grow faster, producing the opposite movement: the corolla opens, the flower becomes fully exposed and is ready for pollination.
Alongside these nightly closing movements, seen in species such as the yellow crocus (Crocus aureus), rabbit sorrel (Oxalis acetosella), tulip (Tulipa hybrida), bindweed (Convolvulus vulggris), queen of the night (Nicotiana alata) and others, there are also characteristic bending movements of the flowers. These can be observed in the pansy (Viola odorata), the pasque flower (Anemone ranunculoides) or the wild carrot umbel (Daucus carota), where the entire flower head inclines as part of its nyctinastic behavior and adapts to changing light and temperature conditions.
The purpose of these nyctinastic movements depends on the ecological and life characteristics of each plant species and is closely linked to survival, reproduction and adaptation to the environment.
In general, for spring flowers that are sensitive to temperature fluctuations, closing at night helps prevent heat loss and protects delicate floral tissues from cold damage. In other plants, nyctinasty protects the pollen from excess moisture. The bending of the corollas during the night is itself a way of preventing dew from penetrating the interior of the flower. In self-pollinating plants, the corollas open when the sun is strong so that the anther membrane can dry thoroughly and crack, efficiently releasing and spreading the pollen grains and ensuring successful fertilization.
Nyctinastic movements also play an important role in insect pollination and plant reproduction. Flowers that open during the day can only be visited by diurnal insects, while those that open at night are adapted to visits from nocturnal insects such as moths. In this way, the timing of flower opening and closing helps ensure effective pollination, supports biodiversity and contributes to the continuation of each plant species.