Nyctinasty is a mechanism that causes plant movements in response to the day-night cycle. Nyctinasty is therefore one of the movements of plant waking and sleeping.
This mechanism, which affects leaves and petals, allows plants to protect themselves against unfavourable conditions by closing their leaves and flowers. When flowers close the plant's reproductive organs are protected.
Plants close at night in environments in which nights are cold and wet, or even during the day in too dry environments.
Nyctinastic movements are caused by different stimuli which may include variations in light (perceived by phytochromes), temperature or humidity, as well as the plant's circadian rhythm.
These stimuli cause modifications in the cellular concentration of calcium ions Ca2+ which differ between the underside and the top of the plant organ (leaf, flower). This differential modification results in a variation in cell turgidity.
When the cells on the underside of the organ swell up more than those on the top, the organ closes. When the opposite occurs the organ opens.
This curious capacity has led to the idea of a floral clock. Indeed, a careful plantation of flowers, depending on their opening and closing conditions, creates a garden - sundial brought to life depending on the time of day and night. Although this type of floral clock is not really accurate, the effect remains impressive.
Sorrel flowers generally close at night. © Sarah Smith, Geograph CC by-sa 2.0