Photonasty

Photonasty is a type of nastic movement in plants where the movement is a response to light. Nastic movements are non-directional responses to stimuli, meaning the direction of the movement is not determined by the direction of the stimulus. In the case of photonasty, the plant moves in response to light, but the movement itself is not directed towards or away from the light source. This is different from phototropism, which is a directional growth response to light.

Photonasty can manifest in various ways. For example, the opening and closing of flowers in response to light intensity is a common form of photonasty. Many flowers, such as those of the buttercup family, open in the presence of sunlight and close when light levels decrease, often in the evening or on cloudy days. This behavior is thought to be related to attracting pollinators, as many diurnal insects are active during periods of bright light.

Another example of photonasty can be seen in the leaves of some plants. Certain species may orient their leaves in a specific way at different times of the day based on light intensity, either to maximize or minimize light absorption. The exact mechanisms triggering photonasty involve photoreceptors within the plant cells that detect light and initiate a biochemical cascade leading to changes in turgor pressure in specialized motor cells. These changes in turgor pressure cause the movement, such as the bending of a petiole or the opening/closing of a flower. The sensitivity to light and the specific response can vary greatly between different plant species.