Question

Photonasty is the opening and closing of ______ in response to light.
A) Stomata
B) Guard cells
C) Flower
D) None of these

Answer 1

Hint: Nastic movements are non-directional movements. They occur in response to stimuli but do not show any growth.

Complete solution:
Let’s discuss the movements in plants. So plants show two different types of movement – one dependent on growth and the other independent of growth.
So, what are these tropic movements? The movements dependent on growth are called the tropic movements, such as growth of roots in response to gravity also increases the length of the root. Similarly, the growth of shoot towards the light increases the height of the seedling. Such movements are directional. The stimuli causing tropic movements, like light, gravity, water, and chemicals are external and unidirectional.
On the other hand, the growth independent movements are called the nastic movements. The stimuli causing these nastic movements, like touch, darkness, sunlight, etc. are non-directional. One such nastic movement is seen in dandelion flowers that open in response to sunlight and close in the evening. Such a movement is called photonasty. Here you can see, there is no growth of flowers, it is simply following the absence and presence of light.
The opening and closing of stomata is dependent on the water concentration in the guard cells.

Note: A) Nastic movements occurring due to various stimuli are:
B) Nyctinastic movements: In response to darkness
C) Thigmonastic movements: In response to touch
D) Photonastic movements: In response to light