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Let us make in-depth study of the period of stomatal opening and types of stomatal movement with diagram.
The opening and closing of stomata is known as stomatal movement. When guard cells are turgid, stomata open and when flaccid, stomata close. The outer wall (convex) of guard cell is thin and inner wall (concave) is thick.
Due to endosmosis, TP (turgor pressure) increases in guard cell and it move towards thin wall that leads to opening of stomata. Similarly, in flaccid condition stomatal aperture (stoma) closes as the outer thin wall comes to its original position.
Period of Stomatal Opening:
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Von Mohl (1856) prepared stomatal clock and observed that stomata open during day and close at night. Loft-field has recognized the following five categories in accordance with their daily movement.
1. Alfalfa type:
Stomata remain open through-out the day, close at night, e.g., Beans, Pea, Mustard, etc.
2. Barley type:
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Stomata open only for a few hours in day and remain closed in rest period, e.g., Cereals.
3. Potato type:
Stomata remain open through-out the day and night, closing only during the period of water stress, when rate of transpiration exceeds the rate of absorption, e.g., Allium, Cabbage, etc.
4. Equisetum type:
Stomata seldom close, usually always remain open.
5. Succulent type:
Stomata open at night and close during daytime, e.g., Opuntia, Bryophyllum, etc.
Types of Stomatal Movement:
Considering variable behaviour of stomatal movement, five categories have been recognized:
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1. Photoactive stomatal movement:
Stomata remain open during day time and close at night (dark).
2. Scotoactive stomatal movement:
Stomata remain close during day time and open during night.
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3. Hydro-active stomatal movement:
Stomata open due to excessive loss of water from epidermal cells and close due to turgid condition of epidermal cell. It is usually seen in mid day.
4. Autonomous movement:
Stomata open and close at rate of 10 – 15 minutes showing diurnal or rhythmic pulsation.
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5. Passive and active stomatal movement:
Opening of stomata is considered as active process while closing is the passive process and this is caused by the turgor changes in the guard cells.
