- Since most of the water is lost through stomata, plants regulate the degree of stomatal opening and closing to reduce the water loss.
- It has been seen that stomata show periodic opening and closing during the day (diurnal variation) depending upon the heat and light, water content of the cell and humidity. They are generally closed during the night.
- From early morning till midday, the stomata are open and hence the transpiration increases till midday.
- During sunny afternoon, the stomata are closed and hence the transpiration decreases.
- From late afternoon till evening, the stomata are open again and hence the transpiration increases. At night, the stomata are closed and hence the transpiration is very low.
Structure of Stomata
- Each stomata consists of a minute pore called stoma surrounded by two guard cells.
The stoma acts as a turgor-operated valve, which closes and opens according to the turgidity of guard cells. The guard cells have unevenly thickened walls. The cell wall around stoma is tough and flexible and the one away from stoma is thinner. The shape of guard cells differs in dicots and monocots, though the
mechanism remains the same.
Mechanism of Stomatal action
- The opening and closing of stomata depends upon the turgor pressure in the guard
cells. When the guard cells are turgid, the stoma opens and when guard cells
lose water, stoma closes.
The mechanism of dicots and monocots is as give below:
(a) The dicotyledonous plants have kidney shaped guard cells. The inner walls
around the stoma are thicker than the outer walls.
- A. When guard cells → Guard cells expand → Tough inner walls → Stomata open
get distended by become convex turgor pressure
- B. When the turgor → Guard cells sag → Inner cell walls come → Stomata close
pressure in guard closer cells decreases
(b) In monocotyledonous plants, the guard cells are dumb bell shaped with
thickened walls towards the inflated region.
- A. When the guard cells → The region with thin → The thick walls → Stoma opens
become turgid walls bulges and gets move apart inflated
- B. When the guard cells → The inflated part sags → The thick walls → Stoma close
lose water collapse