Ch#14 |Transportation | Lec #10 | OPENING AND CLOSING OF STOMATA (FSc Bio part 1)

Ch#14 |Transportation | Lec #10 | OPENING AND CLOSING OF STOMATA (FSc Bio part 1) @riffatjahan

OPENING AND CLOSING OF STOMATA:
Stomata are pores in the epidermis of aerial parts of plants.

Through these pores gaseous exchange occurs between the internal tissues and the atmosphere.

Each stomatal pore is surrounded by a pair of guard cells.

The cells function, as multisensory

They are sensitive to environmental factors such as light intensity, light quality, temperature, relative humidity and intracellular CO₂ concentration.

The stomata open or close in response to these signals.

There are two hypothesis which may explain the opening and closing of stomata.


(i) Starch Sugar Hypothesis:
The German botanist H. Van Mohl proposed that the guard cells are the only photosynthetic cells of epidermis of leaf.
They are involved in the opening and closing of stomata.
During Day Light:
As a result of photosynthesis sugars are produced in the guard cells during day time when light is available.
When sugar level rises (i.e. solute concentration increases) the water potential decreases, the guard cells become turgid, separate from one another and the stoma pore opens.

During Night:
During night there is no photosynthesis and the sugars are either converted into insoluble (starch or are used in respiration.
As a result free sugar decreases in the cell.
So the osmotic potential of the guard cells in lowered and the water leaves the guard, cells.
The guard cells become flaccid and the stoma or pore between them closes.
By these processes there is not rapid rise in turgor.


(ii) Influx of K' Ions Hypothesis:
Potassium concentration is involved in the opening and closing of stomata.
Potassium concentration in the guard cells increases several times, depending upon plant species.
Stomata open due to active transport (pump) of potassium ions (Potassium) into the guard cells from the surrounding epidermis.
The accumulation of Potassium decreases the osmotic potential of the guard cells.


Water enters the guard cells by osmosis.
The guard, cells, becomes more turgid and stretched and stomata are opened.
The stoma closes by reverse process:
(i) Passive diffusion of Potassium from the guard cells (during night).
(ii) Then water-moves out by osmosis (exosmosis).
(iii) The guard cells become flaccid and stoma or pore between guard cells closes.

Control of K ions Into And Out of Guard Cells:
Level of carbon dioxide in the spaces inside the leaf and light, control the movement of Potassium ions, through:
Level of Carbon Dioxide:
A low level of carbon dioxide favours opening of the stomata, thus allowing an increased carbon, dioxide level and increased rate of photosynthesis.

(ii) Light:
Exposure to blue light (which, is also effective in photosynthesis) acidify the environment of the guard, cells (i.e. pumps out protons).
Therefore the guard cells take up Potassium followed by water uptake.
As a result the turgidity of guard cells is increased.
In general, the stomata are open during the day and closed at night.
In this way water loss is prevented when the photosynthetic process is not going on.

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