Absorption Transport and Water Loss in Plants: Structure, Action, Function and Mechanism of Stomatal Action

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Structure of Stomata

Each stoma 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 are the only epidermal cells that contain chloroplasts. 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 mechanism namely the opening and closing of stomata depends upon the turgor pressure in the guard cells. When the guard cells are turgid, the stoma open and when the guard cells lose water, stoma closes.

Stomatal Movement in Dicot Plants

The Dicot plants have kidney shaped guard cells and have dorsiventral leaves.

Image showing stomatal action in Dicots.

Image Showing Stomatal Action in Dicots

Stomatal Movement in Monocot Plants

The Monocots plants have dumb bell-shaped guard cells and have isobilateral leaves.

Image showing stomatal action in Monocots.

Stomatal Action in Monocots

Changes in turgidity bringing about opening and closing of stomata has been known for a long time but the mechanism that leads to turgidity needs to be explained.

Starch- Sugar Hypothesis

Lloyd (1908) proposed this hypothesis and according to this, the opening and closing of stomata is due to changes in turgidity of guard cells, which is associated with the conversion of starch to sugar in guard cells and the amount of starch in guard cells increases during the night times (stomata closed) and decreases during the day time (stomata open).

Steps Involved in Opening and Closing of Stomata

Table Showing Steps Involved in Opening and Closing of Stomata
Table showing Steps involved in opening and closing of stomata.

Day Time

Night Time

CO2 released due to respiration consumed during the daytime in photosynthesis

CO2 released from the respiration is accumulated in the inter cellular spaces due to non-occurrence of photosynthesis

pH of the guard cells increases

pH of the guard cells decreases

An increase in pH favours hydrolysis of starch into sugars

A decrease in pH favours formation of starch from soluble sugars

Due dissolution of sugars in the cell sap of guard cells the osmotic concentration increases

Due to conversion of sugars to starch the osmotic concentration decreases

Due to endosmosis water enters into the guard cells from the neighboring cells

Water moves out of the guard cells to subsidiary cells

The turgor pressure of the guard cells increases

The turgor pressure of guard cells decreases. The guard cells become flaccid

As a result, the stomata open

As a result, the stomata close

Picture of open stomata

Open Stomata

Picture of closed stomata

Closed Stomata

This theory cannot explain the stomatal movement where starch is absent in the guard cells or guard cells lack chloroplasts and opening of stomata at night and closing by the day in some plants like structures (Cacti).

Effect of Potassium Ions (K+) on Stomata

It was observed by Fuji no (1967) that opening of stomata occurs due to the influx of K+ ions into the guard cells. The increase in K+ ion concentration increases the osmotic concentration of guard cells, thereby leading to stomatal opening. It has been convincingly proved that the accumulation of K+ ions brings the opening of stomata and loss of K+ ions, the closing of stomata. Following steps as under.

Table showing opening and closing of stomata in day light an …

Opening and Closing of Stomata in Day Light and Night/Dark

The uptake of K+ ions is balanced by:

  • Uptake of chloride (Cl-) ions as anions to balance the influx of K+ ions.

  • Transport of H+ ions released from organic acid (malic acid): In some plants the guard cells contain starch; there is accumulation of organic acid like malate by conversion of starch into malic acid in light.

  • By negative charges of organic acids when they lose H+ ions.

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