Stomata, Mechanism of opening and closing of Stomata

 

Transverse section of leaf reveals that Stomata are minute pore  present  on the lower epidermis of dorsiventral leaf or dicotyledonous leaf  and on both epidermis of isobilateral leaf or monocotyledon leaf on  the upper epidermis. But in Nelumbo or Lotus stomata are present on upper epidermis. The lower surface of a dorsiventral  leaf has a greater number of stomata while an isobilateral  leaf has about equal stomata on both surfaces of leaf.

Stomata are enclosed by two kidney shaped cells, called guard cells. Flaccidity or turgidity causes opening or closing of stomata.

For Types and classification of stomata, Julien Joseph Vesque's model is widely accepted.This model was later developed by Metcalfe and Chalk. 

Function of stomata

Stomata functions as - To remove excess amount of water from plant body by the transpiration. To exchanges of gases like carbon dioxide and oxygen. To maintain the internal temperature of the plant by cooling,To  help in regulating water movement through transpiration. To help in inhibiting the process of Photorespiration.

Mechanism of opening or closing of the stomata:

Potassium ions concentration, Amount of water, Change in shape in guard cells etc are prominent factors that causes opening and closing of stomata aperture. Out of these, the immediate cause of the opening or closing of the stomata is a change in the turgidity of the guard cells. In addition to this, An increase in concentration of Potassium ions causes the opening of  stomata . In contrast  a decreased  concentration of potassium  ions causes closing of  stomata.

The inner wall of each guard cell is pointed  towards the stomatal pore. This inner wall  is thick and elastic. When turgidity increases due to presence of more water within the two guard cells, the thin outer walls swell up and generate the  turgor  pressure  on the inner walls of guards cells .This pressure changes the shape of guards cells from kidney shape to a crescent shape. As a result the stomata  are opened.

When the guard cells lose turgor pressure  due to  loss of water then the  inner walls  of guard cells resume their  original shape. And the  guard cells become flaccid and the stomata are  closed.

The opening of the stomata is also done due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially making it easier for the stoma to open.

Factors affecting stomach activity :

Amount  of Water causes the loss or gain  of water causes reduction or increase  of turgor pressure. As a result guard cells close or open  the stomata.

Increased the concentration of Carbon Dioxide between the cells of the spongy mesophyll cells closing of  the stomata.

Temperature above  the Thirty  degree Celsius closes the stomata because An increase in temperature raises  the rate of respiration which enhances  the concentration of carbon dioxide. With an increase in temperature there is an increase in water stress.Both Carbon dioxide and water affect the activity of stomata.

Light, mainly blue  light spectrum, opens the guard cells for uptaking the Potassium ions in guard cells. That stimulates the active transport of hydrogen  ions towards the outside from the guard cells. This triggers  the movement of Potassium ions  into the guard cells and causes the opening of Stomata.

Biological  Clock causes   openings and closing of stomata  at the same time day after day.

Xerophytic adaptaion in plants and stomatal activity

Plants which adapt themselves in dry conditions like desert  or arid environments are called xerophytes. They generally have small  leaves, thick  and waxy cuticles, and stomata on the lower  surface of leaves. The stomata can be found in deep pit  hence termed sunken stomata. Some plants will shed their leaves during the dry seasons or store water during the rainy season. Crassulacean acid metabolism is another adaptation to xerophytic plants.Some plants open their stomata at night and close during the day. These plants use carbon dioxide  during the night and synthesise   malic acid or isocitric acid. During the day, the carbon dioxide is released from the organic compound and used in photosynthesis. This is known as crassulacean acid metabolism.