STOMATAL MOVEMENT

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Theories: • Starch-sugar hypothesis• pH regulates stomatal movement • Role of k⁺ • Extrusion of proton

• The epidermal surface of a leaf bears a great number of pores called stomata.

• The stomata are microscopic and are bordered by two specialized epidermal cells called guard cells, which control the opening and closing of stomata.

• The stomatal movement is generally understood to be a direct response to increase or decreases in the osmotic potential of the guard cells.

• As a result of this changes in water potential cause water to move in or out of the guard cells.

• If water moves in, the cells expand, the stoma is open.

• If water moves outcalls go flaccid, the stoma is closed.

Anatomy and cytology of stoma• Although the changes in turgor provide the motive force for the

opening and closing of stomata, an unusual feature of the ground cell wall causes the stomata to open in the manner that they do.

• The cellulose microfibrils making up the walls of a guard cell are arranged radially (from the pore outward and relatively particular to the pore) rather than longitudinally , or along its length.

• This orientation of the cellulose microfibrils is termed radial micellation.

• In addition , the guard cell wall adjacent to the stomatal pore is thicker than the outer wall.

Factors affecting stomatal movement

• The environmental factors having the greatest influence on the opening and closing of stomata are light; presence of potassium, chloride and hydrogen malate; CO₂ concentration; water deficits and abcissic acid; and temperature.

• light: generally, the stomata of a leaf are open when exposed to light and closed in dark.

• The amount of light necessary to achieve maximal stomatal opening is usually considerably less than is needed for maximal photosynthesis activity.

• The stomata of some species may be induced to open by bright moonlight. eg CAM plants.

• It was observed in tobacco leaf stomata that no opening occurred when stomata were exposed to either far-red or ultraviolet irradiation . Good stomatal opening was obtained in the red and blue light but no opening in the green light.

How does the light bring about the opening of stomata?

• Early worker assumes that guard cells, when exposed to light and warmth, increased their output of osmotocally active substances through the process of photosynthesis.(more –ve osmotic potential and a subsequent increase in turgor cause the stomata to open).

• pH : sayre in his work with rumex patientia noted that stomata are sensitive to changes in pH . Generally a high pH favors opening and a low pH favors closing of the stomata.

• A high pH is accompanied by a decrease in starch and increase in osmotically active sugars and results in an increase in turgor.

• This PH effect is explained by Yin and Tung , who obtained evidence of the presence of a phosphorylase in chloroplasts that favors the degradation of starch at pH 7.0 and favors starch synthesis at pH 5.0.

• Potassium, chloride, hydrogen and organic acids: Evidence indicate that the turgidity of guard cells of many species of plants is regulated by K⁺, Cl⁻, H⁺ and organic acids.

• When plants are exposed to light the guard cells accumulate large amounts of K⁺. Light stimulate the active movement of K⁺ ions from surrounding cells into the vacuoles of the guard cells.

• Potassium accumulation is also accompanied by starch degradation, organic acid(mostly malic acid) build up and an increase in pH.

• The K⁺ accumulation seems to be due to the operation of an active exchange process in which the protons(H⁺) are “pumped” out of the guard cell into the accesory cells.

• In some species, as the K⁺ ions migrate, they are accompanied by Cl⁻; the anion apparently moves in response to electric difference created by the K⁺ uptake into the guard cells.

• A malic acid build up in the guard cells of illuminated leaves as protons (H⁺)move from the guard cells into the accesory cells.

• The malate ions within the guard cells are neutralized by the influx of potassium.

• Thus in guard cells, osmotic and hence water potentials become very negative due to the presence of K⁺ Cl⁻ potassium malate or dipotassium malate or both.

CO₂ concentration: stomata are quite sensitive to variations in CO₂ concentration.

• An increase in CO₂ concentration above that found in air will cause stomata to close even in the light.

• Indeed, stomatal closure can be induced by merely breathing on leaves.

• It appears that CO₂ concentration in the leaf intercellular spaces rather in the external air primarily controls stomatal movement.

Water deficits and abcissic acid : • Whenever the rate of transpiration exceeds

the rate of absorption for any period of time, a water deficit is created in the plant.

• Under conditions of water deficit, the stomatal pores of many mesophytes will close, thereby reducing transpiration significantly.

• Abcisic acid : ABA causes the opening of anion specific channels in the plasma mambrane of guard cells. Acco. To one model , ABA activates calcium channels in plasma membrane. Calcium moves into the cytosol from the cell wall and acivates protein kinase. These protein kinases cause opening of anion channels in the plasma membrane and anion primarily Cl⁻ and malate⁻² move out from the cytosol

• K⁺ also exudes out. • In response to this , water also moves out of the guard cells

leading to stomatal closure.• When this response to ABA is to be reversed and ABA is removed

, gradually the K⁺ and Cl

From the discussion it becomes clear that a number of endogenous and environmental signals influence stomatal opening and all these signals work through regulating water content by the discussed mechanism.