Role of Abscisic Acid (ABA)

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The plant hormone abscisic acid (ABA) is produced in the roots during drought stress. It has been observed that during water shortage in the soil or by intense solar radiation, a plant hormone abscisic acid accumulates in the leaves leading to closing of stomata, thus preventing an excessive water loss. The hormone is transported to the leaves and reaches the guard cells of a stoma, they lose their turgidity and the stoma closes.

Factor affecting stomatal movement

Stomatal opening is influenced by light, temperature and water availability.

Light: Stomata open in the presence of light and close in the darkness. Very low light intensity is sufficient for stomatal opening. Light causes photosynthesis in guard cell by the chloroplasts and hence accumulation of sugar in the guard cells.

Temperature: As the temperature increases, the stomata open and with a decrease in temperature, they close. At , the stomata remain closed even in the presence of light. At temperatures higher than stomata tend to close.

Water availability: When water is available, transpiration is high, this is because the guard cells become turgid and stomata are open. When water availability is less and rate of transpiration is high, plants undergo water stress/water deficit.


Antitranspirants are the chemical substances that are used to reduce the rate to transpiration without causing any adverse effect on their growth and other metabolic activities. These substances are sprayed on crop plants during dry season to avoid wilting when the rate of transpiration is high. The reduction in transpiration is achieved by two means;

Metabolic Inhibitors: These are phenyl mercuric acetate (PMA) and abscisic acid (ABA), helps in reducing the stomatal opening.

Film Forming Substances: These include mainly silicon emulsions which form a film or the surface of leaves and thus reduce the transpiration.


In herbaceous plants, when root pressure is high and transpiration is low, the water is forced out in the form of drops from the margins or tips of leaves. This process is called guttation. Guttation is seen occurring at night in herbaceous plants growing under conditions of high soil moisture and high humidity. Guttation takes place through specialized pores called Hydathodes or water stomata. Hydathodes are found at the tip of a xylem/vascular strand. It consists of a water pore in the epidermis. Guttation is commonly seen in the plants of tomato, potato, Colocasia, grasses and garden nasturtium.

Image showing guttation to the leaf of Nasturtium (A) and section of leaf through a hydathode (B).

Image Showing Nasturtium Leaf.

Image showing guttation to the leaf of Nasturtium (A) and section of leaf through a hydathode (B).

Difference between Transpiration and Guttation

Table showing difference between Transpiration and Guttation.
Table showing difference between Transpiration and Guttation.



Water is lost in the form of water vapor.

Water is lost in the form of water drops.

Occurs through stomata, cuticle and lenticels.

Occurs through special pores Hydathodes.

Stomatal opening and closing are regulated.

Hydathodes remain open always.

Occurs during day time and at high temperature.

Occurs at night and at low temperature.

Water vapour lost is pure water and does not contain minerals.

Water lost has substances dissolved in water. It contains sugars, salts and amino acids.

Increased transpiration is physical process.

It is due to root pressure that develops in the living cells of the plants.

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