Question

How does Abscisic acid play a role in stomatal opening and closing?

Answer 1

The mechanism of loss of water from the surface of plants is known as "Transpiration".90% of water loss in plants is through the process of transpiration. Guard cells along with the stomata are the structures that help in the process of transpiration. The loss of water is mainly through the stomata and is controlled by the turgidity of the guard cells .

When the water is freely available the guard cells the stomatal openings are large,guard cells are turgid and hence they are open. The turgidity in the guard cells is due to the accumilation K⁺ in the guard cells. The increase in the   K⁺ levels decreases the water potential of the guard cells hence water enters the guard cells and are they are open.

When the guard cells loose water then the cells become flaccid the guard cells collapse and the opening is closed.This occurs mainly when there is excessive loss of water due to transpiration.

ABA is a plant hormone that is involved in the response of plants to reduce water availability. It helps in the reduction of turgor and hence diminishes the stomatal aperture and thereby contributing to the ability of plant to conserve water during drought.It is believed that ABA brings about the reductions in turgor by promoting the effllux of potassium ions from the guard cells .

Mechanism:

ABA binds to the surface of receptors of the plasma membrane of the guard cells which activate several interconnecting pathways which converge to produce an increased pH in the cytosol followed by the transfer of ca²⁺ from the vacuole to the cytosol. these changes stimulate the loss of negatively charged anions like NO₃⁻ and Cl⁻, from the cell and also the loss of K⁺ from the cell. The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and hence the turgor leading to the closure of stomata.It is believed that ABa operates through ca²⁺ dependemt signal transduction pathway.