Q.Why is abscisic acid also known as stress hormone?

Abscisic Acid: The Plant's "Stress Manager"

Imagine you are a plant. You cannot run away from a drought, you cannot put on a coat when it gets cold, and you cannot close the windows when a storm is coming. So how do you survive? You need a chemical signal that tells every part of your body to shut down non-essential activities and conserve resources until conditions improve. That signal is abscisic acid (ABA).

ABA is often called the stress hormone of plants. When a plant faces water shortage, extreme cold, or other harsh conditions, ABA levels rise dramatically. Its job is to put the plant into a protective, low-energy state — like a bear going into hibernation.

The Two Main Functions (Precisely)

ABA has two primary, well-established roles in plant physiology:

1. Stomatal Closure (Drought Response)

   When roots detect that the soil is drying out, they send ABA through the xylem to the leaves. ABA binds to guard cells surrounding the stomata (the tiny pores on leaves). This triggers an outflow of potassium ions (K⁺) and water from the guard cells, causing them to become flaccid and close the stomatal pore. Closing stomata prevents water loss through transpiration. This is the fastest and most critical survival response to drought.

2. Seed Dormancy and Inhibition of Germination

   ABA accumulates in developing seeds and keeps them dormant — even if conditions are temporarily favourable. It prevents premature germination (vivipary) while the seed is still on the parent plant or during unfavourable seasons. ABA works antagonistically with gibberellins (GA): high ABA/low GA means dormancy; low ABA/high GA means germination. Only when ABA levels drop (e.g., after a cold period or sufficient rain) can the seed germinate.

How It Works (The Mechanism)

ABA is a sesquiterpenoid (15-carbon) hormone. It binds to specific receptor proteins (of the PYR/PYL/RCAR family) inside the cell. This binding triggers a signalling cascade that:

• Opens calcium channels in the plasma membrane
• Increases cytosolic calcium ion (Ca²⁺) concentration
• Activates protein kinases (SnRK2s) that phosphorylate ion channels
• Results in the efflux of anions and potassium ions (K⁺) from guard cells

The net effect: guard cells lose water, become flaccid, and the stomatal pore closes.

Other Notable Functions

• Promotes root growth under water stress (roots grow deeper to find water) …