14.2.4—Abscisic acid and stomata

During water stress, plants can increase the hormone abscisic acid (ABA). ABA acts on guard cells to promote ion loss, water loss, reduced turgor and stomatal closure, helping limit further transpiration.

ABA closure chain:

1. Reduced water supply or high water loss is associated with increased ABA in the relevant plant tissues.
2. ABA binds to receptors on guard-cell membranes and inhibits the proton pumps that normally move H+ out.
3. ABA also causes Ca2+ ions to enter the guard-cell cytoplasm. Ca2+ acts as a second messenger: it opens channels for negatively charged ions to leave, promotes further K+ efflux and closes channels that bring K+ in.
4. Loss of ions raises guard-cell water potential, so water leaves by osmosis.
5. The guard cells become flaccid and the stomatal pore closes.

Closure reduces the route for water-vapour loss, but it also restricts carbon-dioxide entry and can limit photosynthesis. ABA therefore shifts the gas-exchange trade-off towards water conservation during stress; it is a signal, not a permanent switch that makes photosynthesis impossible.

Calcium ions transmit the ABA signal; they are not the final mechanical cause of closure. The immediate mechanical outcome is ion loss followed by water loss and falling guard-cell turgor. The general guard-cell wall and turgor structure is covered in the preceding card.