Stomata closures (OCCUR QUICKLY)

• When specific environmental stimuli like lower light intensity and lower temperature are detected proton pumps in the guard cells’ plasma membranes stop H+ ions from being pumped out by abscisic acid/ABA (a stress hormone)

• The now-activated ABA receptors then cause Ca²⁺ ions to increase in concentration within the guard cells’ cytoplasm + vacuole (via the plasma membrane + tonoplasts)

• The second-messenger Ca²⁺ ions then stimulate the activation of specific channel proteins that let anions and K⁺ ions out of the guard cells while also closing the channel proteins that let K⁺ ions in

• This net loss of ions results in the guard cells’ water potential increasing thus increasing the osmosis rates out of those cells - this makes them flaccid to the point of the stomata closing