Hormones stimulate stomata to close by:
abscisic acid binds to complementary receptors on guard cell membrane causing calcium ion channels on tonoplast to open. Ca 2+ ions diffuse from vacuole into cytoSol.
Positive feedback triggers other ion channels to open. So K+ diffuses out of guard cell.
Water potential of guard cell becomes more positive. Water diffuses out via osmosis. Guard cells become flaccid so stomata close.