How ABA Causes Stomata to Close

An osmotic pressure decrease triggered by abscisic acid (ABA) causes stomata to close.

1 ABA binding to its receptoron a guard cell membrane causes the release of nitric oxide (NO). The NO activates calcium ion channels in the membrane, so these ions enter cytoplasm.

2 The influx of calcium ions activates transport proteins that pump negatively charged malate and chloride ions out of the cells. Thus, the overall charge of cytoplasm increases, and the overall charge of extracellular fluid decreases.

3 The resulting voltage change across the guard cell plasma membranes opens gated transport proteins that allow potassium ions to exit the cells.

4 Water follows the solutes by osmosis. The guard cells lose turgor and collapse against one another, so the stoma closes.