The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata

• Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
  

• When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
  

• Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
  

• A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
  

Transpiration rates will be higher when stomatal pores are open than when they are closed (gas exchange impacts transpiration) 

• Variations in the density of stomata may arise due to genetic factors or growth under different environmental conditions

Stomata