Evaporation

• Water is lost from the leaves of the plant when it evaporates and diffuses from the stomata
• Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
• Vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
• Negative pressure creates a tension force in leaf cell walls which draws water from the xylem
• Water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
• The 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
• 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
• Stomatal pores are responsible for gas exchange in the leaf and hence levels of photosynthesis will affect transpiration
• Other factors that will affect transpiration rates include humidity, temperature, light intensity and wind