Brent Cornell

Transpiration is the loss of water vapour from the stems and leaves of plants

• Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata 
• New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
• Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)

Evaporation (Water Loss)

Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata

• Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
• This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
• This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
• The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls

Root Uptake (Water Gain)

Plants take up water and mineral ions from the soil via their roots and thus need a maximal surface area to optimise this uptake

• The epidermis of roots may have protrusions called root hairs, which increases the available surface area for absorption
• Roots may actively upload mineral ions from the soil to increase water uptake (water will follow the ions via osmosis)

When water is absorbed by the epidermis, it diffuses across the cortex towards a central vascular cylinder (where the xylem is found)

• The water may move continuously through the cytoplasm of the cortex cells via connected plasmodesmata (symplastic pathway)
• The water may alternatively move via the extracellular spaces between the cell wall and plasma membrane (apoplastic pathway)
• The vascular cylinder is surrounded by a hydrophobic ring of tissue (Casparian strip) that blocks water movement via the cell wall
• This forces water to move into the cells, which contain water channels (aquaporins) that allow hydrostatic pressure to be regulated

Xylem Transport (Water Movement)

The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream

• Water rises through xylem vessels due to two key properties of water – cohesion and adhesion

Cohesion:

• Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
• Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
• This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream

Adhesion:

• Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
• The xylem wall is also polar and hence can form intermolecular associations with water molecules
• As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension

The Most Amazing Thing About Trees

Video created by Derek Muller at Veritasium

Stomata (Water Regulation)

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

• 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