3.3.7 Transpiration in Plants

• Within a plant mineral ions and organic compounds (eg. sucrose) are transported by being dissolved in water. The dissolved mineral ions are transported in the xylem tissue and the dissolved organic compounds are transported in the phloem tissue
• The plant roots are responsible for the uptake of water and mineral ions and can have root hairs to increase the surface area for absorption of the substances
• The uptake of water is a passive process and occurs by osmosis (the diffusion of water from a higher (less negative) water potential to a lower (more negative) water potential
• The uptake of minerals can be passive or active and occurs by diffusion or active transport respectively
• Plants must take in a constant supply of water and dissolved minerals to compensate for the continuous loss of water via transpiration in the leaves, and so that they can photosynthesise and produce proteins
• There are two pathways that water (and the dissolved solutes) can take to move across the cortex (and molecules can change between routes at any time):
  • Apoplastic
  • Symplastic

Apoplastic pathway

• Most water travels via the apoplastic pathway (when transpiration rates are high), which is the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
• The water moves by diffusion (as it is not crossing a partially permeable membrane)
• The water can move from cell wall to cell wall directly or through the intercellular spaces
• The movement of water through the apoplastic pathway occurs more rapidly than the symplastic pathway
• When the water reaches the endodermis the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplastic pathway
• This band is called the Casparian strip and forms an impassable barrier for the water
• When the water and dissolved minerals reach the Casparian strip they must take the symplastic pathway. The presence of this strip is not fully understood but it is thought that this may help the plant control which mineral ions reach the xylem and generate root pressure
• As the plant ages the Casparian strip thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions

Symplastic pathway

• A smaller amount of water travels via the symplastic pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells
• The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata
• The movement of water in the symplastic pathway is slower than the apoplastic pathway

Water (and any dissolved substances) can travel from a high water potential (soil) to a low water potential (xylem) via the apoplastic or symplastic pathways. As the plant ages the apoplastic pathway can be blocked by the presence of the casparian strip helping the plant control which mineral ions can move into the xylem vessels.

• The movement of water through a plants xylem is largely due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules
  • Otherwise known as the cohesion-tension theory

• It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant's cells
• Plants are constantly taking water in at their roots and losing water via the stomata (in the leaves)
• Around 99% of the water absorbed is lost through evaporation from the plant's stem and leaves via transpiration

The loss of water vapour from the leaves of plants (transpiration) results in a lower water potential creating a concentration gradient between the roots and leaves causing water to move upwards

Movement of water through leaves

• Certain environmental conditions (eg. low humidity, high temperatures) can cause a water potential gradient between the air inside the leaves (higher water potential) and the air outside (lower water potential) which results in water vapour diffusing out of the leaves through the stomata (transpiration)
• The water vapour lost by transpiration lowers the water potential in the air spaces surrounding the mesophyll cells
• The water within the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull
• This transpiration pull results in water moving through the mesophyll cell wall (apoplastic pathway) or out of the mesophyll cytoplasm (symplastic pathway) into the cell wall
• The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas), which then causes water to move up the xylem vessels (due to the cohesive and adhesive properties of the water). This movement is called transpiration stream

The role of the stomata

• Transpiration is mainly controlled by the pairs of guard cells that surround stomata (plural, stoma is singular)
• Guard cells open the stomata when they are turgid and close the stomata when they lose water
• When the stomata are open there is a greater rate of transpiration and of gaseous exchange
• When the stomata close transpiration and gaseous exchange decrease
• As stomata allow gaseous exchange (CO₂ in and O₂ out) they are generally open during the day

Water movement through a leaf. Water enters the leaf as a liquid and diffuses out as water vapour through the stomata. This loss of water by evaporation and transpiration results in a water potential gradient between the leaves (low) and roots (high) causing water to move up the plant in a transpiration stream.