Transpiration

• Is the loss of water from plants in the form of water vapor

• This evaporative process is dependent on energy

• 97-95% of water taken up is lost in transpiration

• Heat of Vaporization: 539 cal $g^-1

Importance

1. Keeps cells hydrated

2. Maintains favorable turgor pressure for the transport of nutrients absorbed by the roots from the soil

3. Cools the plant

   • Heat load is dissipated in the process due to the high heat of vaporization of water

   • If transpiration is extremely high → dehydration and desiccation → death

Types

• Classified based on the avenue of exit water vapor

• Cuticular Transpiration - Loss of water through the epidermis covered by a cuticle

• Lenticular Transpiration - pores in the outer layer of woody plant stem

• Stomatal Transpiration - Through the stomata

  • Guttation — Water released by plants in liquid form. Water droplets are secreted through the hydathodes due to very high root pressure.

  • Hydathodes — usually located along the margin of the leaves

Stages

1. Evaporation - water from cell structures (phase change of water)

2. Diffusion - water vapor from leaf intracellular spaces to the atmosphere\

   1. Is the movement of substances from a region of higher concentration to a region of lower concentration.

Soil-Plant-Air Continuum of Water

• Movement of water from the Soil to the Root Xylem

  • Water and nutrients are absorbed by root hairs due to difference in water potential

  • Water is then transported radially towards the xylem

Extracellular (Apoplastic Route) - Water moves through non-living parts, e.g. capillary spaces of the cell walls and intercellular spaces

Intracellular Route

1. Symplastic Pathway - plasmodesmata

2. Transmembrane or Transcellular Pathway - vacuolar membrane and plasma membranes

• Movement from Root Xylem to Leaf Xylem

  • Transpiration-cohesion-adhesion-theory

    1. Water vapor leaves the air spaces of the plant via the stomates

    2. This water is replaced by evaporation of the thin layer of water that clings to the mesophyll cells

    3. Tension (pulling) on the water in the xylem gently pulls the water toward the direction of water loss

    4. The cohesion of water is strong enough to transmit this pulling force all the way down to the roots

    5. Adhesion of water to the cell wall also aids in resisting gravity

• Movement from Leaf Xylem to Air

  • Influenced by Rh and VPD

  • Towards lower water potential

Factors Affecting Transpiration

• Leaf number: more leaves, more transpiration

• Number, size, position of stomata: more and large, more transpiration, under leaf, less transpiration

• Cuticle: waxy cuticle, less evaporation from leaf surface

• Light: more gas exchange as stomata are open

• Temperature: high temperature, more evaporation, more diffusion

• Humidity: high humidity, less transpiration

• Wind: more wind, more transpiration

• Water availability: less water in soil, less transpiration (e.g. in winter, plants lose leaves)