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
- Keeps cells hydrated
- Maintains favorable turgor pressure for the transport of nutrients absorbed by the roots from the soil
- 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
Evaporation - water from cell structures (phase change of water)
Diffusion - water vapor from leaf intracellular spaces to the atmosphere\
- 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
- Symplastic Pathway - plasmodesmata
- Transmembrane or Transcellular Pathway - vacuolar membrane and plasma membranes
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)