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AGRI 31 (LECTURE B)
TRANSPIRATION
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TRANSPIRATION
Process where water moves in liquid form in plants, and released in vapor form through aerial parts, but mostly in leaves, to the atmosphere
TRANSPIRATION
Energy dependent process
The transformation of liquid to gas phase involves use of energy
97-99.5
% of water taken up is lost in transpiration
Importance of Transpiration
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 of Transpiration (Based on the avenue of exit of water vapor)
Cuticular transpiration
Lenticular transpiration
Stomatal transpiration
Types of Transpiration (Based on the avenue of exit of water vapor)
Cuticular transpiration
Loss of water through cuticle
5-10% of the water loss
Types of Transpiration (Based on the avenue of exit of water vapor)
Lenticular transpiration
Lenticels
- pores in the outer layer of a woody plant stem
In
deciduous
species (trees which sheds off leaves) and in some fruits, water loss through lenticels may be quite substantial.
Types of Transpiration (Based on the avenue of exit of water vapor)
Stomatal transpiration
Through the stomata
As much as 90% of the water lost from plants.
What affects diffusion of water from leaf to atmosphere?
Relative humidity
(RH) (%)
Vapor pressure deficit
(VPD) ((pascal (Pa))
What affects diffusion of water from leaf to atmosphere?
Relative humidity (RH
) (%)
actual water vapor in the air: water vapor pressure in leaf
In leaves 🡪 100% RH; in atmosphere, RH rarely exceeds 90%
water diffuses out from the plants to the atmosphere
What affects diffusion of water from leaf to atmosphere?
Vapor pressure deficit (VPD
) (pascal (Pa))
Actual water vapor pressure
- water vapor pressure at saturation at the same temperature
when VPD is 0 Pa (i.e. when RH of the atmosphere is 100%), there is no net movement of water
when the RH of the atmosphere is low, the VPD is
high
, and the rate of transpiration is
faster
Soil-Plant-Air Continuum of Water
Movement of water from the
soil
to the
root
xylem
Movement of water from
root
xylem to
leaf
xylem
Movement of water from
leaf
xylem to the
air
Soil-Plant-Air Continuum of Water
Movement of water from the soil to the root xylem
Extracellular
or
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 (tonoplast)and the plasma membranes
Soil-Plant-Air Continuum of Water
Movement of water from root xylem to leaf xylem
transpiration-cohesion-adhesion
theory
Soil-Plant-Air Continuum of Water
Movement of water from leaf xylem to the air
influenced by
RH
and
VPD
Towards
lower water potential
(Ψ; expressed in megaPascal, MPa)
Factors that Affect Transpiration
Plant Factors
Environmental Factors
Factors that Affect Transpiration
Plant Factors
Efficiency of evaporative
surface
Efficiency of water absorption.
Other surface/stomatal modifications
Phytohormones
Canopy
structure.
Factors that Affect Transpiration
Environmental Factors
Edaphic
(soil) factors
Atmospheric
factors
Light
Relative humidity
Temperature
Wind velocity
Oxygen
and
carbon dioxide
concentrations
The transpiration-cohesion-adhesion theory
water vapor leaves the air spaces of the plant via the
stomates
this water is replaced by
evaporation
of the thin layer of water that clings to the
mesophyll
cells
tension (pulling) on the water in the
xylem
gently pulls the water toward the direction of
water loss
the
cohesion
of water is strong enough to transmit this pulling force all the way down to the
roots
adhesion
of water to the cell wall also aids in resisting
gravity