Ascent of sap via xylem : Ascent of sap via xylem
Slide 2 :
Transpiration Pull and Cohesion Tension Theory : Transpiration Pull and Cohesion Tension Theory The ascent of sap is due to transpiration and the cohesion and adhesion of water
Proposed by H. Dixon and R. J. Joly (1894)
Widely accepted
Theory is based on…
1. Cohesive properties of water
2. Adhesive properties of water and
3. Transpiration pull.
What do mean by transpiration pull : What do mean by transpiration pull The pulling force in xylem due to transpiration is called transpiration pull.
Can Transpiration pull lift water? : Can Transpiration pull lift water? Yes
But how ? : But how ? During transpiration, water evaporates from the leaves to the outer atmosphere through the stomata.
More water is released into the intercellular spaces from the mesophyll cells.
In turn mesophyll cells draw water from the xylem of the leaf.
Due to this, a tension is created in water in the xylem elements of the leaves.
This tension is transmitted downward to water in the xylem of root through the xylem of stem.
Now the water is pulled upward in the form of a continuous unbroken water column to reach the transpiring surfaces up to the top of plants.
In plants the pull is generated by the process of transpiration which in known as transpiration pull.
Cohesion tension hypothesis of water transpiration for the ascent of sap in plants showing path of water and ions from the soil up to the leaves : Cohesion tension hypothesis of water transpiration for the ascent of sap in plants showing path of water and ions from the soil up to the leaves
Factors influencing ascent of sap : Factors influencing ascent of sap 1. Soil-water deficiency in the soil … decreases ascent of sap indirectly by affecting the water absorption.
2. Water transport is regulated by root pressure and transpiration.
High temperature,
Low atmospheric humidity,
High atmospheric pressure and
wind velocity …. rapid transpiration and so these factors also influence upward movement of water
Low water potential in leaves cause transpiration : Low water potential in leaves cause transpiration
Why do you think vapours condensed inside bell jar? : Why do you think vapours condensed inside bell jar? Due to condensation of water vapour produced during transpiration.
Important : Important 98% of water absorbed by a plant is lost in transpiration.
Of the 2% of water retained by the plants, 0.2% is used in photosynthesis
Rest is used for other metabolic function such as growth.
Slide 12 : Transpiration
is a simple evaporation phenomenon
Occurs through the aerial parts of the plant.
Could be demonstrated by experiments
Slide 13 : On the basis of different exits we can classify three major sources of transpiration.
1. Lenticular Transpiration
2. Epidermal Transpiration
or
Cuticular Transpiration
3. Stomatal Transpiration
Lenticular Transpiration : Lenticular Transpiration Occurs in woody plants …..e.g. dicots and gymnosperms.
A little amount of water (0.1%) is lost here through the lenticels of bark (of twig or branch or stem).
Does not occur in monocots where secondary growth does not occur.
Therefore, not allowing free air in which transpiration normally takes place.
Slide 15 :
Epidermal or Cuticular Transpiration : Epidermal or Cuticular Transpiration Water is lost through the cuticle of stem and leaves
Comparatively little.
Sciophytes (shade lovers) in strong sunlight wilt because of great loss (upto 50%) of water by epidermal transpiration through leaf epidermis
As they possess no cuticle or thin cuticle.
Opuntia, Acacia & Euphorbia show such kind.