Ascent of sap

Ascent of sap

1. ASCENT OF SAP
By
Dr.M.V.SureshBabuM.Sc., M.Phil.,Ph.D.,
Assistant Professor of Botany
E_mail: [email protected]

2. ASCENT OF SAP
Definition:The upward movement of water and
mineral saltsin xylem vessels through the roots to
aerial parts of the plants is known as ascent of sap.
Mechanism of Ascent of sap :
A number of theories have been given to explain
about the mechanism of ascent of sap.
A) Vital theories
B) Root pressure theory
C) Physical force theory

3. A) Vital theories
1.Relay pump theory –Godslewski
According to this theory the pumping of water
takes place upwards due to the vital activities of
xylem parenchyma and xylem rays.
2. Pulsation theory --J. C. Bose
According to this theory water is pumped up due
to the contraction and expansion of innermost
cortical cells which creates a pulsation causing
upward movement of water.

4. B) Root pressure theory
The pressure exerted by the root tissues that
would force liquid up the xylem.
The basis for root pressure is a higher solute
concentration, and accordingly a more negative
water potential, in the xylem sap than in the soil
solution.
This water potential draws water into the stele,
once there, the water has nowhere to go but up,
so it rises in the vessels and tracheids.
The best evidence of root pressure is guttation
and bleeding.

5. Manifestation of root pressure:
A) Guttation
Good evidence that shows the phenomenon or root pressure
guttation,
A process inn which liquid water is forced out through openings
at the margins of leaves (Hydathodes) due to root pressure.
Guttationoccurs only under conditions of
1. high atmospheric humidity and
2. plentiful water in the soil,
which occur most commonly
at night.

6. B) Bleeding
Exudation of sap from injured
parts of the plants is called
bleeding.
Examples: Exudation of milky
latex from euphorbia, cut
stumps of some plants, such
as Coleus.

7. Experimental studies of root pressure:
If a well watered plant is cut near its base, the xylem sap is
seen to flow out through cut end with a pressure.
This pressure is actually the hydrostatic pressure developed in
the root system called root pressure

8. Limitations:
Root pressure, however, cannot account for the
ascent of sap in trees. Root pressure seldom
exceeds 0.1–0.2 MPa(1–2 atmospheres).
If root pressure were driving sap up the xylem, we
would observe a positive pressure potential in the
xylem.
Materials can be transported upward in the xylem
even when the roots have been removed

9. 3. Transpiration pull theory
The most accepted theory of water movement
through plants.
Proposed by DIXON and JOLLY in 1894.

11. According to this theory:-
Due to transpiration water lost in the form of vapourfrom
the leaves.
Due to water loss mesophyllcells becomes concentrated.
DPD of mesophyllincreases.
These mesophyllcells draw water from adjoining mesophyll
cells which in turn from xylem elements of leaf.
Xylem filled with continuous water column from leaf to root
due to cohesive force.
A tension is generated at the top called transpiration pull.
This tension transmitted downwardsup toroots through
stem.
Due to which water moves upwards.