MECHANISM OF WATER UPTAKE BY PLANTS....Muhammad Nadeem Ashraf

1. Muhammad Nadeem Ashraf
Soil & Environmental Sciences
Faculty of Agriculture, Rawalakot.

2. MECHANISM OF WATER
UPTAKE BY PLANTS
The mechanism of absorption of water of two types active
and passive often operating independently
• Active Absorption:
• Two types of theories:
1. Osmotic theory
2. Xon Osmotic theory

3. • Osmotic theory:
• It explain that the root hairs act as ohmmeters
and there exist a typical condition so that water
can move accross osmotically from soil solution
to root xylem.
• The theory assume that translocation of water
takes place by means of diffusion along a
gradient of decreasing water potential or in
creasing water solute.

4. • Non_Osmotic:
• Non Osmotic theory assume that when the
driving force are zero.
• This theory explain the absorption of water
by roots when the osmotic potential of soil
water is greater than that of hair cell sap.

5. • Non Osmotic active water uptake can also be
supposed by the following:
1. There is a correlation b/w the rate of water
absorption and respiration.
2. Auxin_induced water uptake.
3. The factors such as low
temperature,oxygen,tension and respiratory
inhibitors which affect respiratory rate and in
turn the absorption rate.

6. Cohesion-Tension Theory
• The major mechanism for long distance water
transport is described by cohesion-tension
theory;
• Whereby the driving force of transport is
transpiration,that is,the evaporation of water
from the leaf surface.
• Water molecules cohere(stick together), and are
pulled up by the tension,or pulling force,excerted
by evaporation at the leaf surface.

7. • Water will always move toward a site with lower water
potential,which is a measure of the chemical free energy
of water.
• By definition,pure water has a water potential of 0 Maga
pascal.
• In contrast 20% relative humididy the water potential of
the atmosphere is 500MPa.
• this difference signifies that water will tend to evaporate
into the atmosphere.
• The water with in plants also has a negative potential,
indicating water will tend to evaporate into the air from
the leaf.

8. • The water in plants can exist at such low water potential
due to the cohesive forces of water molecules.
• The chemical structure of water molecules is such that
they cohere very strongly.
• By the cohesion tension theory, when sun light strikes a
leaf, the resultant evaporation first cause a drop in leaf
water potential.this causes water to move from stem to
leaf,lowering the water potential in the stem,which in turn
causes water to move from root to stem,and soil to root.
• This severs to pull water up through the xylem tissue of
the plant.

9. thanks
nadeemawan17@gmail.co
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