Water-Absorption-ascent-of-sap | uksir-notes |Plant- Physiology
Plant Physiology:
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| water-absorption |
Absorption of Water in Plants:
- Water is absorbed by plant from soil.
- The water absorption mostly occurs by root hairs.
- Root hairs are extension of root epidermal cell , which provide large surface area for absorption.
- They are having high conc. Of cell sap, which help in water absorption by osmosis.
Pathway of water movement:
- water movement mostly occurs by 2 path ways.water movement
Apoplastic water movement pathway:
- movement of water through the non living Part only.- here water move from the root hair to the xylem through cell wall only.
- Apoplast movement include the water flow through cell wall & air space.
Symplastic water movement path way:
- movement of water through the living part.- here water moves through cell membrane and cytoplasm of attached cells.
- it may be
- non-vacuolar symplast movement: here water PAC's from one all to another with out entering to the vacuole. Here water pass from one all to another through the plasmodesmata connection.
- Vacuolar symplast movement: here water enter through the vacuole. Water more through the cytoplasm as well as the tonoplast (membrane of vacuole).
Mechanism of Water absorption:
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It is of two type:
Active absorption of water:
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Here roots have active role.
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Here ATP used for absorption of water.
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It is of further 2 types-
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Osmotic absorption: Here OP of cell sap (in root hair
cells) is more than soil water. For which DPD and Suction pressure increases.
Water enters to root cell and decreases the conc. Now the Adjacent Cortex cell
having more DPD, Suction pressure than the root hair cell. Thus water enter to
cortical cell by osmotic pressure. This process continue up to the leaf
cell. E.g.- in dry soil this type of
water absorption occurs.
§
Non Osmotic absorption: Here OP of soil is higher than
the root hair cell. So plant absorb water against conc. Gradient. Here ATP is
used by the root cell for absorption of water. E.g – water absorption in saline
water area.
Passive Absorption of water:
passive-absorption
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In passive water absorption, plant do not use energy.
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Instead plant develops a suction pressure, by using
Transpiration, DPD and TP.
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Due to transpiration, mesophyll cells of leaf increases
DPD, while TP decrease.
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They draw water from their adjacent cell.
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The adjacent cells again draw water from the xylem to
maintain the deficit.
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This process
continues up to the root cells.
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Which ultimately draw water from soil.
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This form a continuous flow of water due to suction
pressure.
Path-of-water
Factors:
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Soil water amount and concentration.
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Optimal temperature (20-300C)
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Deficiency of oxygen in soil decrease water absorption.
ASCENT OF SAP:
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The Translocation of water from bottom to top of a
plant, may be called as ascent of sap.
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Sap- water with dissolved inorganic ions.
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Thus The transport of sap from root to aerial part/
leaf of plants is called as ascent of sap.
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Various theories have been put forwarded from time to
time to explain the mechanism of ascent of sap:
Vital force theory: (Pulsation theory)
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Relay pump theory: Proposed by Godlewski-
water transport occurs due to a rhythmic change of osmotic pressure of the
living cells.
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J.C. Bose was one of the strong supporters of this
theory.
Pulsation theory
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Pulsation Theory: According to J.C. Bose
(1923) translocation of water occurs due to vital activity of the cells.
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These living cell show pulsatory activity, by which
water is transported.
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This hypothetical theory was later discarded by
Strasburger (1891).
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He killed the stem cells of a plant by using poison (Picric acid),
and proved that there is no role of living cell in water translocation.
absorption
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Imbibition theory: proposed by Sach (1878).
According to this theory water transportation occurs due to the imbibition of
water in wall of xylem.
Root pressure theory:
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It was proposed by Priestley (1916).
Root pressure
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If stem of some plants cut near the base, xylem sap seen
to flow out. It is called as Exudation or bleeding.
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This occurs due to the hydrostatic pressure developed
due to root xylem.
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This occurs due to metabolic activity of the root cells.
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It push the water up ward and transport of water
occurs.
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How ever it is not enough to translocate water to a
height of 300-400 ft.
Capillary
force theory:
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Proposed by Boehm (1863)
Capillary-action
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According to him the vessel and tracheid's act as fine
capillaries. So water transport occurs by capillary action. But its not
sufficient, and can lift up to 3m only.
Cohesation- Tension Theory/ Transpirational pull theory:
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Proposed by Dixon and Jolly (1894).
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Supported by Curtis and Clark (1951), and Levitt (1969)
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Also name as Dixon and Jolly theory/ Transpirtaional
pull theory.
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| Dixon and jolly theory |
Cohesion-
Tension Theory:
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Cohesion- it’s a phenomenon of force of attraction
between similar molecules.
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Here water molecules binds so strongly that, they cant
be separated and form a strong continuous water column.
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Adhesion- Here another force helps, that is – force of
attraction between water molecule and xylem wall.
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This adhesion also help in transport of water.
Transpirational Pull-
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| pull due to evaporation |
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This is the main force, Which help to lift water to
such a great height.
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This force develops due to transpiration in the aerial part
of the plant.
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The mesophyll cells loss water and increase their DPD.
§
Then they absorb water from other cells of leaf and
ultimately from xylem.
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This process continue up to root and soil water.
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This force of attraction is so much high that, it can
lift water upto 130m.
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Thus the water column is maintained.
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