CHAPTER: 8
Ascent
of Sap
Ascent
of Sap:
• The
upward movement of water from the root to aerial parts of the plant body is
called ascent of sap or often called translocation of water.
• It
is fascinating to understand how water moves in plants to such great heights
such as 400 ft. or more.
• For
example, trees like Sequoia semipervians are as tall as 300 to
400 ft. They are the 4000 yrs old and giants among the tree plants.
These plants transport water through their stem must be incredible.
• The
plants absorb a large quantity of water
from the soil by root hairs. From root hairs, it reaches to the top of the
plant through xylem, where approximately the same amount of water is transpired
from the surface of the aerial parts of the plants to the atmosphere.
• Water
and minerals absorbed by the root hairs are called sap.
• The
upward movement of sap from the root to the tip of plant is called transport of
water and minerals or ascent of sap.
• Ascent
of sap takes place against the gravitational force.
Factors
Affecting Ascent of Sap:
- Water Absorption:
• Rate
of Ascent of sap is directly proportional to the rate of absorption of water.
- Transpiration:
• More
the rate of transpiration , more is the upward movement of sap.
- Temperature:
• High
temperature increases rate of ascent of sap by increasing rate of
transpiration.
- Concentration of Soil Solution:
• High
concentration of soil solution reduces the rate of ascent of sap by decreasing
the rate of water absorption.
5
Availability of water:
• Availability
of water in the soil directly influences the rate of water absorption and
thereby ascent of sap.
Mechanism
of Ascent of Sap:
• The
mechanism by which the water and minerals moves upward from roots against the
gravitational force is one of the unsolved problems of plant physiology and has
puzzled botanists for a long time.
• The
mechanism of ascent of sap can be discussed under two headings:
- Path of Ascent of Sap
- Mechanism of Ascent of Sap
- Path
of Ascent of Sap:
• From
various experiment, it is proved that the xylem tracheids and vessels are the
actual path of ascent of sap.
• Tracheids
and vessels form a hollow tube like structure from the root to the tip of the
plant and provide path for the movement of water and minerals.
- Mechanism
of Ascent of Sap:
• A
number of theories have been put forward from time to time to explain the
mechanism of ascent of sap.
• The
various theories can be broadly classified as;
- Vital
Force Theory
- Root
Pressure Theory
- Physical
Theory
- Vital Force
Theories:
• According
to these theories, the living plant cells take an active part in the
movement of water and mineral.
• Some
of the Vital Force theories are as follows:
- Godlewski’s Theory
(Relay Pump Theory): According to this theory, the
upward movement of water takes place by the pumping activity of the
cells of xylem parenchyma and medullary rays. The pumping activity is
brought about by periodic changes in osmotic pressure of these
cells. The xylem tracheids and
vessels act as water reservoirs.
- Pulsation
Theory of Bose: Bose observed that the pulsatory
activity of the innermost layer of the cortex, just outside the endodermis
is responsible for the ascent of sap. According to him, the cells of
this layer were expanding and contracting alternately. When the cell
expand they absorb water from the lower cells and when they contract,
water is pumped into next higher cells.
Objection
to Vital Theories:
• Strasburgher
(1893) showed that water continues to rise in plants even after killing their
living cells.
• Thus,
the vital force theories are no more accepted.
- Root
Pressure Theory:
• A
sort of hydrostatic pressure develops in the roots due to accumulation of
absorbed water. This is called root pressure. Some scientists believe that root
pressure is responsible for Ascent of Sap.
Objection
to Root Pressure:
• Normally
observed root pressure is low which is unable to raise sap in giant tall tree.
• Root
pressure is generally absent in gymnosperm plants, which include some of the
tallest trees in the world.
• Root
pressure is seen only in rainy or spring season.
• Rapidly
transpiring plants mostly show a negative root pressure.
• The
ascent of sap even occur in absence of root pressure.
- Physical
Theories:
• According
to these theories, the living cells are not involved in the ascent of sap.
It is purely a physical phenomenon. The physical forces exerted inside the
cells are responsible for the ascent of sap. Many physical force theories have
been put forward to explain the mechanism of ascent of sap. Some of them are as
follows:
- Atmospheric
Pressure Theory:
• According
to this theory active transpiration creates a vacuum in the plant with the
result that water is forced up from below to equalize the pressure.
• The
principal objection to this theory is that the operation of atmospheric
pressure requires a free surface at the lower end. This does not exist due to
the presence of several layers of living cortical cells which separate the
xylem from the soil water.
• Moreover,
atmospheric pressure would be able to raise water only to a maximum height of
about 30 ft. and will not be significance in tall trees.
2.
Capillary Theory:
• According
to this theory, the upward movement of sap takes place through the narrow
vessels due to the capillary force. The tracheids and vessels form a
continuous capillary tube like structure from root to the leaf of the plant and
cause ascent of sap.
Objection:
• The
average and standard diameter of vessels found among the plants is about
0.03mm. The smaller tracheids with 0.02mm diameter can rise the water column
only up to 150 cm.
• Secondly,
for this phenomenon, one end of the capillary tube should be dipped in water.
This situation does not exist in nature. The xylem tracheids and vessels are
not in contact with soil water.
• A
free surface must be present in the xylem vessels to maintain capillary.
- Imbibition
Theory:
• According
to Sach (1874), the upward movement of water takes place by imbibition through
the thick walls of the xylem cells as well as those of the sclerenchyma of the
phloem.
• It
is observed that the imbibition force is quite adequate for carrying water to
any required height.
• But
the movement of water is extremely slow and the water moves through the lumen
of the xylem ducts and not through the walls.
- Transpiration
Pull or Cohesion – Tension Theory:
• This
theory was put forward by Dixon and Jolly (1894) and Supported by several other
scientist.
• It
is most widely accepted theory, popularly known as Dixon’s Cohesion
Theory.
• According to this theory, water forms a
continuous column from the root to the
parenchyma of leaf through the xylem ducts. Transpiration goes on from the
surface of the parenchyma cells of leaf and causes an increase in the osmotic
pressure of these cells. As a result the water is drawn osmotically from the
xylem elements. The high rate of transpiration creates a high magnitude of the
force with which water is pulled. This is called transpiration pull. The water
column moves upward by mass flow due to transpiration pull and simultaneously,
the process of ascent of sap is accomplished.
• The
continuity of water column in the xylem ducts does not break because of
cohesion force among water molecules and adhesive force between water molecules
and cell wall of the xylem.
Evidence
in support to this theory:
- The
osmotic pressure of the transpiring cells often reaches to 20 atms, which
is quite enough for ascent of sap. It has been calculated that 1 atm osmotic pressure can raise water column
upto 10 metres.’
- Purely
physical process , do not require metabolic energy.
- The
rate of water absorption depends upon rise and fall in the rate of transpiration.
- The
tensile strength of the xylem sap is between 25-300 atm which is
sufficient to maintain continuous water column
Objection
to this theory:
• Transpiration
is not solely responsible for the creation of an upward tension in the water
xylem.
• Water
column break down frequently even in herbaceous plant.
• Air
bubbles are frequently found in the vessels and tracheids may break the
continuity of the water column.
• High
wind velocity, high variation of temperature may cause breaking of the
continuity in support.
It's good information
ReplyDelete