Theory 4.

ABSORPTION AND TRANSLOCATION OF WATER

Water Absorbing Parts of Plant

Major portion of water required by the plants is absorbed by the roots. But the
absorption of water by leaves and stem has also been found in a few plants.

Mechanism of Water Absorption

In higher plants, water is absorbed through root hairs, which are in contact with
soil water and from the root hair zone, a little behind the root tips. Root hairs are tubular
hairs like prolongation of the cells of the epidermal layer of the roots. The walls of root
hairs are permeable and they consists of pectic substances and cellulose which are
strongly hydrophilic (water loving) in nature. Root hairs contain vacuoles filled with cell
sap. When roots elongate, the older hairs die and new root hairs are developed so that
they are in contact with fresh supplies of water in the soil.
Mechanism or mode of water absorption by the plants
It is taking place in two ways:

1. Active Absorption of water

In this process, the root cells play active role in the absorption of water and metabolic
energy released through respiration is consumed. Active absorption may be of two kinds.

a. Osmotic absorption: When water is absorbed from the soil into the xylem of the roots
along the osmotic gradient.
b. Non-osmotic absorption: When water is absorbed against the osmotic gradient.

T.S. of a typical dicot root

The arrow indicates the path of water

Now, the cortical cells adjacent to root hairs have higher OP, suction pressure and
DPD in comparison to the root hairs. Therefore, water is drawn into the adjacent cortical
cells from the root hairs by osmotic diffusion. In the same way, the water reaches the
innermost cortical cells and the endodermis by cell to cell osmotic diffusion gradually.
Osmotic diffusion of water into endodermis takes place through special thin walled
passage cells because the other endodermis cells have casparian strips on their walls
which are impervious to water. Water from endodermal cells is drawn into the cells of
pericycle by osmotic diffusion which now becomes rigid and their suction pressure is
decreased. In the last step, water is drawn into xylem from turgid pericycle cells. It is
because in the absence of turgor pressure of the xylem vessels (which are non-elastic),
the suction pressure of xylem vessels become higher than the suction pressure of the cells
of the pericycle. When the water enters into xylem from pericycle, a pressure is
developed in the xylem of roots, which can raise the water to a certain height in the
xylem. This pressure is called as Root Pressure.

2. Passive Absorption of Water

It is mainly due to transpiration; the root cells do not play active role and they
remain passive. This theory explains that some forces are responsible for absorption of
water into the roots, which are actually governed by other cells. The governing force
originates in the cells of transpiring shoots rather than in root itself. These forces develop
due to transpiration. Passive absorption of water takes place when rate of transpiration is
usually high. Rapid evaporation of water from the leaves during transpiration creates a
tension in water in the xylem of the leaves. The tension is transmitted to water in xylem
of roots through the xylem of stem and the water rises upward to reach the transpiring
surfaces. As a result, soil water enters into the cortical cells through root hairs to reach
the xylem of the roots to maintain the regular supply of water. The force for this entry of
water is created in leaves due to rapid transpiration and hence, the root cells remain
passive during this process.

Difference between Active and Passive Water Absorption

S.No Active absorption Passive absorption
1. The active absorption of water It occurs due to activity of upper part of the
takes place due to the activity of plant, such as shoot and leaves.
root and root hairs.
2. The absorption of water occurs by The water is absorbed due to the
the active osmotic and non- transpiration in aerial parts.
osmotic processes.
3. The root hairs have high DPD as The absorption occurs due to tension
compared to soil solution, and created in xylem sap by transpiration pull.
therefore, water is taken in. Thus, water is sucked by the tension.
4. In the movement of water, living The movement of water is through. free
part of protoplasts (symplast) is spaces or apoplast of root and it may
involved include cell wall and intercellular spaces
(apoplast)..
5. The rate of absorption depends The rate of absorption depends upon
upon DPD or difference in osmotic transpiration rate.
concentrations between the two
6. In non-osmotic type, respiratory Energy is never required.
energy is utilized.

I. External Environmental Factors

a. Available Soil Water
Sufficient amount of water should be present in the soil in such form, which can
readily and easily be absorbed by the plants. Usually the plants absorb capillary water.
Other forms of water in the soil (hygroscopic, gravitational etc.) are not easily available
to plants.
b. Concentration of Soil Solution
Increased concentration of soil solution (due to the presence of more salts in the
soil) results in higher OP. If the OP of soil solution becomes higher than the OP of cell
sap in root cells, the water absorption will be greatly suppressed. Therefore, absorption of
water is poor in alkaline soils and marshes. This is popularly known as physiological
dryness.
C. Soil Temperature
Increase in soil temperature up to about 300C favors water absorption. At higher
temperature, water absorption is decreased mainly due to the death of root cells on
account of protein denaturation. At low temperature also water absorption decreases so
much so that at about 100C, it is almost checked. This is probably because at low
temperature:
1. Viscosity of water and protoplasm is increased.
2. Permeability of cell membranes is decreased.
3. Metabolic activities of root cells are decreased, and
4. Growth and elongation of roots are checked.

d. Soil Aeration
Water is absorbed more efficiently in the well-aerated soil than in poorly-aerated
soils, may be because of the fact that roots fail to respire anaerobically and plants die in
flooded areas. The deficiency of O2 inhibits the growth and metabolism and an
accumulation of CO2 increases the viscosity of protoplasts and decreases the permeability
of cell membrane. Both these factors affect and reduce the rate of water absorption
severely.

II. Internal Environmental Factors

i. Transpiration
The rate of absorption of water is directly proportional to that of transpiration. A
higher rate of transpiration produces a tension or pull called as Transpiration Pull,
transmitted to roots through hydrostatic system of plants creating a favorable condition
for the entry of water.

ii. Absorbing Root System

The efficiency of water absorption depends on the absorbing system or root
system. The presence of more number of root hairs accounts for the increased rate of
water absorption.

iii. Metabolism
The metabolism and absorption are closely related. Factors inhibiting respiration
such as poor aeration, application of anesthetics and KCN reduce the absorption rate.
Thus, metabolic activities are expected to participate indirectly by forming a constantly
elongated root system and always providing newer contacts with soil water.