Download Absorption of Water by Plants

Absorption of Water by Plants
Absorption of water
by
cells and roots
Availability of Water in the
Soil
Soil is the major source of water for
plants. The plants absorb water
through root hairs from the soil.
There are four types of water, but all
of them are not available to the
plants.
Gravitational Water
When the water enters the soil and
passes through the spaces between
the soil particles and reaches the
water table, the type of soil water is
called gravitational water. This
water lies far below and is generally
not available to the plant roots.
Hygroscopic Water
This is the form of water which is held by
soil particles on soil surface. The water is
held tightly around the soil particles due
to cohesive and adhesive forces. These
forces greatly reduce the water potential
and thus this type of water in soil is not
available to plants.
Capillary Water
In smaller pores of the soil, water is
held against the force of gravity by
capillary forces and is called capillary
water. This form of water is most
important to plants and constitutes
the only available source of water to
plants.
Run Away Water
All the rain water falling on the
soil is not retained by it. Run
away water does not enter the
soil and gets drained away from
soil surface and reaches to the
water table.
Plants absorb water mostly from soil by their
roots. As a general rule, a plant's root system is
about as extensive as its shoot system. Water and
minerals enter the plant from the soil through
the surface of the fine branches of its roots.
Several centimeters of the tip of each root are
covered with numerous root hairs. Each root hair
is a tubular projection of an epidermal cell. Root
hairs provide the plant with an extensive surface
area and maximum access to the water supply.
Plants have extensive root system to absorb
water and minerals
Parts of a root
Maximum
absorption takes
place through root
hairs
Corn root with
prolific growth
of root hairs
for uptake of
water and
nutrients from
the soil.
For example, a single rye plant
(Secale cerale) was found to have
a root system that occupied 0.08
m3 of soil. The plant had an
estimated 13.8 million roots
covered with 14 billion root hairs,
providing a combined surface area
of 632 m2 (equivalent to oneseventh the area of a football
field).
As the root tip grows into the soil,
new root hairs emerge behind it,
and thus new reservoirs of water
are continually being tapped.
Meanwhile, older root hairs usually
die.
Water Movement Through A Plant
Plant physiologists discuss water
relations in terms of water potential
(Ψw), an expression of the free
energy of water in a system. In this
concept, water moves from a high to
a low water potential. The water
potential of pure water is set
arbitrarily at 0. In a solution or
under tension, the water potential is
lower, a negative value.
Plant water transport is readily
explained in terms of water
potential. Water enters a root hair
cell from the soil because cell solutes
lower the water potential (Ψw). In
the xylem vessels, water is under
tension and has a still lower Ψw, and
water is pulled from the endodermis
and across the root.
Up in the leaf, the bundle sheath
cells contain solutes and have a
low Ψw; they pull water from the
xylem. Photosynthetic cells in the
leaf, with their high sugar content,
have a still lower Ψw and draw
water from adjacent cells in a chain
extending to the bundle sheath.
As water evaporates from the leaf
cells and diffuses out of the leaf, the
lowered Ψw of these cells pulls water
from adjacent cells, generating the
transpiration pull. Outside the leaf, if
the relative humidity is low, Ψw is low
and water diffuses out of the
intercellular spaces.
Water and minerals move from the root hair cell to cells deeper in the root.
Aquaporins are
specific transport
proteins - aid in
passive movement of
water only.
There are two types of
movement of water in
cell
1. Apoplastic movement
2. Symplastic movement
Mechanism of Water Absorption
Water is absorbed in plants by two
methods:
1. Active absorption
2. Passive absorption
Active Absorption
Water is absorbed due to activities going on in
roots. Absorption of water occurs with the help
of energy in the form of ATP, which is released
due to metabolic activities of root cells such as
respiration. Absorption takes place against
concentration gradient - even when the
concentration of cell sap is lower than that of
soil water.
Passive Absorption
Passive absorption is by osmosis.
Passive absorption takes place along
the concentration gradient - when
the concentration of cell sap is
higher than that of soil water. Water
is absorbed when transpiration rate
is high or soil is dry.
Mycorrhiza
Mycorrhizae (“Myco = fungus, and “Rhiza
= root) are fungi that grow in mutually
beneficial (symbiotic) or sometimes
harmful relationships with the roots of
vascular plants. More than 90 percent of
plants have a mycorrhizal symbiont that
plays an important role in biochemical soil
processes.
Some mycorrhizae penetrate into the cells
of plant roots (endomycorrhiza) while
others (ectomycorrhiza) grow between
the roots of woody plants.
Mycorrhizae facilitate water and mineral
exchange with soil, contribute to disease
and toxicity resistance and help in the
colonization of barren soil or degraded
landscapes.
In exchange for these benefits, the
fungi gain access to carbohydrates
produced by the plant and transferred
from the leaves to the root system.
Mycorrhizal plants are at a particular
advantage in nutrient-poor soils
where they enable plants to exploit
the limited resources of water and
nutrients available to them.
Amanita
A mycorrhizal
fungus
Sequoia sempervirens
The tallest tree of the
word
The End