06 Plant Water Relation and Mineral Nutrition
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Plant obtain variety of substances like water, minerals, nutrients, food and gases like O2
and CO2 from surrounding. Plant productivity is mainly affected by the non-availability of water.
Water is the most important factor for all the vital activities of plants. Water is considered as
'elixier of life'. Plant cells and tissues contain about 90 to 95 % water. All the plants absorb water
and mineral salts from the soil by their root system. Water maintain the turgidity and shape of
cells.
Properties of water: -
1. It is in the liquid form at room temperature.
2. It is the best solvent for most of the solutes also called as universal solvent.
3. It is inert inorganic compound having neutral pH when in pure form.
4. Water is the best transporting medium for dissolved minerals and food molecules.
5. Water is the best aqueous medium for all biochemical reactions occurring in the cell.
6. Water is essential raw material for photosynthesis.
7. Water has high specific heat, high heat of vaporization and high heat of fusion, thus acts as
thermal buffer.
8. Water molecule have good adhesive and cohesive forces of attraction because of which it can
easily rise in capillaries.
These various properties of water are due to hydrogen bonds between the water
molecules.
Thus, water is a significant molecule which connect physical world with
biological processes.
-: Water absorbing organs: -
Root: - Root is the main organ of water and mineral absorption. In terrestrial plants water is
absorbed in the form of liquid from soil. While in epiphytes like orchids water is absorbed in the
form of vapours from air by specialised epiphytic roots having velamen.
Regions of root: - The typical root has four regions as: –
1. region of cell division, 2. region of elongation,
3. region of absorption (root hairs) 4. region of maturation.
The absorption of water takes place at region of absorption or region of root hairs. 1
, Structure of root hair: -
1. In region of absorption epidermal cells (epiblema cells) from unicellular hair
like extensions called as root hairs.
2. Each root hair is unicellular, elongated, delicate, unbranched, prolongation of epidermal cell. It
is tubular, about 1 to 10 mm long and colourless.
3. Root hair contain peripheral cytoplasm, nucleus and central large vacuole.
4. The cell wall of root hair is made up of cellulose and pectin.
5. The cell wall is freely permeable to solute and water. Inside cell wall selectively permeable
plasma membrane is present.
Water available to roots for absorption: -
Soil is the main source of water for plants. Plants absorb water from the microenvironment
surrounding the root called as rhizosphere.
All water present in soil is not available for plants.
Different types of water present in the soil as follows: -
Hygroscopic water: - The water which is held very tightly around the soil particles by adhesive
forces is called as hygroscopic water.
Combined water: - The water present in the form of hydrated oxides of silicon, aluminum
etc. is called as combined water.
Hygroscopic water and combined water are also called bound water and it is not available to
plants.
Gravitational water: - Because of gravitational force some amount water goes down through large
pores between soil particles and reaches water table is called as gravitational water. It goes
beyond the reach of roots of most of the plants.
Capillary water: - The water held in very fine spaces (capillaries) between small, non-colloidal soil
particles is called capillary water. Capillary water is the only water available to the plants for
absorption.
Absorption of water by roots from soil: -
Root hair absorb water by applying three physical processes i.e. imbibition, diffusion and osmosis.
Imbibition: -
1. The swelling of hydrophilic colloids due to adsorption of water is called imbibition.
2.Substance which absorbs water or liquid is called as imbibant and the water or liquid which get
absorbed is called as imbibate.
3.In imbibition water molecules get tightly adsorbed on the surface of compounds without
forming a solution. Imbibition continues till the equilibrium is reached.
4. The hydrophilic compounds like cellulose and pectin in root hair cell wall imbibe water.
5. Thus, imbibition is the first physical process involved in water absorption by plants.
e.g. Soaking of seeds, swelling of dried raisins, kneading of flour, swelling of wooden doors and
windows in rainy season etc.
Diffusion: - (Diffusion = disperse )
1. Diffusion is the movement of ions, atoms or molecules of solute, liquids or gases from
the region of their higher concentration to a region of their lower concentration till equilibrium is
Maintained.
2
--------------------------------------------------------------------------------------------------------------------
Plant obtain variety of substances like water, minerals, nutrients, food and gases like O2
and CO2 from surrounding. Plant productivity is mainly affected by the non-availability of water.
Water is the most important factor for all the vital activities of plants. Water is considered as
'elixier of life'. Plant cells and tissues contain about 90 to 95 % water. All the plants absorb water
and mineral salts from the soil by their root system. Water maintain the turgidity and shape of
cells.
Properties of water: -
1. It is in the liquid form at room temperature.
2. It is the best solvent for most of the solutes also called as universal solvent.
3. It is inert inorganic compound having neutral pH when in pure form.
4. Water is the best transporting medium for dissolved minerals and food molecules.
5. Water is the best aqueous medium for all biochemical reactions occurring in the cell.
6. Water is essential raw material for photosynthesis.
7. Water has high specific heat, high heat of vaporization and high heat of fusion, thus acts as
thermal buffer.
8. Water molecule have good adhesive and cohesive forces of attraction because of which it can
easily rise in capillaries.
These various properties of water are due to hydrogen bonds between the water
molecules.
Thus, water is a significant molecule which connect physical world with
biological processes.
-: Water absorbing organs: -
Root: - Root is the main organ of water and mineral absorption. In terrestrial plants water is
absorbed in the form of liquid from soil. While in epiphytes like orchids water is absorbed in the
form of vapours from air by specialised epiphytic roots having velamen.
Regions of root: - The typical root has four regions as: –
1. region of cell division, 2. region of elongation,
3. region of absorption (root hairs) 4. region of maturation.
The absorption of water takes place at region of absorption or region of root hairs. 1
, Structure of root hair: -
1. In region of absorption epidermal cells (epiblema cells) from unicellular hair
like extensions called as root hairs.
2. Each root hair is unicellular, elongated, delicate, unbranched, prolongation of epidermal cell. It
is tubular, about 1 to 10 mm long and colourless.
3. Root hair contain peripheral cytoplasm, nucleus and central large vacuole.
4. The cell wall of root hair is made up of cellulose and pectin.
5. The cell wall is freely permeable to solute and water. Inside cell wall selectively permeable
plasma membrane is present.
Water available to roots for absorption: -
Soil is the main source of water for plants. Plants absorb water from the microenvironment
surrounding the root called as rhizosphere.
All water present in soil is not available for plants.
Different types of water present in the soil as follows: -
Hygroscopic water: - The water which is held very tightly around the soil particles by adhesive
forces is called as hygroscopic water.
Combined water: - The water present in the form of hydrated oxides of silicon, aluminum
etc. is called as combined water.
Hygroscopic water and combined water are also called bound water and it is not available to
plants.
Gravitational water: - Because of gravitational force some amount water goes down through large
pores between soil particles and reaches water table is called as gravitational water. It goes
beyond the reach of roots of most of the plants.
Capillary water: - The water held in very fine spaces (capillaries) between small, non-colloidal soil
particles is called capillary water. Capillary water is the only water available to the plants for
absorption.
Absorption of water by roots from soil: -
Root hair absorb water by applying three physical processes i.e. imbibition, diffusion and osmosis.
Imbibition: -
1. The swelling of hydrophilic colloids due to adsorption of water is called imbibition.
2.Substance which absorbs water or liquid is called as imbibant and the water or liquid which get
absorbed is called as imbibate.
3.In imbibition water molecules get tightly adsorbed on the surface of compounds without
forming a solution. Imbibition continues till the equilibrium is reached.
4. The hydrophilic compounds like cellulose and pectin in root hair cell wall imbibe water.
5. Thus, imbibition is the first physical process involved in water absorption by plants.
e.g. Soaking of seeds, swelling of dried raisins, kneading of flour, swelling of wooden doors and
windows in rainy season etc.
Diffusion: - (Diffusion = disperse )
1. Diffusion is the movement of ions, atoms or molecules of solute, liquids or gases from
the region of their higher concentration to a region of their lower concentration till equilibrium is
Maintained.
2
