1. Water Relation

Diffusion :

• It is defined as the process of random movement of molecules from a region of their higher concentration to a region of their lower concentration.
• It is a slow process.
• The pressure exerted by diffusing the ions or the molecules is called diffusion pressure.

Experiment of diffusion

To show the simple process of diffusion in a solution ,a beaker  is filled with water. A small  crystal of potassium permanganate (KMnO₄) is placed into water. The purple color of permanganate is spread and distributed evenly throughout the water until all the water becomes the same shade of purple color.

                                                    fig: Demonstration of diffusion process in a liquid

Significance of diffusion:

1. Diffusion process helps in the gaseous exchange during the process of photosynthesis and respiration.
2. The process of diffusion is involved in the transpiration in which water is lost in  the form of water vapor.
3. It helps in pollination as the insects are attracted to the pollinating agent due to the diffusion of aroma of flowers.
4. During passive uptake of salts, the ions are absorbed by the process of diffusion.
5. Diffusion helps in translocation of the food materials in the different parts of the plants.

Terms related to diffusion

Diffusion pressure(DP) :

• The pressure exerted by the diffusing particles is called diffusion pressure.
• This pressure is proportional to the concentration of diffusing particles.
• Pure water has the maximum diffusion pressure. 

Osmotic pressure (OP) : The maximum amount of pressure that can be developed in a solution separated from pure water by a semi-permeable membrane is called osmotic pressure.

Turgor pressure (TP):

The hydrostatic pressure developed inside the cell on the cell wall due to endosmosis is called turgor pressure.

Osmosis :

• It is defined as the process of movement of water(solvent) from the region of its higher concentration to the region of its lower concentration through a semi permeable membrane until equilibrium is maintained.

Types of osmosis

Exosmosis:

• It is the process of outward movement of water from a cell when it is placed in hypertonic (of higher concentration) solution.
• for example: when some fresh grapes are immersed in a concentrated salt solution, grapes show shrinkage after a few hours due to the loss of water content by the process of exosmosis.

Endosmosis :

• It is the entry of water into a cell when it is placed in hypotonic solution.
• for example: if some dry raisins are immersed in a dish of water then after few hours it is observed that the raisins have swollen due to a passage of outside water into the raisins by the process of endosmosis.

Significance of osmosis

1. By the process of osmosis ,roots absorb the water.
2. Osmosis helps the movement of water from one cell to another.
3. The rigidity of plant organs is maintained by turgidity caused due to osmosis.
4. Osmosis helps to control dehiscence of anthers, sporangia, fruits etc.

Difference between diffusion and osmosis

Plasmolysis:

• It is the shrinkage of protoplasm from  the cell wall under the action of some strong solution is called plasmolysis.
• It proves permeable nature of cell and semipermeable nature of plasma membrane.

Significance of plasmolysis:

1. It explains the process of osmosis.
2. It also proves the permeability of the cell wall and semi-permeability of the plasma membrane.
3. It is also used to determine osmotic pressure of cell.

                                                           fig: stages of plasmolysis of a plant cell

De-plasmolysis

When a plasmolysed cell soon after plasmolysis is placed in pure water ,the protoplasm regain its original form by the process of endosmosis. therefore the phenomenon of absorption water by a plasmolysed cell through endosmosis is called de-plasmolysis.

Wilting

Wilting is the loss of turgidity of leaves and other soft aerial parts causing drooping, folding and rolling. Wilting is of three types:

1. Temporary wilting – It is the temporary drooping down of leaves and young shoots due to the loss of turgidity during noon. It is very well revised simply after the pace of transpiration diminishes joined by the renewal of water around root hairs.
2. Incipient wilting – Partial and temporary loss of turgor in a plant that occurs in the presence of adequate soil moisture and is associated with excessive water loss through transpiration.
3. Permanent wilting –It is that state in the loss of turgidity of leaves when they do not regain their turgidity even when being placed in a suitable atmosphere.

Imbibition

The process of increase in the volume of solids due to the absorption of water is called imbibition.

Ascent o f sap

It is the upward movement of water and dissolved mineral salts from roots upwards to the leaves and other aerial parts against the gravitational force. It takes place through the xylem.

Mechanisms of ascent of sap

A. Theory of vital force

• This theory suggests that the metabolic activity of the living cells is very much responsible for ascent of sap.

Root pressure theory

This theory suggests that the sap in the xylem is forced upwards under hydrostatic pressure i.e. root pressure developed in roots.

B. Theory of physical force

According to this theory, the ascent of sap for most part is purely involving physical forces.

1.Capillary force theory

Christian wolf purposed this theory in 1873 and he suggests that water rises up in the narrow tubes of xylem vessels by surface tension.

2.Atmospheric pressure theory

According to this theory, atmospheric pressure is responsible for ascent of sap.

3.Transpiration pull-cohesive force theory

This is the most widely accepted theory proposed by Henry H. Dixon and John H. Jolly in 1895. It is based upon following considerations:

1. There is continuous water column right from the vein endings iof leaves down to root hairs.
2. There is a great mutual attraction among the water molecules which is called cohesive force.
3. There is a great adhesive force between the water column and then walls of xylem vessel.
4. The energy for the upward movement of water column is provided by transpiration from leaf cells.

Transpiration

• It is the process of loss of water vapor from the internal tissues of living plants through the aerial parts such as the leaves, green stem, flower etc.
• The rate of transpiration is measured by an instrument called photometer.
• It is the vital and unavoidable phenomenon of plants.

                                          fig: Transpiration in plants

Types of transpiration

1. Stomatal transpiration: It is the evaporation of water from the stomata of the plants. The water near the surface of the leaves changes into vapor and evaporates when the stomata are open.

2. Lenticular transpiration: It is the type of transpiration in which water vapors is lost through the lenticles. Lenticles are minute openings in the bark of branches and twigs.

3.Cuticular transpiration: It is the type of transpiration in which water vapors is lost through the cuticle. Water lost through it is less.

4. Bark transpiration: It is the type of transpiration in which water vapors is lost through the bark. The surface of bark is generally covered by cork. Cork is generally impermeable to water.

Structure of stomata

Small gateways in the epidermal cells are called as stomata. Stoma is controlled or guarded by specially modified cells called guard cells. These guard cells may be kidney shaped (dicot) or dumbbell shaped (monocot), collectively called as stomata. Guard cells control the size of stomata by changes in their turgidity. Change in turgor pressure of guard cells causes opening and closing of stomata, which enables exchange of gases and water vapor. Stomata are further covered by subsidiary cells. Stoma, guard cells and subsidiary cells form a unit called stomatal apparatus. Stomata are the site of gaseous exchange for respiration and photosynthesis. They are absent in root and non green parts of plants.

Significance of transpiration

1. Advantages of transpiration:

1. It creates suction force and helps in the ascent of sap.
2. It helps in distribution of water throughout the plant.
3. It helps in evaporating excess amount of water.
4. It secures the concentration pf cell sap and thus helps in osmosis.

2.Disadvantages of transpiration:

1. Excessive transpiration leads to stunted growth if plants.
2. Excessive transpiration causes wilting and drying of plants.
3. Excessive transpiration causes water deficit and injury to the plant by desiccation.
4. Higher rate of transpiration reduces photosynthesis which causes growth to reduce.

Anti- transpirants:  They are the chemical substances that are used to reduce the rate of transpiration.

Guttation

It is the process of loss of water in the form of water droplets along with minerals from the tip of the leaf veins.

                                                fig: Guttation in plants

Significance of guttation

1. It helps plants to dispose of the unwanted solutes.
2. It helps the plants to improve acquisition of nutrients.
3. It helps in maintaining water balance for the proper growth and development of the plant body.