Water provides medium in which many substances are dissolved. Protoplasm of cell is consist of water in which different molecules are dissolved and suspended.

A watermelon consist of 92%

Herbaceous plants have only about 10-15% of water as dry matter.

Transpiration:- Loss of water through the leaves of plant by the process of evaporation is called transpiration.

Water is limiting factor for plant growth due to high demand of water.

Water potential: It is the difference in free energy of water molecules in solution and that of pure water at the same temperature and pressure.

• Indicated by Ñ°_w(psi), for pure water Ñ°_w = 0

For solution Ñ°_w = -ve

Solute Potential: The amount by which water potential is reduced as a result of presence of solute is called solute potential. Also known as osmotic potential.

• Represented in bars.
• Osmotic potential and osmotic pressure are numerically equal but osmotic potential is –ve.
• Indicated by Ñ°_S.

Pressure potential:-

It operates in plant cells as wall pressure and Turgor pressure.

• Usually has +ve value.
• Indicated by Ñ°_p.

Ñ°_w = Ñ°_s + Ñ°_p + Ñ°_g                           Here Ñ°_g = gravitational potential

Ñ°_w = Ñ°_s + Ñ°_p + Ñ°_m                          Here Ñ°_m = Matrix potential

Usually

So, Ñ°_w = Ñ°_s + Ñ°_p

Osmoses:-  The process of diffusion though semi-permeable membrane is called osmosis.

• Discovered by Pfeiffer
• Solvent moves from its high conc. to lower concentration.
• The net direction and rate of osmosis depends upon pressure gradient and conc. gradient.

Osmotic pressure:- When semi-pressure membrane respects concentrated solution from weaker solution, pressure in atmosphere required in opposite direction to stop the entry of water molecules.

Plasmolysis:-

• Three types of solution:-

1. Isotonic solution:- When conc. of cell sap is equal to conc. of solution in which cell is dept.
2. Hypertonic solution:- When cell sap conc. is less than conc. of solution in which cell is kept.
3. Hypotonic solution:- When cell sap conc. is more than conc. of solution is which cell is kept.

• In hypertonic solution, water moves out from cell and cell membrane of plant cell shrinks away from cell wall. This process is called plasmolysis. Water moves out from area of high potential to low potential of water.
• Cells are said to be flaccid if water moves in and out from the cell is in equilibrium.
• Turgor pressure: It is pressure built against the cell wall when water diffuses into the cell. This is called pressure potential (Ñ°_p). This pressure is responsible for enlargements and extension growth of cells.

Imbibitions:-

• A special type of diffusion when water is absorbed by solids:- colloids is called imbibitions that results in increase in volume.

e.g. Absorption of water by wood

• Pressure produced during imbibitions is called Imbibitional pressure.

Imbibition involves adsorption i.e. partial absorption and is property of colloids.

Significance of Imbibition:

• First step of water absorption.
• First phenomenon in seeds germination.

Characteristics of Imbibition:

• Pressure is developed.
• Heat of wetting is developed.
• Increase in volume of Imbibant.