The absorption and movement of water inside the plant tissue depends upon several environmental and internal factors. Water is the major component of living cells and constitutes more than 90% of protoplasm by volume and weight. So plant water relation is an important factor for the living plants. Water acts as medium for all biochemical reaction that takes place in the cell, and also acts a medium of transportation from one region to another region. Water is a remarkable compounded made up of Hydrogen and oxygen (2:1) and it has high specific heat, high heat of vaporization, high heat of fusion and expansion. Cells of all organisms are made up 90% or more of water. And all other components are either dissolved or suspended in water to form protoplasm, which is often referred to as physical basis of life.
Major physico-chemical phenomena of water movement in plants on which the plant water relation is depending, we are going to briefly discuss here.
Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration by random movement of the molecules or ions of the substance. The molecules move alonga concentration gradient. This movement continues till the concentration of the molecules becomes the same in all the available space. The force responsible for the diffusion of molecules is their kinetic energy, which keeps them in constant motion. There is an inherent tendency in the molecules of a substance to move from the region of their higher kinetic energy to the region of their lower kinetic energy. Thus diffusion can also be defined as the movement of molecules from a region of their higher kinetic or free energy to a region of their lower kinetic or free energy. Diffusion is one of the vital factors to keep up the plant water relation.
Affecting Factors of Diffusion
In general diffusion is a slow process, but the rate of diffusion is faster in following conditions:
i) If the difference in concentration between the two regions (concentration gradient) is increased.
ii) If the distance between the two regions is decreased.
iii) If the area over which diffusion occurs is increased.
iv) If the molecules diffusing are small and fat-soluble.
v) If the number of the pores in the cell membrane is higher and their size is larger.
Role of Diffusion in Plant Water Relation
Diffusion plays an important role in several vital processes in plant water relation. It helps in the exchange of gases during photosynthesis and respiration; and in transpiration water vapors diffuse from the intercellular spaces toll outer atmosphere. During the passive salt uptake ions are absorbed by the simple process of diffusion.
What is Diffusion Pressure?
Diffusion pressure is the pressure exerted by the diffusing particles. This is directly proportional to the concentration of the diffusing particles, that is, higher the concentration of diffusing particles greater the diffusion pressure. Thus the diffusion pressure of a pure solvent is greater than that of the same solvent in a solution.
To maintain plant water relation osmosis also plays some definite role. Diffusion takes place when there is no barrier to the free movement of molecules or ions. If two sucrose solutions of different concentrations are carefully layered over each other, an equalization of the concentration will occur. Molecules of the solvent (water) diffuse into more concentrated sucrose solution and moll of sucrose into the less concentrated solution until a uniform concentration is attained. Diffusion of this when occurs through a semipermeable membrane, is known as osmosis. Membranes of biological system not equally permeable to all molecules; they may allow the solvent to pass through it but not the substance dissolved in it.
What is osmotic pressure?
The hydrostatic pressure which balances and prevents the osmotic inflow of water into concentrated solution is called osmotic pressure. Thus, osmotic pressure may be defined as the potential capacity of a solution to attract and gain more solvent molecules when the solution is separated from the solvent by a differentially permeable membrane.
Plant Water Relation as an Osmotic System
The plasma membrane of a leaving cell is a differentially permeable membrane and the cell sap contained in the vacuoles is an aqueous solution of salts and sugars with a high osmotic potential. Since cells of a plant are in contact with a watery medium or with other cells containing water, they may absorb or lose water by osmosis. When a cell is placed in a hypotonic solution, ( i.e. a solution whose osmotic potential is lower than that of the cell sap) water enters the cell. This inward movement of water into a cell is said to be endosmosis. On the contrary, when a cell is placed in a hypertonic solution, ( i.e. a solution whose osmotic potential is higher than that of the cell sap) water moves out of the cell. This outward movement of water from the cell is called exosmosis. In making of plant water relation in a plants life osmosis plays very important role It helps in absorption of water from te soil and maintains the turgidity of cells.It also assist in closing and opening of stomata.
Permeability means the ability tof a membrane to allow passages of gases, liquid and the substance dissolved in liquid through it. A permeable membrane may be (i) fully-permeable, (ii) semi-permeable or selective permeable. The selective permeability is vital in maintaining the life of cell as well as the plant water relation. Cell membrane provides only one passage for certain materials and blocks their passage out. When the cell loses its property of semi-permeability, all soluble substances quickly leak out and therefore become dead.
The shrinkage of cytoplasm along with plasma membrane from the cell wall under the action of a hypertonic solution is known as plasmolysis. Plasmolysis is a vital phenomenon, and it proves permeable nature of the cell wall and semi-permeable nature of the plasma membrane. It also helps in determining the osmotic pressure of a cell and maintain the plant water relation.
When water enters into a living cell, a pressure is developed within the cell due to turgidity. The hydraulic pressure developed inside the cell on the cell wall due to endosmosis is called turgor pressure. The pressure is responsible for pushing the membrane against the cell wall. Turgidity of cells is essential for the plant water relation to survive the plant and grow normally. Turgor pressure aids in cell enlargement and consequently in stretching of shoots, and in keeping leaves erect and fully expanded.
The chemical potential of water is said to be water potential, and this potential can be taken as a measure of energy available for reaction or movement of water to maintain the plant water relation. Water always moves from a region of higher water potential to a region of lower. Pure water has the maximum water potential. Water molecules will always move from pure water into any solution on the other side of a partial permeable membrane.