How Water is Absorbed by Land Plants?

Water is the principal constituent of living cell and constitute about of protoplasm. Water is the principal raw material for primary production in photosynthesis. All the biochemical process takes place with the aid of water or in the water medium. It helps as a principal transport media in plant body. Hence water is indispensible for plants and plants care to up take land water through the root system.

Root hairs of root hair zone of roots are the site for absorption. The cell walls of root hairs made up specific substances which are strongly hydrophilic in nature and sucks water from the soil capillary water. Cell membranes of root cells are permeable to water.

Mechanism of Water Absorption:

Absorption of water by plants is a complex physiological process which principally involves the entry of water from the surface of soil particle in to the xylem of root through root hair cells and path of water to the xylem. The mechanism of water absorption has been explained under two distinct modes of absorption.

Active Absorption of Water by Plants:

Active absorption process is that in which root cells play active role in the absorption which utilise metabolic energy from the cells produced during respiration.

Further, active absorption takes place with or without involvement of osmosis a physical process of solute transport across the membrane.

Active Osmotic Absorption:

Atkins (1916) Priestley (1920) and other explained that simple osmosis involves in the root cells in absorption of water osmotic pressure difference in the cell sap solution and outside soil solution initiate the process of absorption it includes following steps.

i. Soil water first imbibed by the root hair cell wall due to its hydrophilic nature.

ii. Osmotic pressure of the cell sap of root hair cell is usually higher than the osmotic pressure of soil water. It creates diffusion pressure deficit (DPD).

iii. As a result suction pressure in the root hair cell becomes higher. So that water from the cell wall enters into cell sap of root hair cell by osmotic diffusion.

iv. Then cortical cells adjacent to the root hair cell have higher osmotic pressure, therefore water is drawn into the cortical cells from root hair cell.

v. In some way water passes through other cortical cell to the endodermis region. Water from inner most layer of cortical cell pass to the pericycle and xylem through the passage cells.

vi. Due to absence of target pressure in xylem vessels suction pressure is much higher than the surrounding cells. Thus from xylem water never returns back and ascended up.

In this way water from outside pass to the root xylem passing through cell to cell along the concentration gradient.

Active Non-osmotic Absorption:

In some cases it is an observed fact that absorption of water takes place even when the soil solution is much concentrated with minerals than the cell sap solution of root hair cell.

It has been reported by different workers that the external solutions which cause incitement plasmolysis in root cells posses’ considerably higher osmotic pressure than the osmotic pressure of cell sap.

Clarke (1936), Thiemann (1951) Rosen (1950) reported that the absorption of water is an active process but occurs due to some non-osmotic process. This view has been supported by following facts:

i. The factors which inhibit respiration also decrease water absorption.

ii. Poisons which retard metabolic activates of root cells also retard water absorption.

iii. Auxin, the growth hormone which increases metabolic activities of the cells stimulates absorption of water.

iv. There is a distinct diurnal variation of water uptake like respiration.

v. The temperature coefficient of water absorption varies from 2-3 where as for a strictly physical process the Q10 would be 1.2 to 1.3.

Passive Absorption of Water by Plants:

Passive absorption is entirely different physiological process rather than the active. It takes place when there is rapid rate of transpiration in the leaf.

During this period root cells never exhibit important involvement i.e. plants do not show positive root pressure. In fact water can be absorbed if the plant cut near the sol level and deeper into water. Under such conditions water is absorbed passively without expenditure of metabolic energy by root cells. It involves following mechanism.

i. Rapid evaporation of water from the leaves during transpiration creates a tension in water in the xylem of leaves.

ii. This tension is transmitted to water in xylem of root through the xylem of stem.

iii. As a result of which soil water enters into the cortical cells through the root hairs to reach the xylem of roots to maintain the water supply & to combat the tension.

iv. The force for entry to water is created in leaves due to rapid transpiration and hence, the root cells remain passive throughout the process.

Factors Affecting Absorption of Water:

Absorption of water by the root is influenced by the following external conditions.

Availability of Soil Water:

Increase in soil capillary water content of sol greatly influence the rate of absorption. Other forms of water like hygroscopic water, combined water and gravitational water are not readily available for absorption.

Soil Air:

Proper aeration of soil indirectly influence the water absorption i.e. availability of O2 increase the root respiration which produced more ATP and rate of absorption increased thereby poorly aerated soil minimize the rate of absorption.

Soil Temperature:

Increase soil temperature up to 30 degree delicious favours absorption. But at higher temperature and at very low temperature rate of absorption decrease. At 0 degrees Celsius absorption checked up under such conditions:

• Viscosity of water and protoplasm increased
• Permeability of cell membrane is decreased.
• Metabolism root cells decreased.
• Growth and elongation of root checked.

Concentration of Soil Solution:

Increase in concentration of soil solution results in higher osmotic pressure. This cause great suppression of absorption. Absorption is high if soil solution concentration is low.