Both plant factors (internal factors) environmental factors (external factors) affect the absorption of water by the plants through their roots. These include soil factors (external factors) such as soil temperature, solute potential (osmotic potential) of soil solution, soil aeration (02 concentration and CO2 accumulation), and soil water, whereas presence of efficient absorbing root system, transpiration and metabolic activities of root cells are the plant factors (internal factors) contributing to the absorption of water by roots from the soil.
Soil Factors Affecting Absorption of Water
Although atmospheric conditions may also affect absorption of water but conditions in soil usually act as limiting factors in the absorption of water: These include:
Soil temperature affects the rate of absorption of water. The plants growing in soils with low temperatures absorb less water. At lower temperatures the root growth is inhibited due to slower rate of metabolic activity of the root cells, the protoplasm becomes less permeable to water and also the water becomes more viscous and its mobility is reduced. Very high temperatures are lethal to the cells. The optimum temperature at which maximum absorption can take place ranges from 20 to 30 C.
Soil aeration is one of the most important factors in water absorption. It is commonly observed that rate of water absorption is higher in well aerated soils whereas in poorly aerated soils, such as flooded soils, the leaves of the plants fall down (wilting). Because air spaces among soil particles are occupied by water and the roots fail to respire and die. The deficiency of oxygen results in retardation of root growth and metabolic activity of its cells. Reduced metabolic activity seriously affects the accumulation of salts which lower the water potential of the cell sap and cause water potential gradient.
Accumulation of CO2 causes an increase ¡n the viscosity of protoplasm making it less permeable to water; therefore the rate of water absorption is retarded.
3. Solute Potential of Soil Solution
The solute potential or osmotic potential of soil solution is the driving force that creates water potential gradient responsible for absorption of water by the roots. Therefore, if the water potential of soil solution is more negative (lower) than that of the cell sap of the root cells, water will move out of the plant cells instead of being absorbed. This is why most of the plants fail to grow in soils having high salt contents. However, halophytes (plants growing in saltish areas) have a tolerance for high salt concentrations in the soil solution.
4. Availability of Water
The water is not readily available to the plants at levels above field capacity and level below permanent wilting percentage’. Therefore, the rate of absorption of water is maximum between these two quantities. If water contents increase beyond field capacity the soil aeration is affected resulting in decrease ¡n rate of absorption and decreases of water level below permanent wilting percentage marks the depletion of available water, thus bringing an end to the process of absorption.
Plant Factors Affecting the Absorption of Water
In addition to environmental factors certain plant factors (internal factors) also affect the absorption of water and minerals dissolved in t by the roots. These include:
The presence of a large root system with adequate number of root hair help the plants in absorbing water and minerals dissolved in t from the soil efficiently. In grasses highly branched fibrous root system and in dicots a deep tap root system helps in absorption of water and increasing its rate of absorption. The root hairs play an important role in absorption of water. These are thin-walled therefore, the water moves into them more efficiently from the soil solution. Also the root hairs increase the surface area for absorption. The roots of most of the gymnosperms get infected with fungi present-in the soil and form mycorrhizae. The fungi help in absorption of water from the soil.
The rate of absorption of water is almost equal to the rate of transpiration. At higher rate of transpiration the rate of absorption also increases. Transpiration from aerial parts of the plant result in transpiration pulls that causes tension in the water column held in apoplast and symplast. This pull is transmitted down to the soil solution and result in water potential gradient. This gradient helps in absorption of water.
iii. Metabolic Activities of Root Cells
Although doubt exists regarding the use of energy in absorption of water yet the metabolism and absorption of water is closely related. The factors that inhibit or retard metabolism also affect the rate of absorption. These factors include poor aeration and use of respiratory inhibitors such potassium cyanide. The energy produced during metabolic activity of the cell is used for elongation of root system which provides new contact areas with water present in soil. Thus metabolism participates indirectly in absorption.