Availability of soil water for plants:
Soil is the main reservoir of water for plants, which ultimately comes from rain. The amount of water in soil is variable, which depends on the nature of soil, amount of rainfall and vegetation. Water present in the soil is called soil water. A part of rainwater does not enter the soil but is drained away from soil surface along the slope and gets collected in water reservoir such as lakes, river, ponds etc. It is called runaway water and is not available to the plants. The rest of the water which is held by the soil particles occur in the following forms.
- Gravitational water: After a heavy rain, the surface layer of soil is temporarily saturated. It displaces air from large space between soil particles and gradually penetrates through dry layer of soil under the influence of gravity. This water is called gravitational water. It accumulates in the deep situated zone of the earth, called water table or groundwater. It lies beyond the reach of water-absorbing roots and therefore, is not available to the plants.
- Hygroscopic water or imbibed water: A part of soil water which is retained very tightly by colloidal soil particles is known as hygroscopic water. It is the water that is kept absorbed or imbibed by soil colloids. It is held tightly around the soil particles as a result of cohesive and adhesive forces and cannot easily be removed by the plants. Hence, it is not available to the plants.
- Capillary water: It comprises the bulk of water remaining in soil after gravitational water has drained away. It is the water present in micropores of soil or narrow spaces of soil. It occurs in the form of thin films and retained by forces of surface tension. It does not respond to gravitational pull. Capillary water is readily available to plant and is the main source of water to the plants present in the soil.
- Chemically combined water: Some amount of water is chemically combined to minerals present in the soil. It occurs in the form of hydrated oxides of aluminum, iron, silicon etc. This form of water is also not available to the plants.
The amount of available water in a soil depends upon the texture and structure of the soil .For example, the sandy soil which has coarse particles is unable to hold much water due to bigger pore spaces. Clay soil retains much water due to smaller pore spaces, but such soils become easily waterlogged. Loam soil, which is a mixture of sand, silt and clay, is supposed to be the best for plant growth.
After a heavy rainfall or irrigation, a very little amount of water is retained by the soil. Most of the water runs away as a runaway or gravitational water. The amount of water actually retained by soil is called filled capacity or water holding capacity. Soil moisture beyond the field capacity produces water logging.
Field capacity is equal to the sum total of water retained by hygroscopic and retained water.
A plant usually fails to survive if it is conditioned to water deficiency. Wilting is the shrinkage in volume of water in plant cell sufficient to cause them to lose all or most of their turgor. Wilting may be visible in plants as drooping, rolling, folding of leaves in young plants.
When the wilting symptoms are not visible externally and the water content of plant cell decreases, the condition is known incipient wilting. Most of the herbaceous plants usually wilt during hot summer days and regain their freshness or turgor during night. This is known as temporary or transient wilting. If the absorption of water totally ceases or the plant cells do not get water from any sources, the plant undergoes permanent wilting.
PERMANENT WILTING PERCENTAGE
Permanent wiling percentage (PWP) or permanent wilting coefficient (PWC) is the percentage of soil water or water in the soil when a plant or plants growing in it has just reached a condition of permanent wilting. It varies in different types of soil, for example, clay has higher Permanent wiling percentage (PWP) than sand. At permanent wilting percentage, the soil contains about 10% of water which is either present in extremely fine micropores or in the non available stage.
The total water content present in the soil is called holard. The water available to plant is chresard. It consists roughly of 75% of capillary water. The rest of soil water (hygroscopic water, combined water, water vapor and 25% of capillary water) is called echard water or unavailable water.
IMPORTANCE OF WATER FOR PLANTS
Water plays a vital role in the life of a plant. Some of the important functions of water are listed below:
- Water is an excellent solvent. It is a solvent in which various minerals, organic compounds and gases are dissolved. These substances are distributed throughout the plant body in the form solutes.
- Water constitutes more than 80% of the cell protoplasm. Such high proportion of water in the protoplasm is essential for its normal functioning.
- It helps in the maintenance of turgidity in plant cell, which is necessary for growth and maintenance of shape.
- It acts as a reagent in various chemical reactions in plant cell.
- Water is a raw material in the synthesis of organic compounds. Besides, it is an essential medium for the activity of enzymes.
- Temperature inside the plant body is uniformly maintained by water.
- It helps in translocation of chemical substances.
- Water has high latent heat, and thus they have cooling effect on evaporation.
- Water molecules have great cohesive force among themselves, which help in ascent of sap.
- Water helps in respiration, photosynthesis, growth, absorption of dissolved substances and many other plant processes.