Absorption of water is not dependent of process but it is related to transpiration. Absorption is controlled by rate of water loss in transpiration at least when water is readily available to the roots. Absorption and transpiration are linked by the continuous water column in xylem system of plants. Due to the loss of water in transpiration, it produces the energy gradient which causes the movement of water from soil in to the plants and from plants to atmosphere. In the maintenance of water column in xylem, the cohesive and adhesive properties of water play important role. Moisture enters in to plant roots by process of osmosis (movement of liquid through semi permeable membrane caused by unequal concentration on the two sides). The concentration of soluble material in cell sap of roots is increased because of loss of water through transpiration. When concentration of soluble material in cell sap within roots is greater than the soil moisture, the water passes in the roots to equalize the concentration. A more correct view to consider the concentration of water molecule in cell sap reduced because of quantity of soluble substances present and hence the number of water molecules in the soil solution is greater. As a result more water molecules strike against cell wall and water passes into the roots from the zone of higher concentration of water to a zone of lower concentration of water. When the concentration of soluble substances in the soil moisture exceeds that cell sap, situation will be reserved and water will pass out of the roots to the soil. Plants growing in saline soils with high concentration of soluble salts absorb water with difficulty due to high osmotic pressure of the soil solution. The absorption of water by plants is closely related with transpiration. The sun provides energy for vaporization of water from leaves. Loss of water from leaf cells cause an increase in interior osmotic pressure which causes water to move in to them from xylem vessels. The xylem vessels of leaf are continuous with that of stem and roots and cause a tension created by loss of water from leaf to be transmitted to roots. Increased osmotic pressure in root cells occurs and uptake of water is encouraged. The absorption of water takes place in terminal portion of roots but the maximum absorption takes place in the zone of root hairs, 1 to 10cm behind root tip. In other words, water is absorbed mainly through roots hairs. Root absorbs water both passively and actively. Passive absorption takes place when water is drawn into the roots by negative pressure in the conducting tissues created by transpiration. Under the conditions during which there is little transpiration, the roots of many plants absorb water by spending energy that is called active absorption. Under normal conditions of transpiration, the contribution of active absorption to the water supply of plant is negligible and it is usually less than 10 percent of total absorption. Certain plants are able to absorb moisture from the atmosphere when soil is at permanent wilting point. This is known as aerial absorption or negative transpiration. Direct absorption of water by leaves that are wetted by rain, dew or overhead irrigation can help to resaturate dehydrated leaf tissue. The leaves are borne throughout the stem in all plants which are mainly responsible for the loss of water. The leaf surface shows small pores surrounded by two cells. The pores are called stoma and cells surrounding them are called guard cells. The stoma (stomata) regulates the loss of water as vapour and exchange of CO2 in leaf and other organs. It is thus the efficiency of these structures which possibly determine water loss from plant. The efficiency of the stomata up on their size and number per unit area.
FACTORS AFFECTING ABSORPTION OF WATER
- A) PHYSICAL FACTORS:
The soil and atmosphere are the chief physical factors which determine the flow rate of water through plant.
- I) SOIL WATER CONTENT:
The plant roots can easily absorb the soil moisture in between field capacity and permanent wilting point. When the soil moisture decrease below the wilting point, plant roots have to exert more pressure and thus rate of absorption decreases. On the other hand, when the soil is completely saturated with water, then soil temperature and aeration are poor and this condition also affects the absorption of water.
- II) SOIL TEMPERATURE:
Soil temperature is known to influence water absorption and ultimately transpiration to a considerable extent. In many plants, water absorption below a soil temperature of 10 oC is reduced sharply and 25 oC soil temperature up take of water is slowed down. In most instances, temperature above 40 oC does not support water absorption and plant can show signs of wilting. A freezing temperature reduces water absorption because of following causes.
- a) Decreased root growth
- b) Increased viscosity of water
- c) Increased resistance to movement of water in to roots. thus is caused by decreased permeability of cell membrane and the increased viscosity.
III) SOIL AERATION AND FLOODING:
Most of crop plants are not able to water while standing under water logged conditions. The following are the possible reasons of flood injury.
- a) Poor availability of oxygen and occurrence if higher CO2 concentration around roots.
- b) Accumulation of toxic substances either in the submerged roots or around them.
- c) Changes in pattern of ion up take resulting in the accumulation of some toxic ions.
In water logged condition, the availability of oxygen is reduced which affects respiratory actively of roots. In addition, CO2 concentration is increased and it affects permeability of membranes and adversely influences water up take. Reduced oxygen also affects root growth adversely.
- B) ATMOSPHERIC FACTOR:-
Classification of Crops According To Root Depth, Rooting Characteristic And Moisture Use Of Crops. The amount of soil moisture that is available to a plant is determined by the moisture characteristics of the soil, the depth to which the plant roots extend and the proliferation or density of the roots. Soil moisture characteristics, such as field capacity and wilting percentage are peculiar to a soil and are a function of the texture and organic matter. Little can be done to alter these limits to any great extent. Greater possibilities lie in changing the characteristic of the plant enabling it extend its rooting system deeper into the soil, thereby enlarging its reservoir of water. The density of roots proliferation is important. Water is an unsaturated soil moves very slowly, and only a distance of a few cm. To utilize effectively the moisture stored in the soil profile, roots must continue to proliferate into unexploited zones throughout the plants growth cycle. During favorable growing periods, roots often elongate so rapidly that satisfactory moisture contacts can be maintained even when the soil moisture content declines. Where transpiration is effected due to the different atmosphere factors such as wind velocity, humidity, sunlight, etc when temperature and wind velocity are more sunlight for longer period and humidity are less, under such conditions, transpiration is more. The increased rate of transpiration results more water uptake.
- C) BIOLOGICAL FACTORS:
Root system is the plant factor which is directly related to the absorption of water from soil. Under favorable soil water, potential soil temperature, aeration, and roots system of the plants strongly influence the uptake of water. When growth of roots (root system) is more, uptake of water is also more under favorable soil conditions. Root growth is influenced by soil and more therefore agronomic management practices can help to improve root growth. Other plant factors such as morphology of leaves, stomatal mechanism and growth stage of the crop influence the rate of transpiration. The increased rate of transpiration results more water absorption. Good root system has developed during favorable growing periods; a plant can draw its moisture supply from deeper soil layers. Plants vary genetically in their rooting characteristics. Vegetable crops such as onions and potatoes have a spare rooting system and are unable to use all the soil water within the root zone. Forage grasses, sorghum, maize and such other crops have very fibrous, dense roots. Lucerne has a deep root system. Whether plant is an annual or perennial is another factor affecting its its moisture relations. An annual plant must extend its roots down into the soil to make availability root depth, and needs only to extend its small roots and hairs to be able to utilize the entire amount of available soil water. Plants may be limited in their rooting by factors other than genetic. High water table, shallow soils and an impermeable formation near the ground surface restrict the depth rooting. Fertility and salt status of the soil influence the rooting of plants crop management practices, such as cutting the top growth at different physiological stages and the cultivation and cutting of surface roots after rooting habits. The rooting pattern of common and crop plants vary widely from soil. For example, roots of maize crop have been found to extend as deep as 1.5 meters in medium to textured soils, while in a fine textured soil the crop has a shallower root system. Effective Root zone: Effective root zone is the depth from which the roots of average mature plant are capable of reducing soil moisture to the extent that it should be replaced by irrigation. It is not necessarily to have maximum root depth for ant given plant especially for plants that have a long taproot. Root development of any crop varies widely with the type of soil and other factors. Table: Effective root zone depth of some crops and their classification. Rooting Characteristic Shallow Rooted= Rice, Potato, Cauliflower, Cabbage, Lettuce, onion Moderately Deep Rooted=Wheat, Castor, Ground Nut, Pea, Bean, Chili, Tobacco Deep Rooted= Maize, Cotton, Sorghum, Bajara, Soybean, Sugar Beet, Tomato Very Deep Rooted=Sugarcane, Citrus, Coffee, Apple, Grape Vine, Safflower, Lucerne
MOISTURE EXTRACTION PATTERN WITHIN ROOT ZONE:-
The moisture extraction pattern shows the relative amounts of moisture extracted from different depths within the crop root zone. It is seen that about 40 percent of the total moisture used is extracted from first quarter of the root zone, 30 percent from the second, 20 percent from third and only 10 percent from last quarter. This indicates that the need for making soil moisture measurements at different depths within the root zone in order to have estimate of soil moisture status.