Mechanism of Absorption of Water:
- In higher plants water is absorbed through root hairs which are in contact with soil water and form a root hair zone a little behind the root tips. Root hairs are tubular hair like prolongations of the cells of the epidermal layer (when epidermis bears root hairs it is also known as pestiferous layer) of the roots. The walls of root hairs are permeable and consist of pectic substances and cellulose which are strongly hydrophilic (water loving) in nature. Root hairs contain vacuoles filled with cell sap.
Mechanism of water absorption is of two types:
(1) Active Absorption of Water:
- In this process the root cells play active role in the absorption of water and metabolic energy released through respiration, is consumed.
Active absorption may be of two kinds:
(a) Osmotic absorption i.e., when water is absorbed from the soil into the xylem of the roots according to the osmotic gradient.
(b) Non-osmotic absorption i.e., when water is absorbed against the osmotic gradient.
(2) Passive Absorption of Water:
- It is mainly due to transpiration, the root cells do not play active role and remain passive.
(c) Active Osmotic Absorption of Water:
- First step in the osmotic absorption of water is the imbibition of soil water by the hydrophilic cell walls of root hairs. Osmotic Pressure (OP) of the cell-sap of root ha its is usually higher than the OP of the soil water. Therefore, the Diffusion Pressure Deficit (DPD) and the suction pressure in the root hairs become higher and water from the cell walls enters into them through plasma-membrane (semi-permeable) by osmotic diffusion. As a result, the OP, suction pressure and DPD of root hairs now become lower, while their turgid pressure is increased.
- Now, the cortical cerise adjacent to root hairs have higher O.P., suction pressure and D.P.D. in comparison to the root hairs. Therefore, water is drawn into the adjacent cortical cells from the root-hairs by osmotic diffusion.
- In the same way, the water by cell to cell osmotic diffusion gradually reaches the innermost cortical cells and the endothermic, Osmotic diffusion of water into endothermic takes place through special thin walled passage cells because the other endodermal cells have caspian strips on their walls which are impervious to water.
- Water from endodermal cells is drawn into the cells of pericycle by osmotic diffusion which now becomes turgid and their suction pressure is decreased. In the last step, water is drawn inte xylem from turgid pericycle cells. (In roots the vascular bundles are radial and protoxylen elements are in contact with pericycle).
- It is because in absence of turgid pressure of the xylem vessels (which are non-elastic), the Suction pressure of xylem vessels becomes higher than the suction pressure of the cells of the pericycle. When water enters into xylenm from pericycle, a pressure is developed in the xylem of roots which can raise the water to a certain height in the xylem. This pressure is called s root pressure.
(d) Active Non-Osmotic Absorption of Water:
- Sometimes, it has beern observed that absorption of water takes place oven when the OP of he soil water is higher than the OP of cell-sap. This type of absorption which is non- osmotic and -against the osmotic gradient requires the expenditure of metabolic energy prohably through respiretion.
Following Evidences support This View:
(I) The factors which inlhibit respiration also decrease water absorption.
(ii) Poisons which retardi metabolic activities of the rcot cells also retard water absorptions
(iii) Auxins (growth hormones) which increase metabolic activities of the cells stimulate absorption of water.
(2). Passive Absorption of Water:
- Passive absorption of water takes place when rate of transpiration is usually high. Rapid evaporation of water from the leaves during transpiration creates a tension in water in the xylem of the leaves. This te nsion is transmitted to water in xylem of roots through the xylem of stem and the water rises upward to reach the transpiring surfaces.
- As a result, s oil water enters into the cortical cels through root hairs to reach the xylem of roots to maintain the supply of water. The force for this entry of water is created in leaves due to rapid transpiration and hence, the root cells remain passive during this process.
- During absorption of water by roots, the flow of water from epidermis to endodermis may take place through three different pathways:
(I) Apoplastic pathwvay (cell walls and intercellular spaces),
(ii) Trans-membrane pathway (by crossing the plasma membranes)
(iii) Symplast pathway (through plasmodesmata).
- The mechanism of water absorption described earlier, in-fact belongs to the second category. The relative importance of these three pathways in water absorption by roots is not clearly established. However, a combination cof these three pathways is responsible for transport of water across the root.
External Factors Affecting Absorption of Water:
(1) Available Soil Water:
- Sufficient amount of water should be present absorbed by the plants. Usually the plants absorb capillary water i.e., water present in films in berween soil particles. Other fonns of water in the soil e.g.. hygroscopic water, combined-water, gravitational water etc, are not easily available to plants. Increased amount of water in the soil beyond a certain limit results in poor acration of the soil which retards metabolic activities of root cells like respiration and hence, the rate of water absorption is also retarded.
(2) Concentration of the Soil Solution:
- Increased conc. of soil solution (due to the presence of more salts in the soil) results in higher osmotic pressure. If the O.P. of soil solution will become higher than the O.P. of cell sap in root cells, the water absorption particularly the osmotic absorption of water will be greatly suppressed. Therefore, absorption of water is poor in alkaline soils and marshes.
(3) Soil Air:
- Absorption of water is retarded in poorty aerated soits because in such soilsi deficiency of O1 and consequently the accumulation of CO2 will retard the metabolic activities of the roots like respiration. This also inhibits rapid growth and elongation of the roots so that they are deprived of the fresh supply of water in the soil. Water logged soils are poorly aerated and hience, are physiologically dry. They are not good for absorption of water.
(4) Soil Temperature:
- Water absorption is decreased, At low temp also water absorption decreases so much so that at about 0°C it is almost checked. Increase in soil temperature up to about 30°C frvours water absorption. At higher temperatures water absorption is decreased.
This is probably because at low temp:
(1) The viscosity of water and protoplasm is increased,
(2) Permeability of cell membranes is decreased,
(3) Metabolic activities of root cells are decreased,
(4) Growth and elongation of roots are checked.
Relative Importance of Active and Passive Absorption of Water:
- There are two views regarding the relative importance of active and pa ssive absorption of water in the water economy of plants. Many workers in the past regarded the active absorption of water to be the main mechanism of water absorption and gave very little importance to the passive absorption. But according to Kramer (1969) the active absorption of water is of negligible importance in the water economy of most or perhaps all plants.
- He regards the root pressure and the related phenomena involved in the active absorption of water as mere consequences of salt accumulation in the xylem of different Ikinds of roots. The salt nccumulation produces a difference in water potential which brings about the inward movement of water (osmotic uptake) and development of a pressure in the xylein sap (root pressure).
There are many reasons for regarding the active absorption as unimportant:
- The volume of exudates from the cut stump is very small in comparison to the volume of water lost in transpiration by the similar intact plants under conditions favourable for transpiration.
- Intact transpiring plants can absorb water from more concentrated and drier soil solutions more easily than the similar de-topped plants.
- No root pressure can be demonstrated in rapidly transpiring plants. Such plants may show even a negative root pressure (1.e.. il a littie water is placed over the cut stump it is absorbed by the latter).
- In conifers root pressure has rarely been observed. It is held by certa in workers that though the active absorption is not important quantitatively, it occurs all the time and supplements passive absorption. Two main arguments are against this view. Firstly, during periods of rapid transpiration the sans are removed from the root xylem so that their concentration be:omes very low.
- Under such conditions the osmotic uptake of water cannot be expected to occur. Secondly, even if we suppose that the salts are not removed during periods of rapid transpiration, the latter reduces the water potential ofthe cortical cells in roots to such a low level that the osmotic entry of water from cortex to xylem is not possible.
- The available evidence suggests that usually the water is pulled passively into the plant through the roots by forces which are developed in the transpiring surfaces of the shoot. But under. certain conditions such as warm moist soil and low rate of transpiration, salts accumulate in xylem of roots resulting in active osmotic absorption of water.
Field Capacity or Water Holding Capacity of the Soil:
- After heavy rainfall or irrigation of the soil, some water is drained off along the slopes while the rest percolates down in the soil. Out of this latter water some amount of water gradually reaches the water table under the force of gravity (gravitational water) while the rest is retained by the soil. This amount of water retained by the soil after the drainage of gravitational water has become very slow is called as field capacity or the water holding capacity of the soil.
- The field capacity is affected by soil profile, soil structure and temperature. For instance a fine textured soil overlying a coarse textured soil will have a higher field capacity than a uniformly fine textured soil. Similarly, the field capacity increases with decreasing temperature and vice versa.
Permanent Wilting Percentage or Wilting Coefficient:
- The percentage of the soil water left after the plant growing in that soil has permanently wilted is called as permanent wilting percentage or the wilting coefficient. The permanent wilting percentage can be determined by growing the seedlings in small containers under conditions of adequate water supply till they develop several leaves. The soil surface is then covered and the water supply is cut until wilting occurs. The containers are now transferred to humid chamber.
- If the plants do not recover, they are considered to be permanently wilted. Otherwise, they are again transferred to normal atmospheric conditions. This process is repeated till they are permanently wilted. The percentage of the soil water is determined at this point after removing the plants from the containers and shaking off as much soil from their roots as possible.
- Earlier workers thought permanent wilting percentage to be a soil moisture constant. This view has been strongly criticised by Slatyer (1957) who pointed out that permanent wilting percentage of a soil is dependent on the osmotic characteristics of the plant and is not a soil-moisture constant. Thus the different plants if grown in the same soil wilt at different times depending upon their osmotic potential after the water supply to the soil is stopped.
Soil Texture in Relation to Water Absorption:
- The texture of a soil depends upon the proportion of different sized soil particles in that soil and is a very important factor for the absorption of water in plants.
Depending upon their diameters the soil particles are classified as below:
- Such soils are very rich in sand particles and though well aerated they have poor water holding capacity Sandy soils are, therefore, not good for water absorption.
- These are rich in clay particles and are poorly aerated. Such soils often become water- logged and are, therefore, neither good for water absorption nor for normal growth of the plants.
- Such soils contain almost equal proportion of the different sized soil particles, They are sufficiently aerated and have good water holding capacity. Therefore, they are very good for water absorption and growth. The loam soil in which the proportion of sand is slightly higher is called as sandy loam while a loam soil in which clay particles predominate, is called as clayey loam.
- Many epiphytic orchids develop special aerial adventitious roots which can absorb moisture from the atmosphere. For this purpose, a special water absorbing tissue is present around the cortex of such roots which is called as velamen. It consists of thin walled parenchymatous cells and the moisture absorbed by it is transferred to the root xylem through exodermis, cortex, endodermis and the pericycle.
- In recent years some integral membrane proteins have been discovered which form water selective channels in cell membranes (lipid bilayers) and facilitate faster movement of water across the membranes into the plant cells. These channels have been called as aquaporins. The direction of water transport across the membranes however, is not affected by aquaporins.
- Aquaporin’s are found in both plant and animal membranes but they are relatively abundant in plants. The squaporin’s satisfactorily account for the observed rate of water movement across the membranes which could not be explained earlier simply by direct diffusion of water through lipid bilayer as the later does not allow bulk flow of water across it.
- According to Tyerman et al (2002), expression and activity of aquamarine’s appear to be regulated probably by protein phosphorylation in response to availability of water.