The following points highlight the twelve factors influencing absorption of water by plants. Some of the factors are: 1. Demonstration of the Phenomenon of Passive Absorption in Plants 2. Demonstration of the Phenomenon of Active Absorption in Plants 3. The role of Root Growth in Absorption of Water 4. Influence of Water Content of the Soil on the Rate of Absorption of Water and Others.
Experiment # 1
Demonstration of the Phenomenon of Passive Absorption in Plants:
(i) Absorption of water by plants:
A Peperomia or a tube-rose with its root system is inserted through a cork in a conical flask containing 2% solution of eosin. The whole set-up is kept in an open atmosphere for some time. A second similar set-up is also kept under a humid bell jar.
After about an hour it is observed that considerable translocation of coloured water takes place in the first set-up whereas in case of the second set-up it is significantly inhibited. In the experiment with tube-rose white flower petals turn pink sooner in the first than in the second set-up.
The experiment indicates clearly that the red-coloured eosin solution has been absorbed by the root system and trans located up-ward through stems and leaves. This is a case of passive absorption which depends upon transpiration pull.
A transverse section of the stem shows that the lumen of the xylem vessels is coloured red. In the second set-up which was kept in humid condition, the transpiration is considerably checked and hence the absorption and translocation are delayed.
(ii) Rate of water absorption by plants:
A wide-mouthed graduated bottle fitted with a cork is taken. It is filled with water and a rooted plant is inserted through the central hole of the cork so that the roots dip in water. All connections are made air-tight. The initial water level is marked and the whole set-up is kept in an open atmosphere for a few hours. The level of water is noted at an interval of half an hour.
The level of water in the bottle gradually decreases with time.
Water is absorbed by the roots, translocated through the stem and ultimately reaches the leaves from where it is lost due to transpiration. A suction force is thus developed and the absorption is maintained.
If the factors influencing transpiration are not disturbed, the rate of absorption is uniform throughout. The level of water in the bottle decreases due to loss by transpiration.
N.B. The influence of different plant parts and other factors on the rate of absorption of water may be studied as follows:
(а) Effect of number of leaves.
(b) Effect of dry and humid atmosphere by keeping the plant in dry or humid condition.
(c) Effect of dissolved solutes in water.
(e) Effect of absence of air in Water.
(d) Effect of cutting off the roots.
(f) Effect of removal of root hairs.
Experiment # 2
Demonstration of the Phenomenon of Active Absorption in Plants:
A healthy potted tomato or Vinca plant is selected and the pot is watered well before the experiment is started. The plant is detopped 4 cm above the soil surface.
The stump of the decapitated plant is tightly fitted with a graduated glass tube with the help of rubber tubing. The tube is one-fourth filled with water and its initial level is noted. After 1 or 2 hours the rise in water level in the tube is again noted (Figure 7).
It is observed that the water in the tube is pushed up after some time.
Inference and discussion:
The rise of water in the tube is due to root pressure which is responsible for the upward movement of water through the xylem vessels. The difference between the initial and final readings of the level of water gives the quantitative measurement of the root pressure exerted by the detopped plant within that period.
Root pressure is a pressure which develops in the tracheary elements of the xylem as a result of the metabolic activity of roots. It is, therefore, referred to as active process.
The movement of water up the stem as a result of root pressure is due to osmotic pressures (passive), which are created as a consequence of active absorption of salts by the roots. Root pressure is thus referred to as being an active process in the sense that living roots are essential for it.
Root pressure appears only when the plant is transpiring at slower rate but absorbing water at a higher rate. In highly transpiring plants, a negative root pressure develops.
N.B. That the absorption of water takes place actually at the expense of respiratory energy can be shown by inhibiting the respiration of roots by treatments with poisons like potassium cyanide (KCN), chloroform (CHCl2), ether [ (C2H5)2O ] etc.
Experiment # 3
The role of Root Growth in Absorption of Water:
A glass cylinder (5 x 12 cm) is uniformly filled with air-dry soil. Several seeds of Phaseolus or Vicia are sown 2 cm below the soil surface. Sufficient water is added to moisten the soil. When the hypocotyls come out, the soil surface is covered with a polythene sheet to check surface evaporation.
The polythene sheet contains as many holes as there are hypocotyls so that they may pass through these holes. The holes should be plugged with raw cotton after the emergence of the hypocotyls five seedlings are taken out at a time, washed well to remove adhering soil particles, length and number of roots are measured and fresh and dry weights (at 100°C. for 48 h) are taken.
This is repeated every alternate day for 8 days. Water content is determined from the difference of fresh and dry weights.
The water content is found to increase with increase in growth of the roots.
The absorption of water is found to be directly correlated with the growth of the root system.
N.B. This experiment may also be performed in the following way: Sixteen uniformly growing seedlings of the same age are taken out from the pots and washed well to remove the adhering soil particles. Four conical flasks are filled with water and weighed individually.
Four intact seedlings are put in one flask so that the roots are completely immersed. Half of the roots of another lot of four seedlings is removed and put in the second flask. Similarly three-fourth of the root portion is removed from another lot of four seedlings and put in the third flask.
Roots are completely removed from the base of the remaining lot of four seedlings and placed in the fourth flask. After two hours all the seedlings are taken off carefully and each flask is reweighed. The difference between the initial and the final weights in each case indicates the amount of water absorbed by each lot of seedlings.
The results may be expressed as the amount of water absorbed in gm per gm fresh weight of the seedlings in each treatment. It will be observed that the quantity of water absorbed by the seedlings depends upon the amount of roots present.
Experiment # 4
Influence of Water Content of the Soil on the Rate of Absorption of Water:
Three suitable potted plants of similar age and growth are selected so that each plant bears at least a few fully grown leaves. Older leaves which appear chlorotic may be removed. The soil of the first pot should be saturated with water that of the second pot moderately watered (preferably a little above field capacity).
The third pot should remain un watered (preferably a little above wilting percentage). Arrangements should be made to check surface evaporation from the soil. Pots are kept under bright sunlight and the experiment is started preferably in the morning.
Weighing’s of the pots are made every two hours interval for a period of at least 8 hours. At the end of the experiment aggregate leaf area of each plant is determined.
The rate of transpiration per square centimeter leaf area for each 2 hour interval is calculated. A graph is plotted taking the time as abscissa and amount of transpiration per sq; cm as the ordinate.
The rate of transpiration is maximum in the first pot, intermediate in the second and minimum in the third.
In this experiment it has been assumed that the rate of transpiration is a measure of the rate of absorption. In case of the first pot, maximum absorption of water takes place because maximum available water is present.
In the second pot having moderate water content shows intermediate absorption while the third pot being relatively dry shows minimum absorption of water.
Experiment # 5
Influence of Temperature of the Substratum on the Rate of Absorption of Water:
Three suitable wide-mouthed bottles are filled with water. Water of the first bottle is kept at a temperature of 10°C, the second at about 25°C or room temperature and the third at about 40°C. The temperature may be maintained using ice and hot water.
Three similar plants of any herbaceous species (Coleus, Phaseolus or Helianthus) with large, easily measurable leaves are selected. Plants which have been grown in water culture should preferably be used in this experiment but pot-grown plants may also be used if great care is taken to avoid damage of the root system.
Roots of one plant are inserted in each bottle so that it is completely immersed in water. Each plant is supported in its respective bottle with cotton plug. The cotton plug is covered with polythene sheet in order to check evaporation from the bottle.
Each bottle is weighed and kept under bright sun light. The bottles are reweighed separately at an interval of 1 hour for a period of 4 hours. The rate of transpiration per sq. cm of leaf area per hour is calculated. A graph is plotted in a similar way as done in the previous experiment.
The rate of absorption increases with rise in temperature of the substratum.
Here the rate of transpiration has been taken as an index of rate of absorption. When substratum temperature decreases, rate of water absorption by roots is also decreased.
Changes in substratum temperature affect the growth rate of roots, movement of water in the soil, permeability of membranes and walls of root cells, viscosity of protoplasm of root cells, viscosity and density of water, vapour pressure of water, rate of metabolic reactions inside root cells, etc. Within certain limits, high temperature facilitates all these criteria causing greater absorption of water.
Experiment # 6
Influence of the Concentration of the Soil Solution of the Rate of Absorption:
200 ml each of the following molar solutions of NaCL is prepared from 1 M NaCl (dissolve 58.5gm NaCl in 1000 ml water): 0.3, 0.2, 0.1, 0.05 and 0.025. The solutions are poured into five separate bottles of 250 ml capacity.
The sixth bottle is filled with 200 ml of distilled water. Six equal sized uniforms aged vigorously growing sunflower seedlings are selected and weighed separately. The soil from the roots is carefully washed.
Better results will be obtained with seedlings grown in water cultures. One seedling is supported in each bottle immersing its root system well into the solution and the portion of the stem which passes through the mouth of the bottle is plugged with cotton.
The bottles are kept in moderately strong diffused light. After 4 hours, the seedlings are taken out, washed and reweighed. Increase in water content of each seedling is determined by taking the difference of final and initial weights and is expressed as gm of water absorbed per gm fresh weight. A graph may be suitably plotted.
Absorption of water decreases with increase in molar concentration of the solution.
Water absorption by roots becomes difficult or impossible if the concentration of soil solution is higher than that of the protoplasm of the root cells. Concentration of soil solutions depends upon the presence of soluble substances in the soil. If water absorption by roots is due to osmotic forces, then the D.P.D. of water in the soil solutions should be lower than that of the root cells.
Experiment # 7
Influence of Substratum-Air on the Rate of Absorption of Water:
Two suitable wide-mouthed bottles are filled with distilled water. Air-bubbling arrangements are made in one bottle for aeration of the substratum. For convenience, instead of bubbling, fresh Hydrilla plants may be introduced into the bottle for oxygenation of the substratum.
Five healthy uniform-sized Phaseolus seedlings are weighed and put in each bottle so that the roots are completely immersed in water. Both the bottles are kept under bright sun light. After 4 hours seedlings are taken out and reweighed.
Difference between the final and the initial weights in both the sets gives the amount of water absorbed by five seedlings in 4 hours. From this data the amount of water absorbed per gm of seedling per hour is calculated. Two bar graphs are drawn for comparison of results.
From the results it is clear that the aeration of substratum increases the water absorption by roots. When water is absorbed actively, some extra energy which is produced by metabolic activity of living cells is required.
By utilisation of this energy water is thought to be pumped or forced into the xylem vessels. Thus water absorption process is partly dependent upon the respiratory activity of the root cells and is favoured by the availability of oxygen in the substratum. If aeration of soil is not sufficient, it will produce an inhibitory effect on the metabolism of root cells leading to less absorption of water.
Experiment # 8
To Demonstrate the Influence of Soil Texture on Water Absorption:
Three types of soil, viz., sandy, loamy (garden soil) and clayey (collected from tank bed) are sun-dried for 48 hours. 500gm of each type of soil is taken separately in three small earthenware pots having holes at the bottom.
Several Phaseolus or Vigna seedlings (10-day old) of uniform size are selected. The average water content of the seedlings is determined by taking fresh and dry weights of a few seedlings. Now, 10 fresh seedlings are transplanted in each pot giving equal spacing.
Each pot is watered with 50 ml of water and kept in an open space. Care is to be taken to check evaporation from the soil surface. After 2 or 3 days the seedlings are taken out and the average water content is determined.
The difference between the final and the initial water content of the seedlings in each case indicates the amount of water absorbed by the seedlings growing in each pot. Maximum water content is obtained in the seedlings growing in loamy soil and minimum in sandy soil.
The best soil for the growth of a plant is that which has normally sufficient water and air holding capacities. Loamy soil satisfies this condition, as it contains equal proportions of sand, silt and clay. This soil possesses its good porous nature due to presence of sand while clay and silt increase its Water-holding capacity.
The sandy soil has the least, clay the maximum and the loam the intermediate relative values of field capacity. Hence maximum water is absorbed by the plants grown in loamy soil.
Experiment # 9
Effect of Promoter and Inhibitor on the Rate of Water Absorption:
To perform this experiment the following promoters and inhibitors may be conveniently used: IAA (0-01%), glucose (5%), and malic acid (5%) as promoters and chloramphenicol (0-01%), DNP (1%) and mercuric chloride (1%) as inhibitors.
Three wide-mouthed bottles are taken. One is filled with any promoter solution, the second with any inhibitor solution and the third with tap water which acts as a control. Five suitable seedlings of uniform size and age are selected for each set and fish weight is determined. These are then put in each bottle. The bottles are left for 4 hours and the final weight of each set of seedlings is taken again.
The difference between the initial weights in each case gives the amount of water absorbed by a set of five plants in 4 hours. The amount of water absorbed per gm fresh weight of the seedlings per hour is calculated and graphically represented.
From the results it is clear that promoters increase absorption of water over control while inhibitors decrease it. It increases water absorption by softening the cell wall, thus permitting a passive osmotic uptake of water.
Glucose and malic acid, being respiratory substrates, increase active absorption of water by increasing the rate of respiration. DNP being an un-coupler of oxidative phosphorylation-hampers the active absorption process. Mercuric chloride being a toxic substance inhibits respiration of root cells thereby inhibiting active absorption.
Chloramphenicol is a metabolic inhibitor and retards general metabolism particularly protein and RNA synthesis. Studies with promoters and inhibitors show that living cells and their metabolic activities take some part in water absorption process.
Experiment # 10
To Demonstrate the Translocation of Water:
In this experiment both ascent of liquid through the stem and the effect of transpiration on the rise of liquid can be shown. The leafy shoot of a suitable plant having two branches is taken and kept in water.
From one of the side branches all leaves are removed and the stem is cut partly with a knife from the apical region. A narrow glass tube is fitted with (his end with the help of rubber tubing. The glass tube is filled with water and immersed in a beaker containing mercury.
The tube is clamped properly (Figure 8). All connections are made air-tight. Now the water is continuous throughout the whole system extending from the mesophyll cells of the leaf, down through the xylem vessels of the stem up-to the end of the glass tube immersed in the beaker. The whole set-up is kept for 2 to 3 hours under bright sun light.
After the desired time mercury is found to rise in the narrow glass tube.
Under these conditions as water molecules are lost by transpiration from the leaves, an upward pull develops causing mercury to move up into the glass tube. The mercury column remains intact due to cohesive forces.
N.B. The rate of ascent of water may also be determined from this experiment by taking data on rise of mercury in the glass tube per unit time.
Experiment # 11
To Demonstrate that Xylem is the Path of Conduction of Water:
Leafy twigs of Vinca, Ervatamia or any other suitable plant are immersed in red coloured eosin solution. After a few hours cross sections are made from different heights of the stem and observed under the microscope.
It becomes evident that the red colour is primarily confined to the xylem vessels.
This proves that the xylem strands are the main path of upward translocation of water in plants.
Two leaf bearing woody stems, on which the aggregate leaf area appears to be equal, are selected. The basal ends of the twigs are immersed in water immediately after cutting from the plant.
After several hours the stem is cut again-under water; each stem is inserted through the hole of a cork or a rubber stopper of proper diameter to fit in a bottle of approximately 250 ml capacity.
A second smaller hole is also bored through each stopper through which is inserted short bent capillary tubing for passage of air.
The stems should pass through the stoppers for a distance of about 10 cm. All of the tissues external to the xylem are removed from both the stems for a distance of about 3 cm from the cut ends. Stem b is coated with warm paraffin so that only the xylem and pith remain exposed.
Precaution should be taken to avoid adherence of any paraffin on the tip portion of the xylem core. Stem a is also coated with paraffin in such a way that the tissues external to the xylem (i.e., at the point from where tissues have been removed) remain exposed (Figure 9).
Each bottle is nearly filled with a known volume of tap water and each is stoppered with one of the stoppers through which passes one of the stems. Both the sets are kept in bright sun light.
The behaviour of the leaves is observed from time to time until the leaves of one twig show sign of wilting and drying. It is observed that the stem a shows such sign after some time whiles the stem b does not.
The volumes of water in both the bottles are measured. The difference between the initial and the final volumes gives the amount of water absorbed by the stems. Absorption of water is higher in case of stem b compared to stem a.
From this experiment it can be inferred that water is absorbed and trans-located mainly through the xylem and not through the phloem.
A forked twig of Gardenia or Vinca or China rose is taken and its main axis is left intact. From the left hand branch all the cortical tissues are removed from a narrow ring. From the right hand branch the central woody strand of xylem is very carefully severed from a narrow zone leaving the cortical tissues undamaged. The lower end of the main axis is immersed in water coloured with eosin. After 3 or 4 hours observations are made.
It is observed that the leaves of the right hand branch have considerably wilted and no eosin could be detected above the ring of incision. The leaves of the left hand branch are more or less unaffected and eosin could be easily detected above the stripped zone.
Severance of vascular strands stops upward translocation of liquids causing wilting of leaves. Removal of cortical tissues docs; not interfere with the conduction of water or liquid if the xylem remains intact;
Experiment # 12
To Demonstrate That Living Tissues Take Little Part in the Upward Translocation of Water:
Two suitable twigs of Ervatamia are taken and the end of one twig is dipped in boiling water for some time and the end of the other twig is dipped in a strong solution of CUSO4 or HgCL2 or any other toxic substance for the same period.
After such drastic treatment which kills all the living cells of the treated regions, twigs are put in a beaker containing coloured eosin solution. After about 4 hours observations are made.
At the end of the experiment eosin can be detected in the xylem strands throughout the stem and leaf of both the twigs when cross sections of stem or leaf are observed under the microscope. There is very little wilting of leaves in esach case.
This experiment indicates that living cells take very little part, if any, in upward translocation of liquid in vascular plants.