In this article we will discuss about the water relation in plants.
Plant physiology is the study of vital or functional activities of plants. And water is essential for these functional activities.
What for water is needed:
i. Water is absolutely essential for protoplasm because its hydration is essential for its proper organisation and proper functioning of its organelles.
ii. Water is the carrier of various dissolved substances like gases, minerals, organic and inorganic substances etc.
iii. Water is necessary for metabolic reactions of the cell e.g., photolysis of water.
iv. The rigidity and turgidity of cells is maintained by and large by water.
v. Water forms a continuous network throughout the plant through which dissolved substances move up.
vi. Water makes up the loss of water during transpiration and guttation.
Thus almost all the functional activities of plants depend on the water relation.
Under water relation we study the following points:
a) How does water enter into the plant – This functional activity is studied under the heading ‘Osmosis’.
b) How does water move up inside the plant – It is studied under ‘Ascent of sap’ or ‘Translocation of sap’.
c) How is water lost from the plant – It is studied under ‘Loss of water’ or ‘Transpiration’.
Diffusion:
The movement of the molecules of gases, liquids or solutes from the regions of higher concentration to the regions of lower conc., until the molecules are evenly distributed throughout the available space is known as Diffusion.
Diffusion Pressure Deficit/Water Potential (DPD/WP):
Diffusion pressure deficit (DPD) is also called suction Pressure (S.P). It is the ability of a cell to draw water. It is the force per unit area (i.e., pressure) by which water enters into a cell. A pure solvent is supposed to have maximum diffusion pressure.
When certain solute particles are added to the pure solvent, the diffusion pressure of the resulting solution is lowered. The amount by which the diffusion pressure of a solution is lowered than that of its solvent at the same temperature and atmospheric pressure, is called DPD. The term DPD was introduced by Meyer in 1938.
But according to the recent trend, diffusion of water is explained in terms of Water Potential. DPD is the positive value whereas water potential is the negative value.
Turgor Pressure (T.P.):
T.P is the outward pressure exerted by the cell solution on the cell wall which is developed due to osmotic diffusion of water. In equilibrium inward pressure is also given by the cell wall on the cell solution in an equal amount, which is called wall pressure or Hydrostatic Pressure.
Turgor Pressure is also called Pressure Potential (PP) which has positive value.
Chemical Potential:
The free energy per mole of a chemical substance is called its chemical potential. Chemical potential of water in a solution is reduced by the addition of solutes.
Osmosis:
i. Osmosis is essentially a special type of diffusion of liquids.
ii. The term ‘Osmosis’ was given by Abbe Nollel.
iii. Osmosis Means:
a) Movement of solvent.
b) From a region of lower concentration of solution.
c) To a region of higher conc. of solution.
d) Through semi-permeable membrane (SPM).
iv. Plasma or cell membrane is a semi-permeable membrane. At higher temperature plasma membrane becomes permeable.
Illustration:
Here when barrier between two different concentrated solutions is opened and barrier remained only semi-permeable membrane, then the level of higher conc. solution is increased. It is happened only due to flow of water from lower conc., to the higher conc. through semi-permeable membrane.
i. Semi-permeable membrane is such type of membrane through which solutes of solution do not pass but solvent passes e.g., plasma-membrane.
ii. Differentially or Selectively Permeable membrane is such type of membrane through which some particles pass but others do not pass e.g., lipo-protein cell membrane.)
Absorption of Water:
i. Water is absorbed by the root hairs. Root hairs are more developed in xerophytes (arid plants) and absent in hydrophytes.
ii. First step in absorption of water is imbibition.
iii. The absorption of water takes place in the terminal portions of roots but the maximum absorption of water takes place in the zone of root hairs i.e., 1-10 cm. behind the root tip.
a. Root cap is formed by calyptrogen (= Dermatogen + Periblem)
b. Multiple root cap is found in Pandanus (screw pine).
c. Maximum absorption of inorganic salts is through the zone of cell division.
iv. Renner (1912, 1915) – First time recognised and classified the water uptake mechanism into “active absorption” and “passive absorption”.
a. Root hairs play an active role.
b. Water enters into root hair by osmosis.
c. Process occurs against the concentration gradient, and
d. Metabolic energy is spent.
There are two major theories to explain active absorption:-
a) Osmotic Theory:
i) Atkins (1916) and Priestley (1920-22) were amongst the first to postulate an osmotic theory of active absorption.
ii) The value of osmotic pressure of the cell sap of the root hairs is generally 2.0 atm. (but varies between 3 to 8 atm.) and of soil water is less than 1.0 atm.
iii) Higher DPD of the cell sap of the root hairs causes endo-osmosis of soil water.
iv) But a problem is how a sufficient conc. of solutes is maintained in the root xylem to maintain a higher DPD in the xylem sap.
b) Non-Osmotic Theory:
i) Water is absorbed against a concentration gradient.
ii) Absorption requires an expenditure of energy released from respiration.
iii) Renner’s active theory is called ‘osmotic active’.
i. Process occurs along the concentration gradient.
ii. And hence energy is not involved.
iii. Absorption is controlled by transpiration.
T/A Experiment:
T/A means Transpiration/Absorption ratio let,
i) Weight of bottle with plant = 1000g
ii) Initial reading of water inside tube is 5 cc. one drop of oil is poured into side tube to check evaporation. After allowing the minimum time i.e., 30 minutes for transpiration and absorption, again reading is noted down.
iii) Wt. of bottle with plant = 998g.
Means water lost = 1000 – 998 = 2g.
Final reading of water inside tube = 3 cc.
Means water absorbed = 5-3 = 2 cc.
∴ Transpiration/Absorption = 2/2 = 1
Inference:
Water lost in transpiration is equal to water absorbed.
Factors affecting the rate of water absorption:
1) Available Soil Water:
The rate of water absorption is uniform between the field capacity and permanent wilting percentage. The decrease in soil water below the permanent wilting point causes considerable decrease in the absorption of water.
2) Conc. of the Soil Solution:
The lack of water absorption by plants growing in saline water is an example of physiological dryness.
3) Soil Aeration:
The rate of water absorption is rapid in well aerated soils. Oxygen deficiency and accumulation of CO2 increases the viscosity of the protoplasm and decreases its permeability, both of which reduce the rate of absorption. During waterlogging conditions, there is total check on the water-absorption which is an example of physiological dryness.
Ascent of Sap:
Ascent of sap is the translocation of water and inorganic solutes.
Ascent of sap means:
i) Movement of water and inorganic solutes.
ii) From root to the leaves.
iii) Through xylem vessels.
iv) Against the force of gravity.
v) And water column remains in a state of tension.
a. Ringing or Girdling experiment by Malpighi confirmed the ascent of sap through xylem.
b. Mass Flow or Pressure Flow theory for the movement of food was given by Munch.
Mechanism of Ascent of Sap:
To explain the mechanism of ascent of sap, various theories are put forwarded which are classified under 3 headings:
(A) Vital Theories:
i) Relay or clambering Pump Theory:
This theory was put forwarded by Godlewski (1884) to explain ascent of sap. According to this theory, “Rhythmic change in the osmotic pressure of the living cells of xylem parenchyma and medullary rays bring about a pumping action of water in an upward direction.”
Relay pump theory was contradicted by Strasburger (1893). He proved that ascent of sap was independent of living cells because water continued to be transported above even after the killing of living cells by poison (i.e., picric acid) or high temperature.
ii) Pulsation Theory:
Pulsation Theory by Sir J.C. Bose (1923), experiment on Demodium gyrans (Indian telegraph plant, Family- Leguminosae) According to Sir J.C. Bose, “Living cells of the inner most layer of the cortex, just outside the endodermis are in a state of rhythmic pulsations which cause the pumping of water for cell to cell in an upward direction.” Benedict (1927) found that the actual rate of ascent of sap was 8000 to 30000 times as rapid as would be possible according to the Bose theory.
(B) Root Pressure Theory:
‘Root Pressure’ was coined by Stephan Hales (Father of plant Physiology). The hydrostatic pressure developed due to the accumulation of water absorbed by the roots is called root pressure. Root pressure is measured by manometer.
But Root pressure is not sufficient to drive water to a distance of 400 ft. in the trunks of tall trees.
(C) Physical Theories:
All these theories consider the dead cells of plant to be responsible for ascent of sap.
i) Boehm’s Theory (1809):
According to it, the ascent of sap is partly due to the phenomena of capillarity of the trachea and partly due to the atmospheric pressure.
The highest column of water attained by capillary forces is 4 ft. and by atmospheric pressure is 34 ft. only.
ii) Jamin’s Chain Theory:
Air and water are alternately arranged inside the plant. When air expands, it moves up carrying along with it the water column present above it.
iii) Imbibition Theory:
According to Sachs (1878), Imbibition activity of cell walls of xylem is responsible for ascent of sap.
iv) Cohesion and Adhesion Theory or Transpiration Pull Theory:
This theory was given by Dixon & Jolly (1894) and Askenasy (1895). This theory is most accepted theory.
There are three features of this theory:
a) Strong cohesive force or tensile strength of water.
b) Continuity of water column in the plant.
c) Transpiration pull or tension on the unbroken water column.
Imbibition:
i. The term ‘imbibition’ was coined by Sachs. The soaking up of water by dry substances due to hydrophilic colloids is called Imbibition.
ii. It is the first step in the absorption of water.
iii. The rate of imbibition increases with increase in temperature.
iv. Imbibition in oily seed is more than in starchy seed.
Imbibition pressure is also called Matric Potential (Ѱm). Matric potential is the component of water potential which is determined by the attraction between water and hydrating colloid or gel-like organic molecules, cell wall etc. (collectively called matrix).
The matric potential is maximum (most negative) in a dry material. Both living and dead plant cells possess a large amount of carbohydrates, proteins, and polypeptides etc. which are hydrophilic colloids and therefore, have very strong attraction for water. Seeds rich in colloidal materials are very good imbibants.
Loss of Water:
The loss of water from the living tissues of plants takes place either in vapour form (i.e., transpiration) or in liquid form (i.e., Guttation, Bleeding).
The loss of water in the form of vapour from the living aerial parts of the plant is known as transpiration. The principal organ of transpiration is leaf. Transpiration may be foliar or lenticular. Again foliar transpiration is of two types viz., stomatal (through stomata) and cuticular (through cuticle).
Dry cobalt chloride (CoCl2) is deep blue in colour but when comes in contact with water, it turns into red colour.
i) The exudation of sap (water) through hydathode (structure present at the tips of veins of leaves) is called guttation.
ii) The cause of guttation is root pressure (positive hydrostatic pressure developed in the xylem ducts of the root system.)
iii) Necessary condition for guttation:
a) Increased absorption of water and decreased transpiration.
b) Warm soil & humid or cool atmosphere.
c) Warm day and cool night (such condition is available in winter season).
iv) Guttation normally occurs at night.
v) Accumulation of salts at leaf tip or leaf margin of some plants during winter is due to guttation.
Hydathode:
It is a specialised epidermal cell found at leaf tip or leaf margin i.e., end of veins and veinlets. The group of parenchyma beneath hydathode is known as epithem.
i. The loss of sap (water) from the injured parts of the plant is called bleeding.
ii. The cause of bleeding is root pressure.
Plasmolysis:
1) Plasmolysis is the shrinkage of protoplasm due to outward flow of water in a hypertonic solution.
2) The point where plasmolysis just starts (not visible) is called incipient plasmolysis.
Cell wall → either permeable (cellulose cell wall) or impermeable (cork cell, lignified, suberinised)
Cell membrane → always semi-permeable.
3) Space between shrunken protoplasm and the cell wall is occupied by plasmolysing solution i.e., external solution (or salt).
4) When a plasmolysed cell is placed in hypotonic solution or pure water, the protoplasm as well as cell as a whole attain their original shape and size respectively due to endo-osmosis. This phenomenon is called Deplasmolysis.
Examples:
i) Raisins swell in water due to imbibition and endosmosis.
ii) Excess of fertilizer in the soil kills the plant due to plasmolysis (exo-osmosis).
iii) Salted pickle or meat kills the bacteria due to plasmolysis.
Solution:
Is of three types:
i) Hypotonic Solution:
It means weak solution. A cell swells in hypotonic solution.
ii) Hypertonic Solution:
Hypertonic solution strong solution. A cell shrinks or become flaccid in such solution due to outward flow of water.
iii) Isotonic Solution:
Isotonic solution not weak nor strong solution. A cell remains unchanged in it. Eye drops are always isotonic solution.
No comments yet.