# Chapter 6: Plant Water Relation [ Exercise,Solutions,Notes ]

## Multiple Choice Question,

Exercise | Q 1.01 | Page 132
In soil, water available for absorption by root is ______.
gravitational water
capillary water
hygroscopic water
combined water
Solution:
In soil, water available for absorption by root is capillary water.

Exercise | Q 1.02 | Page 132
The most widely accepted theory for ascent of sap is ______.
capillarity theory
root pressure theory
diffusion
transpiration pull theory
Solution:
The most widely accepted theory for ascent of sap is transpiration pull theory.

Exercise | Q 1.03 | Page 132
Water movement between the cells is due to ______.
T.P.
W.P.
DPD
incipient plasmolysiS
Solution:
Water movement between the cells is due to DPD

Exercise | Q 1.04 | Page 132
In guard cells, when sugar is converted into starch, the stomatal pore ______.
closes almost completely
opens partially
opens fully
remains unchanged
Solution:
In guard cells, when sugar is converted into starch, the stomatal pore closes almost completely.

Exercise | Q 1.05 | Page 132
Surface tension is due to ____________.
diffusion
osmosis
gravitational force
cohesion
Solution:
Surface tension is due to cohesion.

Exercise | Q 1.06 | Page 132
Which of the following type of solution has a lower level of solutes than the solution?
Isotonic
Hypotonic
Hypertonic
Anisotonic
Solution:
Hypotonic

Exercise | Q 1.07 | Page 132
During rainy season wooden doors warp and become difficult to open or to close because of ______
plasmolysis
imbibition
osmosis
diffusion
Solution:
During rainy season wooden doors warp and become difficult to open or to close because of imbibition.

Exercise | Q 1.08 | Page 132
Water absorption takes place through ______.
lateral roots
root cap
root hair
primary root
Solution:
Water absorption takes place through root hair.

Exercise | Q 1.09 | Page 132
Due to low atmospheric pressure the rate of transpiration will ____________.
increase
decrease rapidly
decrease slowly
remain unaffected
Solution:
Due to low atmospheric pressure, the rate of transpiration will increase.

Exercise | Q 1.1 | Page 132
Osmosis is a property of ______.
solute
solvent
solution
membrane
Solution:
Osmosis is a property of solvent.

Exercise | Q 2.1 | Page 132
What is osmotic pressure?
Solution:
i. The pressure exerted due to osmosis is called osmotic pressure.
ii. Osmotic pressure is a pressure of the solution, which is required in opposite direction, so as to stop the entry of solvent molecules into the cell.

Exercise | Q 2.2 | Page 132
Name the condition in which protoplast of the plant cell shrinks.
Solution:
Plasmolysis

Exercise | Q 2.3 | Page 132
What happens when a pressure greater than the atmospheric pressure is applied to pure water or a solution?
Solution:
The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. For example: when water diffuses into a plant cell, it causes pressure to build up against the cell wall. This makes the cell wall turgid. This pressure is termed as pressure potential and has a positive value.

Exercise | Q 2.4 | Page 132
Which type of solution will bring about deplasmolysis?
Solution:
Hypotonic solution can bring about deplasmolysis.

Exercise | Q 2.5 | Page 132
Which type of plants have negative root pressure?
Solution:
The plants in which transpiration occurs rapidly especially during midsummer shows negative root pressure.

Exercise | Q 2.6 | Page 132
In which conditions transpiration pull will be affected?
Solution:
For transpiration pull to operate, the water column should be unbroken and continuous. However, due to temperature fluctuations during day and night, gas bubbles may enter in water column breaking the continuity.

Exercise | Q 2.7 | Page 132
Mention the shape of guard cells in Cyperus.
Solution:
In Cyperus, both kidney-shaped and dumbbell-shaped guard cells are present.

Exercise | Q 2.8 | Page 132
Why do diurnal changes occur in osmotic potential of guard cells?
Solution:
1. According to Steward, diurnal changes occur in the osmotic potential of guard cells due to starch-sugar inter-conversion.

2. Whereas according to Levitt active transport of potassium ions into the guard cells and out of them causes diurnal changes in the osmotic potential of guard cells.

3. Endo-osmosis and exo-osmosis occur due to diurnal changes in osmotic potential of guard cells.

Exercise | Q 2.9 | Page 132
What is the symplast pathway?
Solution:
When water passes across from one living cell to another living cell through plasmodesmata, then it is called the symplast pathway. It is also called the trans-membrane pathway.

Exercise | Q 3.01 | Page 132
Describe the mechanism for absorption of water.
Solution:
A mechanism for absorption of water:

1. In plants, water is absorbed mainly by two processes: Passive absorption and Active absorption

2. Passive absorption:

a. About 98% of the total water absorbed in plants occurs passively.

b. In passive absorption, living cells of the root do not play an important role in water absorption.

c. The driving force is transpiration pull and it thus proceeds through the DPD gradient.

d. There is no expenditure of energy (ATP) as water moves in accordance with the concentration gradient. Hence, it is passive absorption.

e. Passive absorption occurs during day time when transpiration is in progress. It stops at night when transpiration stops.

f. Rapid transpiration creates tension in the xylem vessel due to negative water potential. This tension is transmitted to xylem in the roots. Consequently, water is pulled upwards passively.

g. During passive absorption, no ATP is utilized. Thus, the rate of respiration is not affected.

3. Active absorption:

a. In this water is absorbed due to the activity of roots.

b. Root cells play an active role in the absorption of water.

c. The driving force is the root pressure developed, in the living cells of the root.

d. Active absorption occurs usually at night when transpiration stops due to closure of stomata.

e. As water absorption is against the DPD gradient, there is an expenditure of ATP (energy) generated through the respiratory activity of cells.

Exercise | Q 3.02 | Page 132
Discuss theories of water translocation.
Solution:
Theories of water translocation:

i. Various theories have been put forth to explain the mechanism of translocation of water. These theories include Vital force theory, Relay pump theory, Physical force theory, Root pressure theory, etc.

ii. Root Pressure Theory (Vital Theory): This theory was proposed by J. Priestley. According to this theory, the activity of living cells of the root is responsible for the translocation of water.

iii. Capillarity theory (physical force theory): This theory was put forth by Boehm in (1863). According to this theory, physical forces and dead cells are responsible for the ascent of sap.

iv. Cohesion- tension theory (Transpiration pull theory): This theory was put forth by Dixon and Jolly (1894). This is presently a widely accepted theory explaining the ascent of sap in plants. This theory is based on two principles i.e. Cohesion and adhesion, and transpiration pull.

Exercise | Q 3.03 | Page 132
Describe the mechanism of opening and closing of stomata.
Solution:
Mechanism of opening and closing of stomata:

1. The opening and closing of stoma is controlled by turgor of guard cells.

2. During day time, guard cells become turgid due to endo-osmosis.

3. Thus turgor pressure is exerted on the thin walls of guard cells.

4. Being elastic and thin, lateral walls are stretched out.

5. Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma.

6. During night time, guard cells become flaccid due to exo-osmosis.

7. Flaccidity closes the stoma almost completely.

8. Endo-osmosis and exo-osmosis occur due to diurnal changes in the osmotic potential of guard cells.

9. According to starch-sugar inter-conversion theory (Steward 1964), during day time, enzyme phosphorylase converts starch to sugar, thus increasing the osmotic potential of guard cells causing entry of water, thereby guard cells are stretched and stoma widens. The reverse reaction occurs at night bringing about the closure of the stoma.

10. According to the theory of proton transport (Levitt-1974), stomatal movement occurs due to the transport of protons H+ and K+ ions. During the daytime, starch is converted into malic acid. Malic acid dissociates to form malate ions and protons. Protons are transported to subsidiary cells and K+ ions are imported from them.
Potassium Malate is formed that increases osmolarity and causes endoosmosis. Uptake of K+ ions is always accompanied by Cl– ions. At night, uptake of K+ and Cl– ions is prevented by abscisic acid, changing the permeability of guard cells. Due to this guard cells become hypotonic and thereby become flaccid.

Exercise | Q 3.03 | Page 132
What is transpiration?
Solution:
Transpiration:
The loss of water in the form of vapor is called transpiration that occurs through leaves, stem, flowers, and fruits.

Exercise | Q 3.04 | Page 133
Explain the role of transpiration.
Solution:
Role of transpiration:

i. It removes excess of water.

ii. It helps in the passive absorption of water and minerals from the soil.

iii. It helps in the ascent of sap.

iv. As stomata are open, gaseous exchange required for photosynthesis and respiration is facilitated.

v. It maintains the turgor of the cells.

vi. Transpiration helps in reducing the temperature of leaf and in imparting a cooling effect.

Exercise | Q 3.05 | Page 133
Explain the root pressure theory and its limitations.
Solution:
Root pressure theory (Vital theory):

1. This theory was proposed by J. Priestley.

2. According to this theory, the activity of living cells of root is responsible for the translocation of water.

3. When a stem of a potted plant is cut few inches above the soil by a sharp knife, xylem sap is seen flowing out/ oozing out through the cut end.

4. This exudation at the cut end of the stem is a good proof for the existence of root pressure.

5. As water absorption by roots is a constant and continuous process, hydrostatic pressure is developed in the living cells of cortex of the root. This is termed as root pressure (coined by S. Hales).

6. It is due to root pressure water along with dissolved minerals is not only forced into xylem but it is also conducted upwards against the gravity.

7. Root pressure seems to be largely an osmotic phenomenon and its development is an active process.

8. The value of root pressure is +1 to +2 bars which is enough to pump water to a height of 10 to 20 meters.

9. The factors like oxygen, moisture, the temperature of the soil, salt contents, etc. influence the root pressure.

Limitations of root pressure theory:

Although ascent of sap takes place due to root pressure, there are certain objections raised, such as;

1. It is not applicable to plants taller than 20 meters.

2. Ascent of sap can also occur even in the absence of a root system.

3. Root pressure value is almost nearly zero in taller gymnosperm trees.

4. In actively transpiring plants, no root pressure is developed.

5. Xylem sap under normal condition is under tension i.e. it shows negative hydrostatic pressure or high osmotic pressure.

Thus, root pressure is not the sole mechanism explaining the ascent of sap in all plants of varying heights.

Exercise | Q 3.05 | Page 133
What is the significance of transpiration?
Solution:
Significance of transpiration:

1. It removes excess of water.

2. It helps in the passive absorption of water and minerals from the soil.

3. It helps in the ascent of sap.

4. As stomata are open, gaseous exchange required for photosynthesis and respiration is facilitated.

5. It maintains the turgor of the cells.

6. Transpiration helps in reducing the temperature of leaf and in imparting a cooling effect.

Exercise | Q 3.06 | Page 133
Explain capillarity theory of water translocation.
Solution:
Capillarity theory of water translocation:

1. This theory was put forth by Boehm in (1863).

2. According to this theory, physical forces and dead cells are responsible for the ascent of sap. For e.g. Wick dipped in an oil lamp, shows capillarity due to which oil is raised upwards. The conduction of water in a straw dipped in water is raised to a certain height because of capillarity. The height to which water is raised depends on the diameter of the straw.

3. Capillarity is because of surface tension, and forces of cohesion (attraction between like molecules) and adhesion (attraction between unlike molecules).

4. Xylem vessel/ tracheid with its lumen can be compared with straw.

5. Water column exists because of combined cohesive and adhesive forces of water and xylem wall, due to capillarity.

6. Due to capillarity, water is raised or conducted upwards against gravity, to few centimeters only.

Exercise | Q 3.07 | Page 133
Why is transpiration is called ‘a necessary evil’?
Solution:
Curtis (1926) regarded transpiration as ‘a necessary evil’, because;

1. For stomatal transpiration to occur, stoma must remain open, during day time.

2. When stomata are open then only the gaseous exchange needed for respiration and photosynthesis will take place.

3. If stomatal transpiration stops, it will directly affect the productivity of the plant through the loss of photosynthetic and respiratory activity.

4. Hence for productivity, stomata must remain open.

5. Consequently transpiration cannot be avoided.

Exercise | Q 3.08 | Page 133
Explain the movement of water in the root.
Solution:
Journey of water from soil to xylem in roots (from epiblema upto xylem in the stelar region):

1. Water is absorbed by root hair cells through processes like imbibition, diffusion, osmosis which occur sequentially.

2. Water passes through the epidermal cell (epiblema), cortex, endodermis, Casparian strip, pericycle, and then to protoxylem.

3. When the root hair cell absorbs water it becomes turgid. Its turgor pressure increases, but its DPD value decreases.

4. However, the immediately adjacent cortical cell inner to it, has more DPD value, because its O. P. is more.

5. Therefore, cortical cells will absorb water from the turgid root hair cell. It then becomes turgid.

6. The flaccid root hair cell now absorbs water from the soil.

7. Water from the turgid cortical cell is absorbed by the inner cortical cell and the process goes on.

8. Thus, a gradient of suction pressure (DPD) is developed from cells of epiblema to the cortex of the root.

9. Consequently water moves rapidly across the root through loosely arranged living cells of cortex, followed by passage cells of endodermis and finally into the cell of pericycle.

10. Protoxylem is in close proximity to the pericycle.

11. It is due to root pressure, water from pericycle is forced into the xylem.

12. Pathway of water across the root occurs in two types: Apoplast pathway and Symplast pathway

13. Apoplast pathway: When some amount of water passes across the root through the cell wall and the intercellular spaces of cortical cells of the root, it is then called the apoplast pathway. This pathway occurs up to endodermis.

14. Symplast pathway: When water passes across from one living cell to another living cell through plasmodesmata, then it is called the symplast pathway. It is also called the trans-membrane pathway.

## Define and or explain the term:

Exercise | Q 3.09 | Page 133
Plasmolysis
Solution:
Plasmolysis - The shrinkage of cytoplasm of a living cell as a result of exosmosis is known as plasmolysis.

Exercise | Q 3.09 | Page 133
What is osmotic pressure?
Solution:
i. The pressure exerted due to osmosis is called osmotic pressure.

ii. Osmotic pressure is a pressure of the solution, which is required in opposite direction, so as to stop the entry of solvent molecules into the cell.

Exercise | Q 3.09 | Page 133
Diffusion
Solution:
Diffusion means to disperse.
Diffusion can be defined as the movement of ions/ atoms/ molecules of a substance from the region of their higher concentration to the region of their lower concentration till equilibrium is reached.
The movement is due to the kinetic energy of the molecules.
Water passes into the cell by diffusion through a freely permeable cell wall.

Exercise | Q 3.09 | Page 133
imbibition
Solution:
The adsorption of water by hydrophilic compounds is called imbibition.

2. Substance that adsorbs water/liquid is called imbibant and water/ liquid that gets imbibed is called imbibate.

3. The root hair cell wall is made up of pectic compounds and cellulose which are hydrophilic colloids.

4. During imbibition, water molecules get tightly adsorbed without the formation of a solution.

5. Imbibition continues until the equilibrium is reached. In other words, water moves along the concentration gradient.

6. Imbibition is significant in soaking of seeds, swelling up of dried raisins, kneading of flour etc

Exercise | Q 3.09 | Page 133
Guttation
Solution:
The loss of water in the form of liquid is called guttation.
It occurs through special structures called water stomata or hydathodes.

Exercise | Q 3.09 | Page 133
Transpiration
Solution:
i. The loss of water in the form of vapour is called transpiration that occurs through leaves, stem, flowers and fruits.
ii. Transpiration occurs through three main sites - cuticle, stomata, and lenticels.

Exercise | Q 3.09 | Page 133
Ascent of sap
Solution:
The transport of water with dissolved minerals from the root to other aerial parts like stem and leaves, against the gravity, is called translocation or ascent of sap.

Exercise | Q 3.09 | Page 133
Active absorption
Solution:
In this water is absorbed due to the activity of roots.
Root cells play an active role in the absorption of water.
The driving force is the root pressure developed, in the living cells of the root.
Active absorption occurs usually at night when transpiration stops due to the closure of stomata.
As water absorption is against the DPD gradient, there is an expenditure of ATP (energy) generated through the respiratory activity of cells.

Exercise | Q 3.09 | Page 133
DPD
Solution:
Diffusion pressure of pure solvent (pure water) is always more than the diffusion pressure of the solvent in a solution. The difference in the diffusion pressures of pure solvent and the solvent in a solution is called Diffusion Pressure Deficit (DPD) or Suction Pressure (SP).
The term DPD was coined by B.S. Meyer (1938). Nowadays, term water potential is used for DPD.
In colloquial language, the term DPD is actually the thirst of a cell with which it absorbs water from the surroundings.
The water around the cell wall has more diffusion pressure than cell sap. Due to this, water moves in the cell by diffusion.

Exercise | Q 3.09 | Page 133
Turgor pressure
Solution:
Turgor pressure (T.P) is the pressure exerted by turgid cell sap on to the cell membrane and cell wall.

Exercise | Q 3.09 | Page 133
Water potential
Solution:
i. Chemical potential of water is called water potential.

ii. It is represented by Greek letter psi (ψ).

iii. The unit of measurement of water potential is bars/ pascals/ atmospheres.

iv. Water potential of protoplasm is equal but opposite in sign to DPD. It has a negative value.

v. Water potential of pure water is always zero. The addition of any solute in it decreases its psi (ψ) value. Therefore, it has a negative value.

vi. Difference between water potential of the adjacent cells decides the movement of water through plasmodesmata across the cells.

vii. Water always flows from less negative potential to more negative water potential (i.e. from high water potential area to low water potential area).

Exercise | Q 3.09 | Page 133
Wall pressure
Solution:
The cell wall is thick and rigid, exerts a counter pressure on the cell sap. This is called Wall pressure (W. P).

Exercise | Q 3.09 | Page 133
Root pressure
Solution:
During the absorption of water, the continuous flow of water develops hydrostatic pressure in living cells of the root. This is called root pressure.
Root pressure causes water to flow from pericycle into the xylem. It also causes upward conduction of water against gravity.
A manometer is used to measure the root pressure.

Exercise | Q 3.1 | Page 133
Distinguish between Osmotic pressure and Turgor pressure.
Solution:
Osmotic pressure - Osmotic pressure is a pressure of the solution, which is required in opposite direction, so as to stop the entry of solvent molecules into the cell.

Turgor pressure - Turgor pressure is the pressure exerted by turgid cell sap on to the cell membrane and cell wall.

Exercise | Q 3.1 | Page 133
Distinguish between Diffusion and Osmosis
Solution:
 Diffusion Osmosis 1. It takes place in solid, gas, or liquid medium. 1. It takes place only in a liquid medium. 2. It does not require the presence of a semi-permeable membrane. 2. It requires the presence of a semipermeable membrane. 3. In diffusion, the movement of ions/atoms/molecules from a region of higher concentration to the region of lower concentration takes place. 3. In osmosis, diffusion of the only solvent from a lower concentration of solution to a higher concentration of solution occurs 4. It is influenced by the diffusion pressure 4. It is only influenced by the turgor pressure.

Exercise | Q 4.1 | Page 133
Describe structure of root hair.
Solution:
1. Root hair is a cytoplasmic extension (prolongation) of epiblema cell.

2. Each root hair may be approximately 1 to 10 mm long and tube-like structure.

3. It is colourless, unbranched, short-lived (ephemeral), and very delicate.

4. It has a large central vacuole surrounded by a thin film of cytoplasm, plasma membrane, and thin cell wall, which is two-layered.

5. Outer layer is composed of pectin and the inner layer is made up of cellulose.

6. Cell wall of a root hair is freely permeable but the plasma membrane is selectively permeable.

Exercise | Q 4.2 | Page 133
Write on journey of water from soil to xylem in roots.
Solution:
1. Water is absorbed by root hair cells through processes like imbibition, diffusion, osmosis which occur sequentially.

2. Water passes through the epidermal cell (epiblema), cortex, endodermis, casparian strip, pericycle and then to protoxylem.

3. When root hair cell absorbs water it becomes turgid. Its turgor pressure increases, but its DPD value decreases.

4. However, the immediately adjacent cortical cell inner to it, has more DPD value because its O. P. is more.

5. Therefore, the cortical cells will absorb water from the turgid root hair cell. It then becomes turgid.

6. The flaccid root hair cell now absorbs water from the soil.

7. Water from the turgid cortical cell is absorbed by the inner cortical cell and the process goes on.

8. Thus, a gradient of suction pressure (DPD) is developed from cells of epiblema to the cortex of the root.

9. Consequently water moves rapidly across the root through loosely arranged living cells of cortex, followed by passage cells of endodermis and finally into the cell of pericycle.

10. Protoxylem is in close proximity with pericycle.

11. It is due to root pressure, water from pericycle is forced into the xylem.

12. Pathway of water across the root occurs in two types: Apoplast pathway and Symplast pathway

13. Apoplast pathway: When some amount of water passes across the root through the cell wall and the intercellular spaces of cortical cells of the root, it is then called the apoplast pathway. This pathway occurs up to endodermis.

14. Symplast pathway: When water passes across from one living cell to another living cell through plasmodesmata, then it is called the symplast pathway. It is also called the trans-membrane pathway.

Exercise | Q 4.3 | Page 133
Explain cohesion theory for translocation of water.
Solution:
1. This theory was put forth by Dixon and Jolly (1894).

2. This is presently a widely accepted theory explaining the ascent of sap in plants.

3. This theory is based on two principles i.e. Cohesion and adhesion, and transpiration pull.

a. A strong force of attraction between water molecules is called cohesive force.

b. While a strong force of attraction between water molecules and the lignified wall of the lumen of the xylem vessel, is called adhesive force.

c. Due to combined cohesive and adhesive forces a continuous water column is developed (formed) in the xylem right from root up to the tip of the topmost leaf in the plant.

5. Transpiration pull:

a. The transpiration pull developed in the leaf vessel is transmitted down to the root and thus accounts for the ascent of sap.

b. Excess water is lost in the form of vapour, mainly through the stomata found on a leaf.

c. This water loss increases the D.P.D. of mesophyll cells. These cells withdraw water ultimately from the xylem in the leaf.

d. In other words, due to continuous transpiration, a gradient of suction pressure (i.e. D.P.D.) is developed right from guard cells up to the xylem in the leaf. This will create a tension (called a negative pull or transpiration pull) in the xylem.

e. Consequently, the water column is pulled out of xylem. Thus, water is pulled upwards passively against the gravity leading to the ascent of sap.

Exercise | Q 4.4 | Page 133
Write mechanism of opening and closing of stoma.
Solution:
1. Opening and closing of stoma is controlled by the turgor of guard cells.

2. During day time, guard cells become turgid due to endo-osmosis.

3. Thus turgor pressure is exerted on the thin walls of guard cells.

4. Being elastic and thin, lateral walls are stretched out.

5. Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma.

6. During night time, guard cells become flaccid due to exo-osmosis.

7. Flaccidity closes the stoma almost completely.

8. Endo-osmosis and exo-osmosis occur due to diurnal changes in the osmotic potential of guard cells.

9. According to starch-sugar inter-conversion theory (Steward 1964), during day time, enzyme phosphorylase converts starch to sugar, thus increasing the osmotic potential of guard cells causing entry of water, thereby guard cells are stretched and stoma widens. The reverse reaction occurs at night bringing about the closure of the stoma.

10. According to the theory of proton transport (Levitt-1974), stomatal movement occurs due to the transport of protons H+ and K+ ions. During the daytime, starch is converted into malic acid. Malic acid dissociates to form malate ions and protons. Protons are transported to subsidiary cells and K+ ions are imported from them. Potassium Malate is formed that increases osmolarity and causes endosmosis. The uptake of K+ ions is always accompanied by Cl– ions. At night, uptake of K+ and Cl– ions is prevented by abscisic acid, changing the permeability of guard cells. Due to this guard cells become hypotonic and thereby become flaccid.

Plant obtains variety of substances like water, minerals, nutrients, food and gases like O2 and CO2 , from its surroundings. Productivity in plants is mainly affected by the non-availibility of water. Water is considered as ‘elixir of life’. Water constitutes almost 90 to 95% of most plant cells and tissues. Water helps the cells to maintain turgidity and shape. It shows following properties due to which it has great biological importance.

6.1 Properties of water: It is in the liquid form at room temperature and is the best solvent for most of the solutes. It is inert inorganic compound with neutral pH when in pure form. Due to this, water is best transporting medium for dissolved minerals and food molecules. It is best aqueous medium for all biochemical reactions occurring in the cells. It is an essential raw material for photosynthesis. Water has high specific heat, high heat of vaporization and high heat of fusion. Due to this, it acts as thermal buffer. These various properties are due to hydrogen bonds between the water molecules.

hesive forces of attraction. Due to high surface tension and high adhesive and cohesive force, it can easily rise in the capillaries. It is therefore, a significant molecule that connects physical world with biological processes

6.2 Water absorbing organ: Root : Root is the main organ of water and mineral absorption. In terestrial plants, plants absorb water in the form of liquid from the soil however, epiphytic plants like orchids absorb water vapours from air with the help of epiphytic roots having special tissue called velamen. Typical root is divisible into four different regions. In the zone of absorption, epidermal cells (epiblema cells) form unicellular hair like extensions called root hairs.

Structure of root hair : Root hair is cytoplasmic extension (prolongation) of epiblema cell. Each root hair may be approximately 1 to 10mm long and tube like structure. It is colourless, unbranched, short-lived (ephemeral) and very delicate. It has a large central vacuole surrounded by thin film of cytoplasm, plasma membrane and thin cell wall, which is two layered. Outer layer is composed of pectin and inner layer is made up of cellulose. Cell wall is freely permeable but plasma membrane is selectively permeable.

6.3 Water available to roots for absorption: Plants absorb water from the rhizosphere (the microenvironment surrounding the root). Water present in the soil occurs as gravitational (free) water, hygroscopic water, combined water and capillary water. Water percolates deep, due to the gravity, in the soil, is called ‘gravitational water’. This is not available to plants for absorption. Fine soil particles imbibe/ adsorb water and hold it. This is called ‘hygroscopic water’. Roots cannot absorb it. Water present in the form of hydrated oxides of silicon, aluminum, etc., is called ‘combined water’. It is also not available to plants for absorption. Some amount of water is held in pores present between the neighbouring soil particles, due to capillarity. This is called capillary water that is avilable for absorption.

6.4 Absorption of water by roots from soil: Root hair absorbs water by employing three physical processes that occur sequentially- viz. imbibition, diffusion and osmosis.

a. Imbibition: Imbibition is swelling up of hydrophillic colloids due to adsorption of water. Substance that adsorbs water / liquid, is called as imbibant and water/ liquid, that gets imbibed is called as imbibate. The root hair cell wall is made up of pectic compounds and cellulose which are hydrophillic colloids. During Imbibition, water molecules get tightly adsorbed without the formation of solution. Imbibition continues till the equilibrium is reached. In other words, water moves along the concentration gradient. Imbibition is significant in soaking of seeds, swelling up of dried raisins, kneading of flour etc.

b. Diffusion: Diffusion means to disperse. Diffusion can be defined as the movement of ions/ atoms/ molecules of a substance from the region of their higher concentration to the region of their lower concentration. The movement is due to the kinetic energy of the molecules. Diffusion continues till an equilibrium is reached. Thus, water passes into the cell by diffusion through a freely permeable cell wall. Water is now at the interface of cell wall and plasma membrane.

Diffusion results in the diffusion pressure (D. P.) which is directly proportional to the number of diffusing particles. Diffusion pressure of pure solvent (pure water) is always

more than the diffusion pressure of solvent in a solution. The difference in the diffusion pressures of pure solvent and the solvent in a solution is called Diffusion Pressure Deficit (DPD) or Suction Pressure (SP). The term was coined by B.S. Meyer (1938). Now a days, term water potential is used for DPD. In colloquial language, the term DPD is actually the thirst of a cell with which it absorbs water from the surroundings. Water arround cell wall has more diffusion pressure than cell sap. Due to this, water moves in the cell by diffusion. Diffusion is significant in plants in the absorption of water, minerals, conduction of water against the gravity, exchange of gases and transport and distribution of food

c. Osmosis : It is a process by which water enters into the cytoplasm of the root hair cell. Osmosis is a special type of diffusion of solvent through a semipermeable membrane. The cytoplasm of root hair cell contains minerals, sugars, etc. In other words, solution inside the cell is more concentrated (stronger) than outside the cell (weaker). Therefore, solvent from weaker solution enters into cytoplasm (i.e. to stronger solution) of cell through a semipermiable plasma membrane. This migration of solvent is called Osmosis. Thus, water at the interface of cell wall and plasma membrane, enters into the cytoplasm of the root hair cell due to osmosis. With respect to the concentration and osmotic migration, three types of solutions are recognized viz,

i. Hypotonic (weak solution or strong solvent) having low osmotic concentration.
ii. Hypertonic (strong solution or weak solvent) having high osmotic concentration.
iii. Isotonic having such a concentration of solution where there is neither gain nor loss of water in an osmotic system. In other words, concentration outside and inside the cell is same.

Osmosis is of two types viz, Exo-osmosis and Endo-osmosis. Exo-osmosis : It is the migration of solvent from the cell outside. It causes flaccidity of cell. Endo-osmosis : It is the migration of the solvent into the cell. It causes turgidity of cell i.e. cytoplasm becomes turgid. Turgidity increases the turgor pressure (T. P.) of the cell. T. P. is the pressure exerted by turgid cell sap on to the cell membrane and cell wall. In a fully turgid cell, DPD is zero. Cell wall being thick and rigid, exerts a counter pressure on the cell sap. This is called Wall pressure (W. P.). In a fully turgid cell, T. P. = W. P. but operating in opposite direction.

Opening and Closing of Stoma : Opening and closing of stoma is controlled by turgor of guard cells. During day time, guard cells become turgid due to endosmosis. Thus turgor pressure is exerted on the thin walls of guard cells. Being elastic and thin, lateral walls are stretched out. Due to kidney or dumb-bell like shape, inner thick walls are pulled apart to open (widen) the stoma. During night time, guard cells become flaccid due to exosmosis.

Flaccidity closes the stoma almost completely. Endosmosis and exosmosis occur due to diurnal changes in osmotic potential of guard cells. Different theories are proposed to explain diurnal changes in osmotic potential. According to starch-sugar interconversion theory (Steward 1964), during day time, enzyme phosphorylase converts startch to sugar, thus increasing osmotic potential of guard cells cosing entry of water there by gaurd cells are stretched and stoma widens. The reverse reaction occures at night brining about the closure of stoma.

According to theory of proton transport (Levitt-1974), stomatal movement occurs due to transport of protons H+ and K+ ions. During daytime, starch is converted into malic acid. Malic acid dissociates to form Malate and protons. Protons are transported to subsidiary cells and K+ ions are imported from them. Potassium malate is formed that increases osmolarity and causes endosmosis. Uptake of K+ ions is always accompanied with Cl¯ ions. At night, uptake of K+ and Cl- ions is prevented by abscissic acid, changing the permeability of guard cells. Due to this guard cells become hypotonic and thereby become flaccid

Significance of Transpiration :
i. It removes excess of water.
ii. It helps in the passive absorption of water and minerals from soil.
iii. It helps in the ascent of sap.
iv. As stomata are open, gaseous exchange required for photosynthesis and respiration, is facilitated.
v. It maintains turgor of the cells.
vi. Transpiration helps in reducing the temperature of leaf and in imparting cooling effect.

Disadvantages: Excessive transpiration leads to wilting and injury in the plant. It may also lead to the death of the plant.

Transpiration is ‘A necessary evil’ : For stomatal transpiration to occur, stoma must remain open, during day time. When stomata are open then only the gaseous exhange needed for respiration and photosynthesis, will take place. If stomatal transpiration stops, it will directly affect productivity of plant through the loss of photosynthetic and respiratory activity. Hence for productivity, stomata must remain open. Consequently transpiration can not be avoided. Hence, Curtis (1926) regarded transpiration as ‘a necessary evil’.

## plant water relations class 12 questions and answers

• plant water relations notes pdf