Solutions-Class-12-Biology-Chapter-6-Plant Water Relations-Maharashtra Board

(b) capillary water

(d) transpiration pull theory

(c) DPD

(a) closes almost completely

(d) cohesion

(b) Hypotonic

(b) imbibition

(c) root hair

(a) increase

(c) solution

The pressure exerted due to osmosis is osmotic pressure.

During plasmolysis, protoplast of cell shrinks and recedes from cell wall.

The water potential of pure water or a solution increases when a pressure larger than atmospheric pressure is applied to it.

Placing a plasmolysed cell in hypotonic solution will bring about deplasmolysis.

Plants showing excessive transpiration have negative root pressure.

Temperature variations during the day and night might cause gas bubbles to develop, affecting transpiration pull.

Kidney shaped and dumbbell shaped guard cells are seen.

During the day, phosphorylase enzyme activity transforms starch to sugar, and at night, sugar is converted back to starch. This alters the osmotic potential of guard cells.

When water is absorbed by root hair, it travels from one living cell to another via plasmodesmatal connections, which is referred to as the symplast pathway across the root.

• Water absorption occurs in two modes: active absorption and passive absorption.
• Passive absorption accounts for 98% of all absorption.
• Passive absorption requires no energy cost.
• The driving force is transpiration pull, and water travels according to the concentration gradient. Water is drawn upwards.
• It happens during the day when there is active transpiration.
• Active absorption occurs most often at night, when transpiration ceases owing to stomatal closure.
• Water absorption occurs against the D.P.D. gradient, requiring A.T.P. energy, which is obtained by respiration.
• Active absorption may be osmotic or non-osmotic type.
• Root pressure is important for osmotic absorption.

• Water translocation is the movement of water and dissolved minerals from roots to aerial regions.
• The movement is against gravity and is referred to as sap ascension.
• The translocation takes place through the lumen of water carrying tissue xylem, namely vessels and tracheids.
• Different ideas for translocation mechanisms have been considered, such as vital force theory (Root pressure), relay pump, physical force (capillary), and so on.
• The most commonly recognised explanation is the cohesion tension theory, often known as the transpiration pull theory.

• The loss of water as vapour is referred to as transpiration.
• Stomatal transpiration is a major kind of transpiration that involves minute pores.
• Stomata are surrounded by accessory cells and separated by two guard cells.
• The turgidity of guard cells controls the opening and shutting of stomata.
• Endosmosis causes guard cells to become turgid, and their lateral thin and elastic wall bulges or stretches out.
• The inner thick and inelastic wall is pushed apart, allowing the stoma to open during the day.
• When guard cells become flaccid owing to exosmosis at night, the wall relaxes and the stoma closes.
• Endosmosis and exosmosis occur as a result of changes in the osmotic potential of guard cells.

Transpiration : The loss of water from plant body in the form of vapour is called transpiration.

Role of transpiration :

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

Significance of transpiration :

Advantages :

• It removes excess of water.
• It helps in the passive absorption of water and minerals from soil.
• It helps in the ascent of sap.
• As stomata are open, gaseous exchange required for photosynthesis and respiration, is facilitated.
• It maintains turgor of the cells.
• 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.

Root pressure theory :

• Root pressure theory is proposed by J. Pristley.
• For translocation of water, activity of living cells of root is responsible.
• Absorption of water by root hair is a constant and continuous process and due to this a hydrostatic pressure is developed in cortical cells.
• Owing to this hydrostatic pressure i.e. root pressure, water is forced into xylem and further conducted upwards.
• Root pressure is an osmotic phenomenon.

Limitations of root pressure theory :

• Not applicable to tall plants above 20 metres.
• Even in absence of root pressure ascent of sap is noticed.
• In actively transpiring plants, root pressure is not developed.
• In taller gymnosperms, root pressure is zero.
• Xylem sap is under tension and shows negative hydrostatic pressure.

• Capillarity theory of water translocation is proposed by Bohem.
• According to this theory, physical forces and dead cells are responsible for ascent of sap.
• Capillarity is because of surface tension and cohesive forces and adhesive forces of water molecules.
• Xylem vessels and tracheids are tubular elements having their lumen.
• In these elements water column exists due to combined action of cohesive and adhesive forces of water and lignified wall.
• As a result of this capillarity water is raised upwards.

• The loss of water in the form of water vapour is called transpiration.
• About 90 — 93% of transpiration occurs through stomata, small apertures located in the epidermis of leaves.
• For stomatal transpiration to occur, stoma must remain open, during day time.
• When stomata are open then only the gaseous exchange 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.
• The process is necessary evil because water which is important for plant is lost in the process.
• At the same time, it helps in absorption of water and its translocation. Hence it cannot be avoided.
• Hence for productivity, stomata must remain open. Consequently, transpiration cannot be avoided.
• Hence, Curtis regarded transpiration as ‘a necessary evil’.

• Root hairs absorb water by imbibition then diffusion which is followed by osmosis.
• As water is taken inside the root hair cell it becomes turgid i.e. increase in turgor pressure [T.P.]
• Root hair cell has less D.P.D. but adjacent cortical cell has more D.P.D.
• The inner cortical cell has more osmotic potential so it will suck water from root hair cell.
• Root hair cell becomes flaccid and ready to absorb soil water.
• Water is passed on similarly in inner cortical cells.
• Water moves rapidly through loose cortical cells up to endodermis and through passage cells in pericycle.
• From pericycle due to hydrostatic pressure developed it is forced into protoxylem.

Osmosis : It is a special type of diffusion of solvent through a semipermeable membrane.

Diffusion : It is the movement of ions/atoms/molecules of a substance from the

region of higher concentration to the region of their lower concentration. -

Plasmolysis : Exo-osmosis in a living cell when placed in hypertonic solution is called plasmolysis.

Imbibition : It is swelling up of hydrophilic colloids due to adsorption of water.

Guttation : The loss of water in the form of liquid is called guttation.

Transpiration : The loss of water from plant body in the form of vapour is called transpiration.

Ascent of sap : The transport of water with dissolved minerals in it from root to other aerial parts of plant against the gravity is called ascent of sap.

Active absorption : Water absorption by activity of root which is against the D.P D. gradient along with expenditure of A.T.P. energy generated by respiration is the

process of active absorption.

Diffusion Pressure Deficit [D.R D.) : The difference in the diffusion pressures of pure solvent and the solvent in a solution is called diffusion pressure deficit.

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

Water potential : Chemical potential of Water is called water potential.

Wall pressure : Thick and rigid cell wall exerts a counter pressure to turgor pressure developed on the cell sap is called wall pressure that operates in opposite

direction.

Root pressure : As absorption of water by root hair being a continuous process, a sort of hydrostatic pressure is developed in living cells of root, this is called root

pressure.

• Some minerals which are required in large amounts for plant metabolism are macronutrients. E.g. C, H, O, P, N, S, Mg, Ca, K. etc.
• Some minerals which are required in small amounts for plant metabolism are micronutrients. E.g. Cu, Co, Mn, B, Zn, Cl, etc.

• Soil is the chief source of minerals for the plants.
• Minerals get dissolved in the soil water.
• Minerals are absorbed by the plants in the ionic form mainly through roots.
• Absorption of minerals is independent of water.

• Root hairs helps plants in absorbing water from the soil.
• In the absorption zone, root hairs are present.
• Ephemeral (short-lived) structures called unicellular extensions are created by epidermal cells. e.g., root hairs.

• Root hairs are nothing but cytoplasmic extensions of epiblema cell.
• Root hairs are long tube like structures of about 1 to 10 mm.
• They are colourless, unbranched and very delicate structures.
• A large central vacuole is surrounded by thin layer of cytoplasm, plasma membrane and outer cell wall.
• The cell wall of root hair is thin and double layered with outer layer of pectin and inner layer of cellulose which is freely permeable.

• The readily accessible capillary water from the soil is absorbed by unicellular root hairs, which are tubular extensions of epiblema cells.
• Imbibition, diffusion, and osmosis are the three physical processes that deal with the absorption of water.
• Imbibition is the process through which water molecules adsorb on root hair cell walls.
• They diffuse through the cell wall, which is freely permeable, and enter the root hair cell.
• They penetrate through the semipermeable plasma membrane through the mechanism of osmosis.
• The root hair cell becomes turgid, which causes an increase in turgor pressure and a drop in D.P.D. value.
• Since the next cortical cell has a higher osmotic potential, it has a higher D.P.D. value.
• Thus, the cortical cell absorbs water from the turgid epidermal cell. This process continues as a result of the gradient of suction pressure that forms from cell to cell until the endodermis' thin-walled passage cells.
• It will exit the endodermis and enter the pericycle before being driven into the protoxylem cell by hydrostatic pressure.
• The pathway of water is by apoplast and symplast.
• When water passes through cell wall and intercellular spaces of cortex it is apoplast pathway.
• When water passes across living cells through their plasmodesmatal connections it is symplast pathway.

• This is very widely accepted theory of ascent of sap proposed by Dixon and Joly.
• It is based on the concepts of water molecule adhesion and cohesion as well as plant transpiration.
• Cohesion, a strong force of attraction between water molecules, and adhesion, a strong force of attraction between water molecules and the lignified walls of xylem components, are two more strong forces of attraction.
• Sap rises via the Xylem components' lumen.
• A continuous water column is maintained in the xylem from the root to the aerial portions, or leaves, due to cohesive and adhesive forces.
• Stomata allow for transpiration, and leaf vessels acquire a transpiration pull as a result.
• This tension or pull is transmitted downwards through vein to roots which triggers ascent of sap.
• In transpiration, water is lost in vapour form and this increases D.P.D. of mesophyll cells that are near guard cells.
• Mesophyll cells absorb water from xylem in leaf and a gradient of D.P.D. or suction pressure (S.P.) is set.
• Owing to this gradient from guard cell to xylem in leaf, a transpiration pull or tension is created in xylem.
• Hence water column is pulled upward passively against gravity.

• Transpiration takes place through stomata.
• Turgidity of guard cells controls opening and closing of stomata.
• The movement is caused by turgor pressure acting on the guard cell's unevenly thickened wall.
• Stomata open as a result of the outer, thin, elastic wall stretching out and pulling the inner, thick, inelastic wall.
• Stomata close when the guard cells become floppy.
• The enzyme phosphorylase turns starch into sugar during the day, in accordance with the starch-sugar interconversion theory.
• Because sugar is osmotically active, guard cells' O.P. is raised. The water is taken up by auxiliary cells. Turgidity causes the stoma to open and the walls to stretch.
• During night-time sugar is converted to starch and hence guard cells loose water and become flaccid. Hence there is closure of stomata.
• According to proton transport theory, the movement is due to transport of H⁺ and K⁺
• Subsidiary cells are reservoirs of K⁺
• Starch is converted to malic acid which dissociate into malate and proton (H⁺) during day.
• Proton transported to subsidiary cells and K⁺ ions are taken from it. This forms potassium malate in guard cells.
• Potassium malate increases osmotic potential and endo osmosis occurs hence turgidity of guard cells. —> stomata opens.
• The uptake of K+ and CI- ions is stopped by abscissic acid formed during night. This changes permeability. Guard cells become hypotonic and loose water as they become flaccid stomata close.

• Growing plants in aqueous (soilless) medium is known as hydroponics. [Greek
• word hudor = water and ponos = work]
• It is a method for growing plants that involves giving them all the nutrients they need in the water they are supplied.
• Plant roots are suspended in this liquid with the proper support to create a nutritious medium. This medium is made by dissolving the necessary salts of macronutrients and micronutrients in the right proportion.
• The symptoms of various mineral nutrient deficiencies may be studied in great detail using hydroponics.
• With the aid of their root hairs, plants may absorb provided nutrient solutions or dissolved ions of mineral nutrients from their environment.
• While preparing the required nutrient medium particular nutrient can be totally avoided and then the effect of lack of that nutrient can be studied in variation of plant growth.
• Any visible change noticed from normal structure and function of the plant is the symptom or hunger sign considered.
• For e.g. Yellowing of leaf is observed due to loss of chlorophyll pigments or Chlorosis is noticed if Magnesium is lacking as it is a structural component of chlorophyll pigment.

• Plants absorb minerals from the soil with their root system.
• Minerals are absorbed from the soil in the form of charged particles, positively charged cations and negatively charged anions.
• The absorption of minerals against the concentration gradient which requires expenditure of metabolic energy is called active absorption.
• The ATP energy derived from respiration in root cells is utilized for active absorption.
• Ions get accumulated in the root hair against the concentration gradient.
• These ions pass into cortical cells and finally reach xylem of roots.
• Along with the water these minerals are carried to other parts of plant.

• Plants absorb mineral nutrients from their surroundings.
• For a proper growth of plants about 35 to 40 different elements are required.
• Plants absorb these nutrients in ionic or dissolved form from soil with their root system e.g. Phosphorus as PO₄^—, Sulphur as SO₄^2—
• Based on their requirement in quantity, they are classified as major nutrients or macronutrients and those needed in small amounts are minor or micronutrients.
• Macroelements are required in large amounts, as they play nutritive and structural roles e.g. C, H, O, P, Mg, N, K, S and Ca. — Ca pectate cell wall component, Mg component of chlorophyll.
• C, H, O are non-mineral major elements obtained from air and Water e.g. CO₂ is source of carbon, Hydrogen from water.
• Microelements are required in traces as they mainly have catalytic role as co-factors or activators of enzymes.
• Microelements may be needed for certain activity in life cycle of plant e.g. B for pollen germination, Si has protective role during stress conditions and fungal attacks, Al enhances availability of phosphorus.
• The important micronutrients for plant growth are Mn, B, Cu, Zn, Cl.