By going through these Maharashtra State Board 12th Science Biology Notes Chapter 6 Plant Water Relation students can recall all the concepts quickly.
Maharashtra State Board 12th Biology Notes Chapter 6 Plant Water Relation
1. Types of substances required by the plant from its surroundings
- Food (for parasites)
- Gases from the atmosphere : (A) O2 for respiration (B) CO2 for photosynthesis
2. Water is absolutely necessary for all vital activities. Hence referred to as elixir of life.
3. Role of water :
- Major constituent of protoplasm (90-95%)
- Helps in maintaining the turgidity of cells and their shape.
- It is a transporting medium.
- Water affects productivity of plants.
Properties of water-
1. Important properties of water.
- Liquid form at room temperature.
- Best solvent for various solutes.
- Inert inorganic compound.
- Neutral pH (i.e. pH = 7) of pure water.
- High specific heat.
- High heat of vaporization.
- High heat of fusion.
2. Due to these properties water is best transporting medium, best medium for biochemical reactions and acts as a thermal buffer.
3. Hydrogen bonding occurs in liquid water which is mainly responsible for these properties.
4. Good adhesive and cohesive forces exist in water molecule.
5. Owing to high surface tension and these forces, it can rise in capillaries.
6. Water is a molecule that connects or is a link between physical factors and biological processes.
Water absorbing organ-
1. Root system – Chief organ of water and mineral absorption.
2. Terrestrial plants absorb liquid water from soil with root hairs while epiphytes like orchids, have special hygroscopic tissue velamen that absorbs water vapour from atmosphere.
3. Regions of root – four zones.
Root cap is situated at tip behind it is
- zone of meristematic region
- zone of elongation
- zone of absorption or root hair zone and
- zone of maturation.
4. In zone of absorption, thin, delicate, unicellular hair like extensions i.e. root hairs develop from epidermal cells.
5. Structure of root hair : It is cytoplasmic extension, tube like, colourless, unbranched and short lived (ephemeral) structure.
6. Root hair has large central vacuole, thin cytoplasm, plasma membrane and double layered wall of pectin and cellulose.
7. Freely permeable cell wall while selectively permeable plasma membrane.
Water available to roots for absorption-
1. Rhizosphere : Microenvironment surrounding the root, constitutes rhizosphere from which plants absorb water.
2. Soil is the main source of water for plants.
3. Water present in soil is in following forms namely :
- Gravitational water percolated deep in soil due to gravity.
- Hygroscopic water held tightly around soil particles, adsorbed or adhered water on fine particles.
- Combined water present as hydrated oxides of silicon, aluminium, etc.
- Capillary water present in the fine spaces or capillaries between soil particles.
4. Plants readily absorb capillary water from soil.
Absorption of water by roots from soil-
When water is absorbed by plant, all the three physical processes occur simultaneously at root hair.
1. Imbibition :
- Swelling up of hydrophilic colloidal substances.
- Water is adsorbed on the surface.
- Imbibant : Substance that adsorbs.
- Imbibate : Substance that gets imbibed.
- In root hair double layered cell wall of cellulose and pectin is imbibant.
- Water is tightly adsorbed on the surface till the equilibrium is reached.
2. Diffusion :
- Movement of ions/atoms/molecules of a substance from region of high concentration to that of their low concentration.
- Movement results due to kinetic energy.
- It takes place till equilibrium is reached.
- In root cell, diffusion occurs through freely permeable cell wall.
- Diffusion pressure created is directly proportional to number of diffusion particles.
- Pure water has more diffusion pressure (D.E) than solvent in solution.
Diffusion results in diffusion pressure. D.ED. can be considered as thirst of cell, capacity which absorbs water from surrounding of adjacent cell.
- D.ED. (Diffusion Fressure Deficit = S.E (Suction Fressure)
- Difference in D.E of pure solvent (i.e. water) and solvent in solution is termed D.ED.
- D.ED. is capacity to absorb water from surrounding.
- Cell sap has less D.E than water around cell wall. Thus, water diffuses inside.
- It is significant in absorption of water and minerals, transport of food, exchange of gases and conduction of water upwards against gravity.
3. Osmosis :
- A process by which water actually enters root hair (cell interior).
- Special type of diffusion.
- Involves movement of solvent through a semipermeable membrane.
- Cell sap inside the cell is concentrated (minerals, sugars) while solution outside cell is weaker. Hence solvent (water) from outside enters the cell passing through semipermeable plasma membrane.
- In root cell, at interphase of cell wall and plasma membrane, water enters by osmosis.
- Type of solutions based on concentration
- Two types of osmosis
- Turgor pressure (T.P.) : Pressure exerted by turgid cell sap on cell membrane and cell wall.
- Fully turgid cell has D.ED. = 0 (zero)
- Wall pressure (W.P.) : Cell wall exerts pressure on inner cell sap i.e. counterpressure. Hence T.P = W.P but it is in opposite direction.
- Osmotic pressure (O.P.) : Pressure exerted due to osmosis so as to stop entry of water (solvent) inside.
- Pressure of solution in opposite direction.
- To check entry of water (solvent molecules) inside cell.
- D.RD. (thirst of cell) demand or ability to gain water by cell = O.P – T.P and T.P = W.P
∴ D.ED. = O.P – W.P (Osmotic pressure minus wall pressure)
- In flaccid cell T.P is 0 (zero) . .D.RD. = O.E In turgid cell D.PD. is 0 (zero) ..T.R = O.R
Facilitated diffusion :
- Passive absorption of solutes (no expenditure of energy)
- Takes place with the help of carriers (special proteins – porins)
- Diffusion through cell membrane
- Lipid soluble components can easily pass but hydrophilic components need carrier.
- Requirement of concentration gradient for diffusion.
Membrane proteins – aquaporins and ion channels are sites of facilitated diffusion.
Water potential ( Ψ )-
- Free energy is needed to do the work and for movement of water, i.e. osmosis )
- Chemical potential : Free energy per molecule in a chemical system.
Water potential : It is chemical potential of water – Unit bars / pascal (pa) / atmosphere D.RD. is now termed as water potential.
- Water potential of protoplasm is opposite in sign but equal to D.RD. i.e. negative value.
- Pure water has water potential zero. When some solute is added there is decrease in water potential (t//) i.e. negative.
- Flow of water is from less negative potential to more negative potential, i.e. from higher water potential to lower.
- In adjacent cells, plasmodesmata connections are concerned with movement of water.
Factors affecting water absorption :
- Types of water-presence of capillary water.
- Soil temperature-favourable range 20 to 30°C.
- Concentration of solutes in soil water – High solute concentration reduces rate of absorption.
- Soil aeration : If soil aeration is less then there is absorption.
- Rate of transpiration : With increase in transpiration, there is increase in absorption of water.
1. Exosmosis that occurs in living cells upon placing in hypertonic (concentrated) solution is termed plasmolysis.
- Shrinkage of protoplasm
- Separation from cell wall forms a gap between cell wall
- Flaccid nature due to removal of water.
2. Turgor pressure (T.R) is zero in plasmolysed cell.
3. Deplasmolysis : When flaccid cell is kept in hypotonic solution endoosmosis takes place and thus it becomes turgid.
4. In fully turgid cell T.R = O.P and D.RD. is zero, (no absorption of water by cell)
Path of water across the root (i.e. from epiblema up to xylem in the stelar region)-
1. Root hair cell : Absorption of water takes place from rhizosphere by process of imbibition then diffusion and finally osmosis.
2. In turgid cells (root hair) due to absorption of water → Increased turgor pressure (T.P) and lowered D.PD. →adjacent cell (Cortical cell) → more D.PD. more osmotic pressure (O.R) → adjacent cell will take water from turgid root ha.i → root hair cell thus becomes flaccid → absorb water from soil.
3. A gradient of D.PD. or suction pressure (S.R) is formed from root epidermis till the region of cortical cells.
4. Movement of water is from root hair → epidermis → loosely arranged cortical cell → passage cells of endodermis → pericycle → protoxylem
5. Due to continuous absorption of water hydrostatic pressure is developed, i.e. root pressure → Helps in transfer and conduction further in xylem of root.
6. The movement of water from root hair to xylem takes place along two different pathways, viz. apoplast pathway and symplast pathway.
7. Pathway for water across roots:
8. Additional apoplast pathway :
- Direct pathway leading to xylem.
- Secondary roots originate at pericycle inside endodermis.
- Bypass endodermis having Casparian strip. Hence allow direct entry in vascular system.
9. In normal apoplast pathway, suberised layer forces shift to symplast in order to enter xylem.
10. Symplast pathway is transmembrane pathway through plasmodesmatal connections in living cells of cortex. The plasmodesmata interconnect the cytoplasm of cells forming cytoplasmic network called symplast.
Learn this as well :
- Vacuoles in the root cells are interconnected to form intercellular connections.
- Intervacuolar connections are formed between the cells.
- Cytoplasmic connections are towards the periphery of cell.
- Tonoplast, the membrane of vacuole is differentially permeable membrane which allows the passage of certain solutes but not all along with solvent.
Mechanism of absorption of water-
Translocation of water-
1. Ascent of sap : Transport of water along with dissolved minerals from root to aerial part against gravity is called translocation or ascent of sap.
2. Ascent of sap occurs through lumen of xylem tracheids and vessels. Physical forces and activity of living cells is required for ascent of sap. Complex tissue xylem as a path of water is proved by ringing experiment.
3. Root pressure theory (Vital theory) by J. Pristley :
- Living cells of root are responsible for translocation of water.
- Xylem sap exuding out from cut end of stem above the soil indicates existence of root pressure.
- As water is absorbed by root hair constantly and continuously, hydrostatic pressure is set in root cortical cells.
- Owing to this root pressure, water with dissolved minerals is pushed into xylem and also conducted upwards.
- Root pressure is an osmotic phenomenon, develops due to absorption of water.
- Oxygen, moisture, temperature and salt content of soil affect root pressure, Root pressure of +1 to +2 bars is sufficient to carry water upwards to 10 to 20 metres.
Objection to this theory :
- Not applicable to tall plants.
- Ascent of sap occurs even if root system is absent.
- Some tall plants have zero root pressure (Gymnosperms).
- Root pressure is absent in actively transpiring plants.
- Xylem sap shows negative hydrostatic pressure as it is under tension in normal condition.
4. Capillarity theory (Physical force theory) By Bohem :
- Physical forces and dead cells (xylem with lignified wall) are responsible for translocation.
- Water is raised to certain level due to capillarity.
- Capillarity is due to surface tension, cohesive and adhesive forces of water.
- Water conducting elements have lignified walls and are with lumen (xylem vessels and tracheids)
- Combined cohesive forces of water and adhesive forces of water with xylem wall form continuous water column.
- Owing to capillarity, water is conducted upwards against gravity.
Objection to capillarity theory :
- Continuous capillary tube is essential but tracheids have thickened, tapering closed end walls.
- Lower end of capillary tube not in direct contact with soil water.
- Tall trees show wide lumen in xylem vessels. Narrower the capillary tube, higher level of water column is raised.
5. Cohesion – Tension theory (Transpiration pull theory) By Dixon and Joly :
- Widely accepted theory of ascent of sap.
- Based on cohesion and adhesion with transpiration pull developed.
- Strong force of attraction of water molecules : Cohesive force
- Strong force of attraction of water molecules and lignified walls of xylem : Adhesive force
- Water loss is in the form of water vapour, mainly through stomata is transpiration.
- Owing to combined action of cohesive and adhesive forces, a continuous water column is maintained through xylem.
- Transpiration pull developed due to water loss in leaf vessels is transmitted downwards towards root.
- Water lost from stomata causes increased D.RD. of mesophyll cells which in turn takes water from xylem of leaf.
- A gradient of suction pressure or D.ED. is set in, due to transpiration, which causes tension or pull. Owing to this, water column is pulled upwards through xylem.
- It is passive pull of water against gravity which results in ascent of sap.
Objections to transpiration pull theory :
- Formation of gas bubbles due to temperature fluctuations may not keep water column continuous.
- Vessels as tabular structure are much evolved and efficient in conduction but this theory assumes trachieds are more efficient.
- If transpiration is checked due to some artificial means like application of Vaseline, then also ascent of sap occurs, (clogging of stomata due to application of Vaseline)
- Ascent of sap occurs in plants which are deciduous, (leaf fall)
Transport of mineral ions-
- Minerals are elements which play an important role in vital processes in metabolism. Thus they are essential elements for plants.
- Elements required in large amount, Macro elements : e.g. N, P C, H, O, etc.
- Elements are required in small amount, Micro elements : e.g. B, Cu, Mn, Co, etc.
- Soil is a chief source of minerals and they are absorbed in dissolved (ionic) form through root system.
- Minerals are absorbed by plants from their surrounding environment (atmosphere – C, H, O) and soil (inorganic materials).
- Absorption of minerals is independent of that of water.
- Minerals are transported with ascent of sap. Hence root is source and they get lodged at the required organ.
- Unloading of the transported material is by diffusion from veins and cells uptake them.
- Minerals can be remobilized inside plant body from older leaves to young leaves, e.g. R S, N, K, etc. But those parts of structural framework are not disturbed, e.g. Ca.
- Nitrogen in inorganic ion form and amino acids, amides in organic form are transported through xylem.
- Some exchange of material takes place between xylem and phloem.
Transport of food-
- Food is synthesised in chloroplast containing cells.
- Part of plant where food is synthesised is source (leaf) and where it is utilized is sink e.g. root.
- Translocation of food occurs from source to sink through phloem. The movement or transport of food from one part to other part is called translocation of food.
- Sieve tubes (phloem) and vessels (xylem) are ideal for vertical or longitudinal transport. Sieve tubes for downward transport.
- The lateral or horizontal translocation occurs through medullary rays (parenchyma) from phloem to pith or cortex.
- Food is translocated in soluble form sucrose along concentration gradient set from sink.
- Vertical translocation – (longitudinal transport)
From leaves i.e. source to sink (root) in downward direction or growing point (stem) and seed germination, corm, bulbil germination in upward manner.
- Lateral translocation – occurs in root and stem.
- Radial translocation from phloem to pith.
- Tangential translocation from phloem to cortex.
- Phloem transport is bidirectional. Phloem sap has sucrose, and water with other sugar, amino acids and hormones.
- Mechanism of sugar transport through phloem – Mass Flow hypothesis or Munch’s Pressure flow theory is – most widely accepted concept.
- Other theories are diffusion, activated diffusion, electro osmosis, protoplasmic streaming.
- Ernst Munch theory : Glucose synthesised in photosynthesis which increases osmotic concentration of photosynthetic cell → Endo osmosis → water absorbed from adjacent cells and xylem → Turgidity of cell →Increased turgor pressure → sugar from photosynthetic cell forced into sieve tube → This is vein loading.
- Root cell (sink) → utilization of sugar → polymerisation of sugar to starch → osmotic concentration lowered. Exosmosis → hence water lost to adjacent cells → decrease in turgidity → Turgor pressure lowered → Turgor pressure gradient is set → Translocation of food passively along concentration gradient → This is vein unloading.
- Sugar is used at the sink or stored and excess water transported to xylem.
Objections of theory –
- Bidirectional flow is not explained.
- Pressure flow is a physical process.
- From the constant absorption of water 5% is utilized and 95% surplus water is lost through aerial parts in the form of mainly water vapour.
- Guttation : Loss of water in liquid form (1%), occurs from water stomata or hydathode.
- Transpiration : Water lost in the form of water vapour mainly foliar transpiration.
- Types of Transpiration
Structure of stomatal apparatus-
- Stomatal apparatus has guard cells, stoma and accessory cells.
- The elliptical pores (opening – stoma) are bounded by two guard cells, either kidney shaped or dumbbell shaped cells.
- Guard cells are modified epidermal cells, nucleated cells with uneven thick wall – Inner wall thick and inelastic, outer wall is thin and elastic, with chloroplasts.
- Accessory cells/Subsidiary cells –
Specialized epidermal cells that surround guard cells. They are reservoir of K+ ions.
- Opening and closing of stomata Is controlled by turgidity of guard cells.
- During daytime → Thrgld guard cells due to endoosmosls → ExertIon of T.P on outer thin wall → elastic wall stretch out → Thick walls pulled apart → stoma opens.
- During night-time → flaccid guard cells due to exosmosis → outer elastic wall relaxes → Inner thick walls pushed → stoma closes.
- Diurnal changes In osmotic potential are responsible for flaccidity and turgidity of guard cells.
- As per starch-sugar hypothesis → DurIng day time starch gets converted to sugar by enzyme phosphorylase → Increase osmotic potential → entry of water
Reverse reaction during night → stoma close
As per proton pump theory — Transport of H+ and K+ ions
- During daytime – starch converted to malic acid → dissociation into malate and protons (H+) → H+ in subsidiary cells → K+ ions from subsidiary cells to guard cells → open stomata Potassium malate → Increase osmotic potential → endoosmosis (turgidity)
- At night → uptake of K+ and Cl– ions is checked by abscissic acid – change In permeability, osmotic potential → Hypotonic guard cells → exosmosis → flaccid → stoma close
Advantages of Transpiration:
- Removal of excess water
- Helps In absorption of water
- Cooling effect
- Helps in gaseous exchange
- Maintains turgor of cells
- Ascent of sap
Disadvantage – Excessive transpiration causes wilting injury and that may lead to death of plant.
Transpiration : A necessary evil – (By Curtis)
- During daytime stomata remain open thus help in gaseous exchange – for respiration and photosynthesis
- Productivity is adversely affected if stomata remain closed
- When stomata are open transpiration cannot be avoided.
Know the scientists :
Scientists — Their theories/discoveries
- B.S. Meyer – Coined the term Diffusion Pressure Deficit D.RD.
- Atkins and – Osmotic absorption Pristley theory
- Kramer and – Non-Osmotic absorption Thimann theory
- J. Pristley – Root pressure theory
- Bohem – Capillarity theory
- Dixon and Joly – Cohesion Tension theory
- Munch – Pressure flow theory
- Steward – Starch-sugar interconversion theory
- Levitt – Proton transport theory
- Curtis – Transpiration as ‘a necessary evil’
- S. Hales – Term root pressure