Plant Water Relation and Mineral Nutrition

· Study of functional aspects of plant life
· Father of plant physiology is Stephens Hales
· Plant physiology is the science that deals with the functioning of cells, tissue, organ or the plant as a whole.
Includes: Physical, Physiochemical and Special processes.
· Physical: Diffusion, osmosis, plasmolysis, Imbibition, Absorption, Ascent of sap, Transpiration, Guttation.
· Physiochemical: Respiration, photosynthesis, photorespiration.

Instrument Measurement
Auxanometer Growth
Osmometer Osmotic pressure or osmosis
Potometer Transpiration
Porometer Size of stomata
Hygrometer Relative humidity
Microtome Used for making tissue slides
Manometer Root pressure
Clinostat demonstrate geotropism
Respirometer Respiration
Crescograph Growth per second
Oxygen bomb calorimeter The energy contained in macrophytes

1. Diffusion (Passive process)

· It is a passive process in which movement of particles i.e. ions or molecules occurs from higher concentrated region to lower concentrated region.
· Movement of water molecular from higher concentration to lower concentration without a semi-permeable membrane is called diffusion
· Movement of particles is due to kinetic energy.
· Movement of particles in the protoplasm of the cell is due to diffusion.
· Rate of diffusion increases with the increases in temperature, diffusion pressure deficit.
· Flow from From higher diffusion pressure to lower diffusion pressure.
· Diffusion pressure is a hypothetical term that describes the potential ability of the liquid or solid or gas to diffuse from its higher concentration to lower concentration.
· Diffusion pressure of a pure solvent (as water 1236 atm) is always higher than the solution.
· Rate of diffusion increases with the increase in temperature and DPD (diffusion pressure deficit) diffusion of gas > diffusion of liquid > diffusion of solid
· Diffusion involves gaseous exchange during photosynthesis and respiration, distribution of food materials in cytoplasm and translocation of food materials.
· Gaseous exchange (especially O2 and CO2) with the atmosphere through stomata and lenticels occur by the process of diffusion.
· Diffusion, however, is not involved in the opening and closing of stomata. Here, osmosis is involved.
· Osmosis is also diffusion. It is a special type of diffusion.
· Aroma of the flower is due to the diffusion of volatile aromatic compounds.
· Passive uptake of minerals is also diffusion.
· When diffusion of two or more substances takes place simultaneously, it is called independent diffusion.
Eg: Gaseous exchange in respiration and photosynthesis, loss of water vapour in transpiration etc.
· Movement of dissolved substances through a semi-permeable membrane with the help of carrier protein is called facilitated diffusion. Eg; transport of fructose.

2. Osmosis

· Term osmosis was coined by Nollet.
· It is the diffusion of water or solvent molecules across a semi-permeable or selectively permeable membrane from dilute solution to concentrated solution. Osmosis involves the movement of solvent only
· Osmosis is a special type of diffusion, the only difference being the presence of a semi-permeable membrane.
· Cell is an osmotic system in which plasma membrane, tonoplast and all membrane surrounding organelles act as a differentially permeable membrane.
· Cell wall is freely permeable and is not an osmotic barrier.
· Cutinized and suberized cell walls are impermeable.
· In osmosis water moves from lower DPD to higher DPD.
· Rate of Osmosis is measured by Osmometer, Thistle funnel (using egg membrane)
· Osmosis continues until the hydrostatic pressure due to accumulated flow of solvent has attained a valve sufficient to stop the further flow. This excess pressure is known as Osmotic pressure.
· Flows from Turgid cell to flaccid cell, means water moves from dilute solution to concentrated solution.
· Osmotic pressure = - Osmotic potential.
· DPD: This is often called suction pressure (SP). In one sense it is the ability to absorb water.
· So, the osmosis occurs till the hydrostatic pressure becomes equal to osmotic pressure.

Osmosis requires:
a) difference in concentration of the solution.
b) semipermeable membrane.

Osmosis always takes place from:
· Higher water potential to lower water potential.
· Dilute solution to concentrated solution.
· Higher free energy level to lower free energy level.
· Hypotonic solution to hypertonic solution i.e, against the concentration gradient of solution. (osmosis is not possible in isotonic solution)
· Low osmotic pressure to high osmotic pressure.
· Turgid cell to flaccid cell.
· Low DPD to high DPD.
· Osmotically inactive solution to osmotically active solution.
· The osmotically inactive solution is formed due to insoluble solute.
Eg; Starch in water.
· Osmosis takes place from higher water potential to lower water potential, dilute solution to concentrated solution and from higher free energy to lower free energy.
· At least a semi-permeable membrane is necessary for osmosis.
· Osmosis is applicable only to the solvent part of the solution.
VVI Note:
1. Free energy of pure water is 0.
2. It becomes lower due to the addition of dissolved solute.
3. So the free energy of water in the solution is always negative.
4. Osmotic pressure and DPD ∝ Concentration solution (solute)
2. DPD: Water absorbing capacity.
· Osmosis is exhibited only by the living tissue. As in dead cells, semi permeability is lost, so boiled potato does not show osmosis.

Osmosis Involves in:
· Gives rise to turgidity which is responsible for some kinds of plant movements.
· Opening and closing of stomata;
· Growth of plumule & radicle during seed germination;
· Maintaining movements of leaves of Mimosa pudica.
· Cell to cell movement of water.
· Root hairs absorb water from the soil through the process of osmosis.

Various Types of Solution
a. Hypertonic: The osmotic concentration is more than cell sap in a solution. i.e. the solution has a higher solute concentration than cell sap. A cell loses water when placed in a hypertonic solution.

b. Isotonic solution: Has the same osmotic concentration or solute concentration as that of cell sap.

c. Hypotonic: In which the osmotic concentration or solute concentration is less than that of cell sap. A cell gains water when it is placed in a hypotonic solution.

d. Turgor pressure: It is the pressure, which develops inside a cell and is exerted on the cell wall.
· During endosmosis, water enters inside the cell and the turgor pressure increases gradually.
· Turgor pressure is maximum when the cell wall cannot stretch anymore.
· The fully expanded condition of a cell with its wall in a state of tension due to excessive accumulation of water is called turgidity.

e. Wall pressure: The inward pressure exerted on the cell content by the stretched cell wall.
· Normally, Turgor pressure (TP) = Wall Pressure (WP)
Exosmosis – the movement of water out of the cell
Endosmosis – the movement of water inside the cell
Turgid cell – The cell with maximum water concentration in it
Flaccid Cell - cell with less water concentration or solvent concentration inside it.

In the Normal Cell;
· DPD = OP – TP (TP = WP)
· In a fully plasmolysed cell, turgor pressure or wall pressure vanishes.
i.e., TP = WP = O & DPD = OP (OP = Osmotic pressure)
· In a fully turgid cell, DPD vanishes. i.e. DPD = O. So, TP = OP
· Cell in hypotonic solution → Endosmosis, cell become Turgid
· Cell in hypertonic solution
  Exosmosis, cell become flaccid.
· Opening and closing of stomata are due to the turgidity of the guard cell.
· Guard cell turgid – stomata open.
· Guard cell flaccid – closing of stomata.
· Movement of some plant parts like leaves of Touch me not Plant is regulated by the turgidity of their cell.

Osmosis potential (O.P) or Solute Potential:
· Osmosis potential is the decrease in the chemical potential due to the deposition of solute particles in the solution.
· Its value is negative and pure water has zero potential.
· Osmometer measures osmotic pressure.
· Osmotic pressure is the hydrostatic pressure which is just enough to balance and prevent osmosis of water into an osmotically active solution through a semi-permeable membrane.

Water Potential:
· Free energy per mole of the water molecule is called water potential.
· Water potential = – DPD
· It is represented by the Greek letter Ѱ (psi).
· Water potential of pure water is zero and the addition of solute in it decreases its water potential (i.e. negative value).
· Unit of water potential bars.

· It is a special type of phenomenon in which shrinkage of protoplast occurs when a cell is placed in hypertonic solution due to exosmosis.
Types of Plasmolysis:
    a) Incipient plasmolysis: protoplasm just leave the cell wall.
    b) Full plasmolysis: It is the complete shrinkage of protoplasm.
· Best and quickest method to determine the living or dead nature of the cell.
· Helps to determine permeable nature of cell wall and semi-permeable nature of cell membrane.
· Gap between the cell wall and cell membrane in a plasmolysed cell is filled with hypertonic solution.
· During plasmolysis there is no wrinkling of cell wall because space between the cell wall and cell membrane is filled with hypertonic solution.
· Bacteria cannot survive in a salted pickle due to plasmolysis. [IOM]
· Excessive use of fertilizer can kill the plant by plasmolysis.

· Salt is spread in the tennis court to kill the weeds by plasmolysis.
· A plasmolysed cell regains normal condition if placed in a hypotonic solution which is called deplasmolysis.
· Weedicides kills the plant by plasmolysis.

· A plasmolysed cell regains normal condition if placed in a hypotonic solution, it is called deplasmolysis.
Note: Bursting of cell in pure solvent/dilute medium is plasmoptysis.

· It is the surface phenomenon by which solvent is adsorbed(not absorbed) on the surface of particles without forming a solution making them swell.
· Imbibition is the movement of water into imbibant through diffusion as well as capillary action.
· Swelling of seeds, when placed in water, is due to imbibitions.
· It is the first Physiological process during water absorption and seed germination (seed germination is accompanied by the evolution of heat)
· Wooden doors are difficult to open and close during the rainy season due to imbibition.
· Cell wall, dry seed, wood, and velamen roots absorb water by imbibition.
· The solvent adsorbed is called imbitate and the solid which adsorbs is called imbitant.
· Pollen tubes burst inside ovary due to imbibitions.
· Due to imbibition:
a. Volume of imbibant is increased.
b. Pressure is exerted.
c. Heat is released (exothermic process)
· Wooden pieces present in rock crevices, when soaked in water may
rupture the rock due to imbibition pressure.
· Agar is a very efficient imbibant, it imbibes (99 times its mass) of water therefore used in bacterial culture.
· Rubber never imbibes water, it only imbibes ether and other organic solvents like kerosene.
· Imbibition is an exothermic process.
· Increase in temperature causes an increase in imbibition.
· In terms of imbibition, DPD is calculated by, DPD = Imbibition pressure - TP
· There should be some forces of attraction between imbibant and imbibed liquid or affinity between imbibant and imbibed liquid is necessary for imbibition to occur.
Eg: If a wooden piece is soaked in ether, no swelling or no imbibition, but the wooden piece in water shows swelling due to imbibition, Similarly, rubber soaked in water shows no imbibition and rubber in ether shows imbibition.
· The First step in imbibition is adsorption i.e, attachment of liquid on the surface.
· Imbibition involves both diffusions as well as capillary action.
· Adsorption is the property of colloid and thus materials that have a high proportion of colloid are good imbibants.
· It is, therefore, the wood (plant material) is a good imbibant, because it contains protein, cellulose and starch as colloid substances.
· Imbibition capacity of protein > starch > Cellulose
· Imbibition doesn't occur in waxy substances. (eg; cutin)
· Swelling of seed in water is due to imbibition.

Absorption of water:
· It is a movement of water from the soil to the xylem of the root.
· Involves imbibition, osmosis, diffusion and capillary action.
· Mainly from root hairs.
    1. Root hairs
    2. Secondary or Lateral roots

· Absorption of water is through the root hair zone of the root tip. (Root cap has no role in water absorption as it doesn't have any root hair)
· Root hairs & exogenous in origin & secondary or lateral roots are endogenous in nature.
· Transplanted seedlings often get killed because of damage to root hair.
· Water will be absorbed by root hair when the external medium is hypotonic.
· Main source of water: Rainwater.
· Most easily available soil water: Capillary water.
· Hardly available soil water: Gravitational water
· Completely unavailable water: Hygroscopic water.
· Total water contain in soil: Hollard.
· Water available to plants: Chresard.
· Water unavailable to plants: Echard

Special note:
1. Root hair acts as an osmotic system for the absorption of water.
2. Root hairs are most developed in herbs as compared to shrubs and trees.
3. Root hairs are more developed in angiosperm than the gymnosperm because of the presence of mycorrhiza in a gymnosperm.
4. Root hair is having a permeable cell wall, with the outer layer of pectic substances (for attachment of soil particle) and the inner layer of cellulose.

Absorption of water in certain plants:
Plant type Absorption through
Marchantia Unicellular rhizoids
Mosses Multicellular rhizoids
Lichen Rhizines
Sporophyte of fern Adventitious root
Gametophyte of fern Rhizoids (Unicellular)
Algae and fungi General body surface
Pinus Mycorrhizal roots
Higher plants Root hairs
Parasitic plants (Cuscuta) Haustorial roots

· Pathway of water absorption in higher plants:
Soil → Root hair → Cortex → Endodermis (passage cell) → pericycle → Protoxylem → metaxylem.

Mechanism of water absorption
· Roots play an active role.
· Involves symplast movement.
· Root plays an active role and shoots plays a passive role.
· ATP is utilized.
· Transpiration is not responsible for the active process.
· Account for < 10% of absorption.
Symplast – living system formed by plasmodesmata and protoplast.
Types of active absorption:
1. Non-osmotic absorption- Occurs against osmotic gradient by using metabolic energy or ATP.
2. Osmotic absorption
- Occurs according to osmotic gradient without using ATP.


· Root plays a passive role and shoots plays an active role.
· ATP is not utilized in water absorption by this method.
· Rapid water absorption takes place by this method.
· It accounts for 90% of total water absorption.
· It involves both Apoplast and symplast movement of water.
· Apoplast- Non-living system formed by a cell wall and intercellular space.
· Transpiration is responsible for passive absorption.

Rate of water absorption and transpiration:
· Movement of water from the soil to the root of the plant or xylem of the root is known as the absorption of water.
· Rate of water absorption decreases in poorly aerated and highly saline soil.
· Rate of water absorption decreases at very low and very high temperatures.
· Maximum water absorption is at 20°C – 30°C.
· Further movement is known as an ascent of sap.
· Occurs through tracheary elements of xylem (tracheids or vessels)
· Tracheids are the path of the ascent of sap in gymnosperm and vessels in angiosperm.
·  Vessels are absent in gymnosperm except for Gnetales.
·  Salty water – physiologically dry soil
Sandy soil – physiologically dry soil
Note: Physiologically dry soil: Water is present in the soil but plants can’t absorb it due to dissolved salts (saline soil).

Absorption of Minerals:
· Mostly occurs through the zone of elongation (not by Root hairs) in the form of ions.
· Mineral absorption is independent of water absorption.
· Absorption occurs directly by cells of epiblema and not by root hairs.
· Absorption of minerals occurs in the form of ions present in the soil.
· First step in mineral absorption is ion exchange.

Mineral absorption may occur:
i) Passively: Along the concentration gradient without using ATP.
ii) Actively: Occurs against the concentration gradient by using ATP.

Ascent of Sap
· It is the process of the vertical rise of water with dissolved minerals through xylem tracheids and vessels against the force of gravity.
· Path of the ascent of sap is the xylem tracheids and vessels.
· Path of ascent sap in gymnosperm is Tracheids.
· Best experiment to show the path of the ascent of sap is the Ringing or Girdling experiment.
· This experiment is not applicable in plants having Bicollateral, Amphivasal and scattered vascular bundles.

Theories on Ascent of Sap

· Ascent of sap through the involvement of living cell.
i. Godlewski and Janse – Relay pump theory.
· Ascent of sap due to pumping activity of xylem parenchyma.
ii. J.C Bose (1923) – Pulsation theory of the ascent of sap.
· Ascent of sap due to pulsation activity of innermost cortical cell i.e. cell just outside the endodermis.

2. ROOT PRESSURE THEORY: Given by Priestley
· Ascent of sap due to root pressure.
·  Strasburger – proved that living cells weren't involved in the translocation of water through the xylem.
Stephan Hales coined the term root pressure in 1727.

· Given by Dixon and Jolly.

· Most acceptable theory of the ascent of sap.
· Best experiment to show the path of the ascent of sap.
· Ascent of sap is due to cohesive (between water molecules) and adhesive force (between water and xylem vessel)
· Based on the cohesive, adhesive, and capillary properties of water.
· According to this theory, transpiration is responsible for the ascent of sap.
· Ringing experiment – xylem is the path of the ascent of sap.
· Unlike water absorption, minerals are absorbed through the entire surface of the young root.
· Absorption of minerals is in the form of ions through the meristematic zone of the root tip.

4. Capillary theory:
According to this theory, the xylem represents the tube of the narrow bore and hence believe that the ascent of Sap was due to capillarity.

5. Imbibition theory:
· Given by Unger and supported by Sachs.
· It states that water mores up through the walls of xylem elements by imbibition and that the lumen or cavity of xylem is not important.
Note: Atmospheric pressure theory: Given by Boehm
· A/C this theory ascent of sap is due to atmospheric pressure that raises up the water to fill up the accumulation of absorbed water.


· Loss of water in the form of water vapour from the aerial part of the plants.
· Transpiration occurs when the outer atmosphere has less moisture than substomatal cavities.
· The condition upon which the transpiration is maximum is low humidity, high temperature, guard cell turgidity, moist soil.
· Transpiration increases in hot, dry and windy conditions.
· Transpiration is reduced due to the deposition of cutin.
· Rate of transpiration is reduced with a decrease in light intensity.
· Loss of water occur in pure form (water not mixed with ion)
· Transpiration is a “controlled phenomenon”.
· Transpiration does not occur from the root.
· “Transpiration is a necessary evil” by Curtis.
· Transpiration rate of the dorsiventral leaf is higher on the lower surface due to the higher number of stomata.
· Cobalt chloride test is done to observe the unequal transpiration in dorsiventral leaf.
· Change in colour of the cobalt chloride from blue to pink (Hydrated-pink, dry-blue) indicates that the water vapour evaporates from both the surface of leaves. But the quick change in colour of cobalt chloride paper at the lower surface indicates a higher rate of transpiration from lower than that of the upper surface.
· The rate of transpiration is measured by Ganong’s potometer.
· Vital and unavoidable phenomenon, often called Necessary evil.
· Occurs through three sites, stomata, lenticels, and cuticle.
· Helps to maintain plant body temperature constant.
· The plants use only about 1-2% of water absorbed and the rest 98-99% is lost by transpiration.
· Rate of transpiration is measured by using potometer.
· Transpiration is a vital physiological process that is restricted to living bodies.
· Evaporation is a physical process in which water changes from a liquid to a gaseous phase. It is a slow loss of water from the surface of non-living objects to unsaturated atm.

Transpiration occurs through 3 sites:
a. Stomata
b. Cuticle
c. Lenticels
(i) Stomatal transpiration
· Through stomata
· Accounts for 90% of the total transpiration.
· Stomata are generally numerous on the lower epidermis of than upper epidermis.

(ii) Cuticular
· Through cuticle of the epidermis of leaf and stem or through the general body surface.
· Accounts for about 10% of the transpiration.

(iii) Lenticel transpiration
· Through lenticels, lenticels are the minute pores present on the bark.
· Transpiration through leaf – foliar transpiration.
· Transpiration through corky covering of stem-bark transpiration.
· Epidermal cells containing chloroplast are guard cells.
· Stomata open during daytime is photoactive stomata. eg: usual plants.
· Stomata open during nighttime are Scotoactive stomata. Eg: Succulent xerophyte (CAM plant) i.e, opuntia, Bryophyllum etc.
· Scoto active stomata are present in succulent xerophytes like cactus, Bryophylum etc.
· Opening and closing of stomata are due to the turgidity of the guard cell.

Stomata Contain:

a) Subsidiary cells (cells around guard cell)
b) Guard cells
    i) Single in the Apophysis region of the capsule
    ii) Kidney-shaped in dicots
    iii) Dumb bell-shaped in monocots

Various Types of Stomata:
Mainly 5 types
1. Apple and mulberry type
· In bifacial or dorsiventral leaves, stomata are present only on the lower side. eg. wall nut, walnut most of the trees, oak.

2. Potato type- The stomata are more numerous on the under surface than on the upper surface of the leaf. eg. Cabbage, pea, bean, potato, tomato.

3. Oat type- stomata are more or less equally distributed on the two surfaces. i.e. amphistomatic.

4. Water lily type- stomata only on the upper surface i.e. condition is epistomatic eg. Aquatic plants with floating leaves.

5. Potamogenton type- Stomata are absent eg. mostly submerged aquatic plants.

· In the thin leaves of mesophytes, stomata open during the day and close during the night. They belong to the Alfa-Alfa type.
· In most cereals, stomata close during the night and open for a few hours during the day, this comes under Barley type.

Opening and Closing of Stomata:

· Stomata open - Guard cell becomes turgid i.e. stomata open when guard cells show an increase in both osmotic and turgor pressures.
· Stomata close - Guard cell becomes flaccid.
· Photoactive stomata - Stomata opens during daytime eg. all plants.
· Scoto active stomata - Stomata that open during night time. eg. opuntia, succulent xerophytes.

Mechanism of Opening and Closing of Stomata:
(i) Starch – Sugar hypothesis:
· Proposed by Lloyd and elaborated by Sayre and Steward.
· According to this hypothesis the stomatal opening and closing mainly depends upon the activity of the enzyme starch phosphorylase in different pH.
· Conversion of the starch to organic acid is required for stomatal opening.

Fig: Graphical Representation of Starch sugar Hypothesis

(ii) Potassium Malate Theory or Active K+ Exchange Mechanism:
· Most accepted theory.
· Given by Fujian and later modified by Levitt.
· Influx of K+ into guard cell – stomata open.
· Out flux of K+ from guard cell – stomata close.
· Opening of stomata is an active process but the closing of stomata is a passive process.
· Transpiration in old stems and fruit occurs through lenticels.
· Plant growing in high altitude shows xeromorphic (adaptation of minimum transpiration)

Fig: Role of K+ in stomatal opening.

Factors affecting rate of transpiration:
· An increase in temperature increases the rate of transpiration.
· Rate of transpiration increased with the increase in root shoot ratio.
· Wilting takes place when a plant loses more water by transpiration than it can take up from the roots.
· Rate of transpiration increases in gently blowing wind but decreases in violently blowing wind.
· Maximum transpiration occurs in mature leaves than young leaves.
· Anti transpirents – inhibits transpiration.
    E.g. Abscessic acid, (ABA), higher CO2 concentration, phenylmercuric acetate (PMA) etc.
· Bell Jar experiment to demonstrate the phenomenon of transpiration.
· Ganong's potometer is used to measure the transpiration rate.
· Experiment to demonstrate unequal transpiration from two surfaces of the dorsiventral leaf by using cobalt chloride paper.
· Cobalt Chloride Paper colour: Dry: blue, Wet: pink
· Transpiration directly influences the absorption of water from the soil.
· The evaporation of water during transpiration helps in cooling of leaves and protects from heat injury.
· The stomatal and cuticular transpiration collectively termed foliar transpiration.
· Transpiration is also harmful to the plant because it is an unmatched loss of water, which causes wilting and dying of the plant and it results in wastage of energy during the Ascent of Sap.

· Loosening of plant part.
· Wilting occurs when transcription>water absorption, due to loss of turgidity.
· It occurs when the xylem is blocked or removed.
· Percentage of water left in the soil when the plant wilts is known as the permanent wilting coefficient.
· During wilting the sequence of events shall be:
· Exosmosis → plasmolysis → temporary wilting → permanent wilting.

Types of Wilting: 3 types
i) Incipient wilting:
· Not visible due to partial loss of turgidity.

ii) Temporary wilting:
· It happens during hot days when the rate of transpiration is high and the rate of absorption is less and is recovered at night.
· It is reversible.

iii) Permanent wilting:
· Plant fails to recover or return to its original state.
· Xylem vessels are blocked,
· It is an irreversible process.
Note: During the wilting, there is no wrinkling of the cell wall because during
wilting there is only loss of water with no replacement.

· Guttation is the process of loss of water along with minerals (cell sap) from the plant in the form of water droplets from the margin or apex of the leaf lamina.
· It occurs through hydathode.
· Loosely arranged parenchymatous cells around hydathode are epithem.
· Root pressure is responsible for guttation.
· Occurs in the early morning when water absorption and root pressure are high and transpiration rate is low.

· Exudation is the loss of water sap from the incision of plant body parts.
· Exudation of water from the leaf incision: Bleeding
Exudation of water from leaf margin: Guttation

Fig: L.s of leaf apex showing Hydathode.

Mineral Nutrition in plants
· Green plants prepare their organic food from inorganic substances which they take from the soil in the form of minerals, known as mineral elements or mineral nutrients and this nutrition is called mineral nutrition.
· Analysis of plant ash shows that about 92 mineral elements are present in different plants. Out of these, 30 elements are present in each and every plant
· Out of these 30, 16 elements are necessary for plants and are essential elements, which are as: C, H, O, N, P, S, K, Mg, Ca, Fe, Cu, B, Zn, Mn, Mo and Cl.

Essential elements are divided into two types:
· Macroelements or Major elements: They are required in large amounts.
Eg; C, H, ON, S, P, K, Ca, Mg.

· Microelements or Minor elements or Trace elements: These are required by plants in very small amounts, i.e, in a trace.
Eg; Fe, Cu, B, Zn, Mn, Mo, Cl.

The elements having atomic number > 20 are microelements except boron ad chlorine.
· Carbon hydrogen and oxygen are non-mineral elements. Plants Obtain carbon in the form of atmospheric carbon dioxide, hydrogen mainly water and oxygen are obtained from air or water.

· Tracer elements: These are radioactive isotopes of elements, which are used to detect various metabolic pathways in plants. 
    Eg; C(14), N(15), P(32), S(35) etc.

· Critical elements: 
Critical elements are the elements in which soil is generally deficient. eg, N, P, K. These are given in the form of fertilizers.
· C, H, O (Framework elements): They constitute the carbohydrate that forms the cell wall.
· Protoplasmic elements: N, P, S form part of protoplasm along with C, H, O.
· Balancing elements: Ca, Mg and K counteract the toxic effect of the other minerals by ion balancing.

· In addition to 16 elements, some plant requires some more essential elements. These are:
i) Silica- found in grasses and diatoms.
ii) Sodium- found in algae and microbes.
iii) Aluminium- found in fern and lycopodium
iv) Iodine- found in marine algae.

Special point:
· Molybdenum is the least required micronutrient for plants.
· Nitrogen is derived from both mineral and non-mineral sources.

Role of different elements:
1 Nitrogen (N):
· Chief source of nitrogen for plants is nitrates of Ca and K.
· Constitute of proteins, nucleic acids vitamins, hormones, coenzymes, ATP etc.
· Deficiency causes stunted growth, lower respiration rate, chlorosis of older leaves, premature leaf fall.

2. Sulphur (S):
· It is absorbed as sulphate.
· Constitute certain protein vitamins (thiamine, biotin, CoA, ferredoxin)
· Deficiency causes Chlorosis of younger leaves.

3. Phosphorous (P)
· It is absorbed as phosphate.
· Important constitute of certain proteins, nucleic acid, cell membrane, nucleotide required for phosphorylation reaction
· Deficiency causes premature leaf fall.

4. Calcium (Ca):
· It is absorbed in the form of nitrates and sulphate.
· Involved in the selective permeability of cell membrane, activates certain enzymes, required for the development of stem and root apex and as calcium pectate in middle lamella of the cell wall.
· Deficiency causes Disintegration of the growing meristem.

5. Magnesium (Mg):
· It is an important constituent of chlorophyll. (Mg2+)
· It helps in the binding of ribosomal particles where protein synthesis occurs.
· Deficiency: Chlorosis.

6. Potassium (K):
· It is absorbed as its nitrates and chloride.
· It is essential for the process of photosynthesis and respiration.
· It plays important role in stomatal movement.
· Deficiency causes mottled chlorosis of leaves, plant growth inhibition.

7. Iron (Fe)
· Absorbed by plant mostly in ferrous form.
· It is a part of cytochrome, hence play important role in ETS, photosynthesis and respiration.
· It is also essential for chlorophyll synthesis.

8. Boron (B):
· It is absorbed as borate.
· It helps in the translocation of food in plants.
· Deficiency causes Death of shoot tip, suppression of flower formation, stunted root growth, brown heart disease.

9. Manganese (Mn):
· It is absorbed as its oxide.
· it is an activator of several enzymes of Kreb's cycle like malic dehydrogenase.
· It is responsible for photolysis of water during photosynthesis, synthesis of chlorophyll and IAA.
· ETS are dependent on Mn.
· Deficiency causes grey spots in leaves.

10. Zinc (Zn):
· It is essential for the synthesis of tryptophan amino acids which (form IAA)
· Deficiency causes Chlorosis of older leaves.
· Khaira disease of rice and a white bud of maize is due to Zinc deficiency.

11. Molybdenum (Mo):
· It is responsible for the modulation of legumes.
· Deficiency causes the inhibition of fruit formation.
· Whiptail disease of cauliflower is caused by Mo deficiency.

12. Chlorine (Cl):
· It has an important role in photosynthesis, especially in light reaction,i.e.helps in the photolysis of water.
· With Na and K helps in maintaining solute concentration and ionic balance in cells.
· Deficiency: Wilting of leaf tip followed by chlorosis, necrosis (death of tissue)

13. Copper (Cu): Its deficiency causes necrosis of the tip of new leaves.

NOTE: Liebig’s law of minimum: It states that the yield of the crop plant is determined by the amount of that essential element, which is present in minimum quantity, in proportion to the demand of the plant.

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