Crop physiology

Crop physiology :  It can be defined as the science, which deals with the life processes of crop plants or the functions of cells, tissues, organs or the whole plant.

·        Father of plant physiology Stephen Hales

·        Plant cell was discovered by Robert Hook

·        Nucleus was discovered by Robert Brown

·        The term cell was proposed by Robert Hooke

·        Father of modern cytology is known as Carl P.Swanson

·        The term protoplasm was proposed by Purkinje

·        “Protoplasm is the physical basis of life” the concept was proposed by J.Huxley

·        Olegosomes organelle called as storage of oil bodies

·        Cell theory was discovered by Schleiden and Schwann

·        Unit membrane model of plasma membrane was discovered by Daniel and Davson

Plant cell structure

·       Cell wall : Rigid outer layer that provides support and protection.

o  Primary cell wall consists of Cellulose and silica

• Middle lamella of cell wall consists of Calcium and Magnesium pectates

• Secondary cell wall consists of Lignin

• Plasma membrane is made up of Lipoproteins

·        Cell membrane : A thin, semi-permeable layer that regulates what enters and leaves the cell.

• The plasma membrane is selectively or differentially permeable in nature

·        Cytoplasm : The jelly-like substance inside the cell where many metabolic processes take place.

• Plant cell is differ from animal cell having cell wall

• Centrosomes ,centrioles are absent in plants and present in animals.

• Cytoplasm and nucleus together are called as protoplast .

• Cellwall gives definite shape and protection to the protoplasm . It is non-permeable membrane .

• Cellwall space outside the protoplast constitutes apoplast .

• Continuity of protoplasm through plasmodesmata forms the symplast .

Cell organelles

Chloroplasts : Responsible for photosynthesis, converting light energy into chemical energy.

Mitochondria : Generate energy for the cell through cellular respiration.

• mitochondria is the power house of the cell.

•  two membranes,

                  inner membrane finger like folds cristae  and outer membrane matrix.( Enzymes for krebs cycle present in matrix)

 Nucleus : Contains the cell’s genetic material (DNA) and controls cell growth and reproduction.

Endoplasmic reticulum (ER) : Involved in protein synthesis, transport, and storage.

Golgi apparatus : Modifies, sorts, and packages proteins and lipids for transport or storage.

• Golgibodies discovered by Camilio Golgi.

•  The face of cisternae closest to plasma membrane is called as trans face

•  The face closest to centre of cells cis face .

•  The main function of golgibodies is in cell wall formation,

Lysosomes : Contain digestive enzymes that break down and recycle cellular waste.

Vacuoles : Storage compartments that help maintain cell turgor pressure and store nutrients, waste, and water.

•  Tonoplast is also known as vacuolar membrane, selectively permeable,barrier between cytoplasm and the vacuole.

Peroxisomes : Involved in the breakdown of fatty acids and amino acids.

•      Peroxisomes was discovered by Tolbert

•      Peroxisomes’ and Glyoxysomes are often called as microbodies

Glyoxysomes

•      Glyoxysomes first discovered by Beevers.

•      Glyoxysomes contain enzymes like isocitratase and malate synthetase,which are useful for glyoxylate cycle.( Convert fats into sugars).

Ribosomes: An organelle not surrounded by cell membrane is Ribosome

•      Ribosomes are first observed by G.E.Palade.

•      Smallest organelle of a cell is Ribosome

•      Rough Endoplasmic reticulum: Endoplasmic reticulum associated with Ribosomes

•      Smooth endoplasmic reticulum: Endoplasmic reticulum without associated with Ribosomes

•      Ribosomes are site for protein synthesis.

•      Polyribosomes – cluster of Ribosomes

•      Prokaryotes having 70s ribosomes,higher plants having 80s ribosomes.(S =Svedberg unit).

Fluid mosaic model

•      The membrane consists of bilayer which forms a liquid lipid matrix

•      Globular integral proteins dispersed in it

•      Fluid mosaic model of plasma membrane was discovered by Singer and Nicholson

Pigments

•      Leucoplast are colourless plastids ,larger leucoplast are called as amyloplasts -store starch in it .

•      Chromoplast are coloured plastids and contain carotenoid pigments only.

•      Chloroplast are green plastids , helps in photosynthesis.

•      Chloroplast is internally filled with matrix called stroma in which embedded with grana.

•      Pale yellow green precursor pigment which is called as protochlrorophyll.

•      conversion of ADP to ATP through ATP-synthase.

Cytoskeleton

•      Cytoskeleton provides structure and organisation to the cytoplasm and shape to the cell.

•      Cytoskeleton components are actin filaments, microtubules, intermediate filaments.

•      Microtubules madeup of alpha and beta tubilin.

•      Role of microtubules in intracellular transport of cell organelles and vesicles form ER,Golgibodies and plasma membrane .

Photosynthesis

Photosynthesis : Process by which convert light energy into chemical energy in the form of glucose (sugar).

This process occurs chloroplast.

o   Photosynthesis Equation

6 CO2 + 6 H2O + light energy → C6H12O6 (glucose) + 6 O2

o   Blue colour of the light  is absorbed maximum in photosynthesis

Photosynthetic pigments:

o   Photosynthetic pigments are Chl.’a’, Chl.’b’, Chl.’c’, Chl.’d’, Carotenoid, xanthophyll    

o   Chlorophyll: It is a magnesium porphyrin compounds. Important types are

a. Chl.a -C55H72O5NHMg and

b. Chl.b C55H70O6N4Mg

o   It is insoluble in water but soluble in organic acids ( acetone, petroleum ether , alcohol etc).

o   Ratio between chlorophyll a and b I most of the plant is 3:1.

o   Carotenoids: yellow or orange colour pigment.

a. Two types they are Carotenes and Xanthophyll.

b. The main function of Carotenoid is to protect the accessory pigments from photo-oxidation damage.

c. Carotenes: C40H56O.

d. Xanthophyll: C40H56O2

e.  Example of Carotenoid present tuber crop is Carrot

o   Phycobillins:  Red & blue pigments.

a. contain four pyrrol rings and lack of Mg and the phytol chain.

Photophosphorylation

        Process which light energy is used to generate ATP from ADP and inorganic phosphate (Pi) in photosynthetic organisms.

·        Photophosphorylation process was discovered by Arnon.

·        Photophosphorylation occurs in the presence of light.

·        Wavelength of PS II is 680nm and wavelength of PS I is 700  nm .

·        Takes place in the thylakoid membranes of chloroplasts.

·        Splitting of water by light is called as photolysis takes place in lumen side of Grana

·        ATP synthesis takes place in stroma side.

Phosphorescence

         Excitation of electrons from triblet state to ground state is called  as Phosphorescence

Fluorescence

                  Excitation of electrons from singlet state to ground state is called as Fluorescence

Quantum yield

                Number of oxygen molecules released per photon of light is called as Quantum yield

Quantum requirements

                Number of photons required to release one molecule of oxygen is called as Quantum requirements

Z”- Scheme of Photosynthesis :

·        Electrons are removed from water and then donated to the lower (non‐excited) oxidized form of P680.

·        Absorption of a photon excites P680 to P680*, which “jumps” to a more actively reducing species.

·        P680* donates its electron to the quinone‐cytochrome bf chain, with proton pumping.

·        The electron from cytochrome bf is donated to PSI, converting P700 to P700*.

·        This electron, along with others, is transferred to NADP, forming NADPH.

·        So the electron ejected from water is not return back to the same place.

Non-cyclic photophosphorylation

·        Produces ATP and NADPH.

·        PS I and PS II is functional

·        Oxygen is evolved

Cyclic photophosphorylation :

·       Produces only ATP.

·       PS I is functional

·       Oxygen is not evolved

·       Photophosphorylation energy is derived from electrons.

·       Primary electron acceptor of PSI is Ferridoxin.

·       Copper containing protein involving in hill reaction is Plastocyanin

·       Iron containing protein involving in hill reaction is Ferredoxins

Emerson enhancement effect :

• • •  Application of both red and far-red light wavelength increases the photosynthetic rate than the total photosynthetic rate got from the two beams of light used separately.

    • Red drop: The quantum requirement of photosynthesis is fall drastically for Far-Red light of wavelength >680nm is known as Red drop

    • Absorption band of Carotenoid is ranges from 400-500nm

    • Glutamic acid is a precursor of chlorophyll biosynthesis

    • Paraquat a herbicide can able to block the light reaction in chloroplast ( Ferridoxin to NADP)

C3 cycle ( calvin cycle)

·        C3 cycle is otherwise called as Calvin cycle or Blackman reaction or Reductive pentose phosphate pathway.

·        Found in all Photosynthetic plants.

·        CO2 acceptor is Ribulose -1,5-diphosphate .

·        Occurs only mesophyll chloroplast.

·        Optimum temperature -10-25°C.

·        Example of C3 Enzyme is RuBp-case

·        C3 Cycle is 3 PGA (3 Phospho glyceric acid)

·        Six (6) turns of Calvin cycle is required for the synthesis of one molecule of hexose.

·        Synthesis of one glucose molecule in dark reaction 18 ATP and 12 NADPH2 molecules are required.

          ·        Examples for C3 Crops are Rice, Wheat, Black gram etc…

Steps involved in C3 Cycle are

·        Carboxylation,

·        Reduction and

·        Regeneration of RuBp

·        Carboxylation :  CO2 is fixed into a 3-carbon molecule (3-phosphoglycerate) via RuBisCO.

·        Reduction : 3-phosphoglycerate is reduced to form glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.

·        Regeneration : The cycle regenerates its own intermediates (RuBP) to continue fixing CO2.

C4 cycle ( Hatch and slack cycle )

·        Found only in certain tropical plants

·        CO2 acceptor is Phosphoenol pyruvate.

·        First product in C4 Cycle is Oxalated acid,

·        Occurs in bundle sheath

·        Optimum temperature – 30- 45°C.

·        Examples for C4 Crops are Maize, sugar cane, Amaranthus etc…

·        C4 Plants Calvin cycle occurs in stroma of bundle sheath chloroplast.     

·        In C4 plants 30 numbers of ATP molecules are used to synthesis one molecule of hexose.

·        Special type of anatomy present in Most  of C4 Plant is Krantz anatomy

Photorespiration

·        Process occurs more than one organelle.

·        The substrate for this process is Glycolate.

·        Inherent defense mechanism to escape the plant from photooxidative damage.

·        Photorespiration otherwise called as C2 cycle

·        Photorespiration is favored by High O2 and low CO2  

·        Photorespiration is maximum in rice crop

·        Photorespiration is lack in C4 cycle

Cell organelles involved in photorespiration are

·        Chloroplast,

·        Peroxisome ,

·        Mitochondria

Warburg effect:

          The inhibitory effect of oxygen on the rate of photosynthesis

CAM plants

·        Plants that utilize Crassulacean Acid Metabolism (CAM) as an adaptation to arid environments

·        Examples for CAM Plants are Cacti, Pine apple, Agave etc…

·        Water use efficiency is high in CAM Crops .

Phloem transport

Source:

Leaves and other green tissues that produce Photosynthates

Sink:

Organs which utilize the Photosynthates for their growth and then store the Photosynthates

Phloem loading :

Transport of sucrose or photosynthetic assimilates from mesophyll cells to sieve  elements

Phloem unloading :

Transport of sucrose or photosynthetic assimilates from sieve  elements to sink

Munch hypothesis ( Mass flow hypothesis)

Explains how sugars (primarily sucrose) are transported from source (like leaves) to sink (like roots or fruits) within a plant’s phloem

Transpiration :

·        process by which water is transported through a plant, from the roots to the atmosphere, through the leaves.

·        occurs mainly through structures called stomata (small pores) on the surface of the leaves.

Factors Affecting Transpiration

·        Temperature

·        Humidity

·        Wind

·        Soil moisture

Importance of Transpiration

·        Regulates temperature

·        Supports nutrient uptake

·        Maintains plant structure

Respiration

                    The process where plants break down sugars (like glucose) to release energy for growth, development, and other cellular functions

·        Food is converted into  energy at Mitochondria

·        ATP formation in mitochondria was discovered by Krebs and Lipman

·        ATP formed on complete oxidation of a glucose molecule through HMP Pathway :36 ATP molecules.

·        Hydrogen acceptor in HMP Pathway is NADP.

·        ATP are formed from complete oxidation of acetyl CoA:12 ATP.

·        1 NADH2 is equal to 3 ATP molecules

·        1 FADH2 is equal to 2 ATP molecules

·        In cellular respiration FAD transfers electrons during the synthesis of ATP.

·        The ratio of photosynthesis to respiration during day time remains 10:1.

Glycolysis or Embden- Mayer Hof-Paranas Pathway:

·        It is the metabolic pathway that converts Glucose into pyruvate or pyruvic acid.

·        The free energy released in this process is used to form the high –energy compounds of ATP, FADH2 and NADH and the process occurs in cytoplasm.

Three phases of Glycolysis

Phase –I: Glucose molecule is phospharylated. Two ATP are utilized.

Phase-II: Breaking up of Fructose 1,6-diphosphate(6 carbon compound) into of  3Phosphoglyceraldehyde and Dihydroxyacetone phosphate (two molecules of 3-carbon compound).

Phase –III: Degradation of 3-phosphoglyceraldehyde into pyruvic acid with the production of four molecules of ATP. In phase –I  already two molecules are utilized so the net gain is only two molecules of ATP.

o   The connecting link between Glycolysis and kreb cycle is Acetyl CoA.

o   Cell oxidative phosphorylation process takes place in Mitochondria.

o   Final product of oxidative phosphorylation (simultaneous oxidation and phosphorylation) is NAD

Aerobic respiration or Kerbs’ cycle or TCA Cycle:

·        All aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats and proteins into carbon dioxide.

·        It occurs in the matrix of the mitochondrion.

·        The universal Hydrogen acceptor is NAD

·        The efficiency of aerobic respiration is 50%

·        During aerobic respiration the substrate enters into the mitochondria is  pyruvic acid

·        Oxidation of ethyl alcohol is takes place in Mitochondria

·        CO2 is showing inhibitory effect on respiration

·        Mitochondria is considered as polluting organelle

 Anaerobic respiration ( fermentation):

·        In the absence of oxygen, pyruvic acid has undergone anaerobic respiration or fermentation.

·        Two types of anaerobic respiration are alcoholic fermentation and lactic acid fermentation.

·        The ratio of energy released between anaerobic and aerobic respiration is 1:18.

·        The number of CO2 release in between aerobic and anaerobic respiration is ZERO.

Pasteur Effect:

The inhibitions of sugar break down due to the presence of oxygen under aerobic conditions” and the reaction is called as Pasteur reaction. I

Electron transport chain:

·        It is the process of electron transfer between the electron donor and the electron acceptor.

·        The resulting product is ATP.

·        It is synthesized inside the cristae of mitochondria.

Respiratory quotient:

                   The ratio of the volume of CO2 released and O2 taken in   Respiration is called as respiratory quotient. 

·        Lipids are made up of Glycerol and fatty acids.

·        Unsaturated fatty acids (good for health and consumption) content is high in sunflower oil.

·        There are three phases for fat synthesis

·        Synthesis of fatty acids

·        Synthesis of glycerol, and 

·        Condensation of fatty acids and glycerol into fats.

·        Example of cyclic fatty acid: Chaulmoogric acid, Example of saturated fatty acids is: Butyric, Palmitic and Stearic acids.

·        Essentials for fatty acid synthesis: Acetyl CoA, Two enzyme complexes and five cofactors (ATP, Mn++, Biotin, NADPH and CO2.

·        Saponification: In the presence of hydrolytic enzymes, fats hydrolyzed into fatty acids and glycerol. This process is called as Saponification.

 Growth regulators and hormones

Growth hormone :

·        Growth hormones, also known as plant growth regulators (PGRs) or phytohormones

·        naturally occurring substances that regulate plant growth and development.

Types of Plant Growth Hormones

Auxins:

 Promote cell elongation, cell division, and differentiation. Example: Indole-3-acetic acid (IAA).

o   Auxin (IAA) was first coined by F.W.Went (1928).

o   Examples of naturally occurring (other than IAA) auxins are:

o   Indole-3-acetaldehyde

o   Indole -3-pyruvic acid

o   Indole -3 –acetonitrile 5. Indole-3-ethanol

Gibberellins

  Regulate seed germination, stem elongation, and flower formation.

Cytokinins

·        Promote cell division, cell growth, and differentiation.

·        Example of rich source of cytokinin is Liquid Endosperm of Coconut.

·        Kinetin (Cytokinin) hormone is the nucleic acid derivatives. It was first named by Miller  and Skoog  at 1954.

·        Examples of naturally occurring Cytokinin are: N6 – Methyl amino purine, N6N6Dimethyl amino purine and N6 –isopentenyladenine.

Ethylene

·        Regulates fruit ripening, senescence, and stress responses.

·        Ethylene caused disease in plant is called as sleep disease.

Abscisic acid (ABA)

Involved in stomatal closure, stress responses, and seed dormancy.

•      Stomatal closure is response to leaf stress is mediated by Abscisic acid.

Functions of Plant Growth Hormones

•        Cell growth and division: Regulate cell expansion, differentiation, and proliferation.

•        Developmental processes: Influence seed germination, root growth, shoot elongation, and flower formation.

•        Stress responses: Help plants respond to environmental stresses, such as drought, temperature, and pathogens.

Germination

·        Seed germination is the process by which a seed begins to grow and develop into a seedling.

·        germination is an example of growth which shows increase in volume without increase in weight.

Stages of Germination

·        Imbibition : The seed absorbs water, swelling and breaking the seed coat.

·        Activation : The seed’s metabolic processes are activated, and enzymes begin to break down stored nutrients.

·        Emergence of radicle : The primary root (radicle) emerges from the seed, anchoring the seedling.

·        Emergence of cotyledon : The seed leaf (cotyledon) emerges, marking the beginning of photosynthesis.

·        Seedling establishment : The seedling develops its root system and leaf structure, becoming self-sufficient.

Etiolated

         Young plants growing in absence of light develop elongated, but thin stems with narrow leaves and poorly developed root system. Such plants are known as etiolated.

Standard growth curve of plant is a Sigmoidal Curve.

·        On four phases of this curve the maximum growth is occurred in Exponential phase. 

Apical dominance:

        The auxin of the terminal bud is thus responsible for inhibiting the development of lateral buds by a phenomenon is known as apical dominance.

Examples

·        Examples of anti-gibberellin compound: CCC- 2 chloroethyl trimethylammonium chloride). 

·        Examples of Terpenes hormones are:  GA, Cytokinin, ABA and Brassinosteroids. 

·        Abscission of leaves, flowers and fruits can be accelerated by Ethylene.

Stress hormone

·        Naturally occurring growth inhibitor is ABA (Abscisic acid).

·        It is plays an important role in Stomatal closure.

Photoperiodism

·        Response of plants to the length of daylight or darkness, influencing various physiological processes.

·        The phenomenon of Photoperiodism was discovered by Garner and Allard (1920).

Long Day Plants : (14-16 hours).

·        Examples : Wheat, oats, barley, spinach.

Short Day Plants :  (10-12 hours).

·        Examples : Rice, soybeans, poinsettias, chrysanthemums.

Day neutral plants : do not have a specific photoperiod requirement for flowering

·        Examples :tomatoes, cucumbers, sunflowers.

Vernalization

·        Induction of flowering in plants with cold treatment is called as Vernalization

·        The hormone responsible for vernalization is Vernalin.

Senescence :  

·        It involves cellular breakdown and death of seeds.

·        It occurs when the degenerative processes exceeded synthetic ones, or in other words, when catabolism exceeds anabolism.

·        Genetically controlled process.

·        Senescence is caused due to decrease in the amount of auxins and gibberellins in plants.

Stress

·        Biological stress to be any change in environmental Conditions which may have a negative effect on the normal growth and development of plants.

·        Examples of stresses are low temperature, non-availability of water, extreme cold, excess of salt etc.

·        Moisture stress : Water deficit causes

·        Leaf abscission

·        Decreased leaf area

·        Extensive root growth

·        Decreased photosynthesis

·        Increased in wax deposition on the leaf surface

·        Stomatal closure

Xerophytic plants:

·        The plants growing in the deserts are exposed to extremely harsh environment.

·        According to the water stresses responses the plants are classified as four categories.

Embolism :

·        Due to water stress the xylem water column where the water continuity will be break

Drought escape (ephemerals) :

·        Plant's ability to complete its life cycle before the onset of significant soil moisture deficits, effectively avoiding drought stress

Drought resistance (succulents) :

·        The ability to endure periods of water scarcity and maintain satisfactory growth and yield

Physiological and biological changes in seed germination

Physiological Changes 

Water uptake

§  Seeds absorb water, leading to swelling and seed coat rupture.

Increased metabolic activity

§  Enzymes are activated, and metabolic processes accelerate.

Hormonal changes

§  Hormones like gibberellins and auxins play key roles in regulating germination.

Biological Changes

Seed coat rupture

§  The seed coat breaks, allowing the radicle to emerge.

Radicle emergence

§  The primary root (radicle) grows downward, anchoring the seedling.

Cotyledon emergence

§  The seed leaf (cotyledon) emerges, marking the beginning of photosynthesis

Seed dormancy

§  Seed dormancy is a state of temporary inactivity in seeds, preventing germination even when environmental conditions are favorable..

Methods for breaking seed dormancy

Scarification

§  Mechanically scratching or nicking the seed coat to allow water entry.

Stratification

§  Exposing seeds to cold temperatures (usually 30-90 days) to simulate winter.

Soaking

§  Soaking seeds in water to rehydrate and stimulate germination.

Gibberellin treatment

§  Applying gibberellin hormones to stimulate germination.

Light exposure

§  Providing specific light conditions to stimulate germination.

Physiological and biochemical changes during seed germination

Water uptake

·        Seed germination starts with the imbibition of water by dry seed coat. 

·        The seed coats become more permeable to O2 and water and less resistant to outward growth of embryo.

Respiration

·        Rapid increase in respiration rate of embryo occurs.  Sucrose is provided by endosperm.

Mobilization of reserve material

·        As germination progresses, there is mobilization of reserve materials to provide 

Factors causing dormancy of seeds

1. Seed coats impermeable to water

2. Seeds coats impermeable to oxygen

3. Immaturity of the Embryo

4. Germination Inhibitors

5. Chilling or low temperature requirement

6. Light sensitive seeds

 Stress Physiology( Drought ,Heat,cold )

Stress physiology

        The study of how plants respond to and cope with adverse environmental conditions, negatively impact their growth, development, and productivity.

• Abiotic factors (like temperature, water, salinity, and heavy metals)

• Biotic factors (like pathogens and pests).

DROUGHT (Water stress)

    Plant water deficit caused by a shortage of precipitation and soil water availability, leading to impaired plant growth and development.

Classified into two broad categories viz., soil drought and atmospheric drought.

o   Soil drought leads to atmospheric drought. 

o   Atmospheric drought occurs due to low atmospheric humidity, high wind velocity and high temperature which cause a plant to lose most of its water.

Physiological changes occur due to drought

·        Functioning of stomata

·        Carbohydrates metabolism in green leaves

·        Photosynthetic activity

·        Osmotic pressure

·        Permeability

·        Biochemical effects

Adaptation to drought

Drought resistance

      capacity of plants to survive during the period of drought with little or no injury. They are ephemerals, succulents and non-succulent perennials

Ephemerals

o   These are short lived plants and they complete their life cycle within a short favourable period during rainy season.

o   They are called as drought escaping plants.

Succulent plants

      Reducing transpiration such as thick cuticle, reduced leaf area, sunken stomata etc.

Non succulent plants

        They develop many morphological adaptations which are collectively called xeromorphy.

o   They develop, in general, greyish colour, reflecting surfaces, smaller leaves, extensive root system, leaf fall during dry season, sunken stomata and thick cuticle etc. 

o   The stomata remain closed mostly in dry periods.

Methods to overcome drought

·        Selection of drought tolerant species

·        Adjusting the time of sowing in such a way that the crop completes its lifecycle before the onset of drought

·        Seed hardening with KCl, KH2PO4, CaCl2 or Thiourea

·        Mulching to minimize the evaporative loss

·        Foliar spray of antitranspirants such as Kaolin, PMA, Waxes and Silicone oils

·        Foliar spray of KCl

·        Foliar spray of growth retardants such as CCC and MC

TEMPERATURE STRESS

   It includes both high temperature stress and low temperature stress.

·        Low temperature stress causes chilling injury and freezing injury.

Chilling injury

  The injury which occurs due to low temperature but above zero degree centigrade is called chilling injury.

Freezing injury

Freezing injury occurs when the temperature is 0°C or below.

Effect of freezing and chilling injury plants

·        The lipid molecules in cell membrane get solidified

·        Inactivation of mitochondria

·        Streaming of protoplasm is stopped

·        Accumulation of respiratory metabolites which become highly toxic

·        Ice formation inside the cell occurs.

Prevention of cold injury

·        Some plants change the pattern of growth.

·        The growth is completely arrested during this period.

·        In cell membrane, unsaturated fatty acid content is increased.

·        Intracellular ice formation is reduced.

·        The quantity of free enzymes, sugars and proteins increases.

High temperature stress

·        The effect of high temperature is heat Injury. 

·        Heat Injury occurs when plant temperature is higher than that of environment (exceeds 35°C).

General effects of high temperature

·        High temperature affects

·        Seedling growth and vigour

·        Water and nutrient uptake

·        Solute transport

·        Photosynthesis and respiration

·        General metabolic processes

·        Fertilization and maturation

LOW LIGHT STRESS

      The abnormal light intensity and quality causes reduced yield in any crops.

 UV-RADIATION STRESS - UV radiation is divided into three categories

o   UV A – wavelength (320 to 400 nm) less lethal to the plants.

o   UV B – wavelength (280 to 320 nm) lethal to the plants.

o   UV C – wavelength (< 280 nm)highly lethal to all biological systems.

o   In general, 1% reduction in ozone (O3) causes 2% increase in UV radiation.

UV radiation and plant response

o   UV radiation slows down the growth of plants

o   Damage the process of photosynthesis

o   Prevent maturation and ripening process

o   Accelerate genetic mutation.

SALT STRESS

·        It occurs due to excess salt accumulation in the soil. 

·        water potential of soil solution decreases

·        exosmosis occurs. 

·        leads to physiological drought causing wilting of plants.

Saline soil

o   The electrical conductivity is greater than 4 dS/m,

o   Exchangeable sodium percentage is less than 15%

o   pH is less than 8.5. 

o   These soils are dominated by Cl- and SO2-4 ions.

Alkaline soil

o   Alkaline soils are also termed as sodic soils

o   Electrical conductivity is less than 4 dS/m,

o   Exchangeable sodium percentage is greater than 15%

o   pH of the soil is greater than 8.5. 

o   Soils are dominated by CO-3 and HCO-3 ions.

          Halophytes

  Plants that grow under high salt concentrations.

·        Euhalophytes: can tolerate extreme salt stress

·        Oligohalophytes: can tolerate moderate salt stress

·        Glycophytes are the plants that cannot grow under high salt concentration.

Effect of salt stress on plant growth and yield

·        Seed germination

·        Seedling growth

·        Vegetative growth

·        Reproductive stage

·        Photosynthesis

Mechanism of salt tolerance

·        Some plants are able to maintain high water potential by reducing the transpiration rate.

·        Salts are accumulated in stem and older leaves in which metabolic processes take place in a slower rate.

·        Na+ (sodium ion) toxicity is avoided by accumulating -high amount of K+ ions.

·        Accumulation of toxic ions in the vacuole but not in the cytoplasm.

·        Accumulation of proline and Abscisic acid which are associated with tolerance of the plants to salt.

·        Salt Tolerant crops: Cotton, sugar cane, barley

·        Salt Semi tolerant crops: Rice, maize, wheat, oats, sunflower, soybean

·        Salt Sensitive crops: Cow pea, beans, groundnut and grams

Management of salt stress

·        Leaching of salts with adequate water

·        Application gypsum  to convert the highly injurious carbonates to less injurious sulphate

·        Selection of salt tolerant crops

·        Use of FYM and other organic manures

Mineral Nutrition and uptake

Mineral nutrtion:

           The process of plants absorbing and utilizing inorganic mineral elements from the soil for their growth and development

·        The element involved in tertiary structure of protein formation is Sulphur.

·        Elemental nitrogen fixation in Rhizobium by Nitrogenase enzyme.

·        Nitrogen element is taking by plant as both anionic and cationic form

·        Total essential elements 17

·        Examples of macronutrients are N,P,K,S,Mg,Ca / examples of micro nutrients are Fe,Mn,Zn,B,Cu,Mo,Cl,Ni  / and example of structural elements are C, H, O

Criteria of essentiality of an elements: 

          Arnon and stout (1939) proposed three criteria for the essentiality of an element.

·        In the absence of an element, it is not possible for plant to complete its vegetative or reproductive cycle.

·        The role played by an element is specific and it cannot be replaced by any other element.

·        The element is directly involved in nutrition of the plant.

Discoverer of mineral nutrients

• Fe – Sachs

• Mn – McHarge

• B – Warington

• Zn – Summer & Lipman

• Mo – Arnon & Stout

• Cl-  Broyer et al

Deficiency indicator plants

• Nitrogen -Cabbage , Cauliflower

• Phosphorus – Rape

• Potassium – Potato

• Calcium – Cabbage , Cauliflower

• Magnesium – Potato

• Iron – Cabbage , Cauliflower, Potato, oat

• Sodium – Sugar beet

• Manganese – Sugar beet, Oat

• Boron- Sunflower

Non essential and tolerant elements:

· Some elements are absorbed by plants but not essential for their growth and metabolism e.g. Iodine, Cobalt, Sodium, Silicon, Aluminium, vanadium, lithium, strontium, tin, radium, beryllium, barium, mercury, silver and bromine.

· Some of them are useful for plant growth but showing tolerance in some crops e.g. vanadium, silicon, strontium, cobalt and sodium

Mobile elements  - N, P, K, Mg

Immobile nutrients - Ca, Fe, B ( Cu, Mn, Zn , Mo  and S are Intermediate elements)

·        Nitrogen deficiency symptoms is first appears in old leaves or matured leaves 

·        Fe, Zn, Cu, Mn elements are acts as a cofactor for many enzymatic reaction.

·        Nutrient essential for photolysis of water is Mn and Cl

Hidden hunger

Symptomless deficient condition of nutrient

Hyper accumulators

        Accumulation of excess heavy metal in vacuoles

• Freshly cut and exposed fruits and vegetables become dark because of oxidation of tannic acid in the fruits/ vegetables in presence of the trace of iron from the knife.

• Repeated cultivation of crop fields leads to the depletion of N, P and K.

• The assimilatory powers of photosynthesis are ATP and NADPH2

Carbonic acid exchange theory:

• CO2 released during respiration combines with water to form carbonic acid (H2CO3) which dissociates as H+ (Hydrogen ions) and HCO- 3 (bicarbonate ions).

• A cation adsorbed on clay micelle may be exchanged with the H+ of soil solution and this cation diffuses into the root in exchange for H+ ion.

 Deficiency symptoms and deficient element:

          ·        Die back disease in citrus – Copper

·        Rapid deterioration of root and shoot tips -calcium

·        Sickle leaf disease – Phosphorus

·        Khaira disease of paddy – zinc

·        Chlorosis – Mg,Fe,Mn,Cu,N

·        Necrosis – K, Mg, Zn, Ca, Mo

·        Whiptail of cauliflower – Molybdenum

·        Sand drown diseases -Magnesium

·        Top sickness of tobacco, brown heart -  Boron

Essential elements and their important key roles

·        N – Main constituents and important of the synthesis of Nucleic acids, Proteins and chlorophyll

·        P -  Component of ATP, GTP, UTP, constituents of nucleic acids and cell membrane

·        K – Stomatal regulation process 

·        Ca – Important constituent of middle lamellae of cell wall and control the selective permeability of the cell membrane

·        Mg – Constituent of Chlorophyll pigments, synthesis of DNA and RNA

·        S – It is essential for the production of many amino acids .e.g. Cystine

·        Zn – Activators of many enzymes e.g. dehydrogenase enzyme and Tryptophan synthase, Synthesis of Auxin(IAA) and chlorophyll

·        Fe – Component of chlorophyll, catalysts of many enzymatic reactions, biological nitrogen fixation in plants

·        Mo – Important constituent for nitrogenase, nitrate reductase and xanthine dehydrogenase enzyme

·        Ni – Important for urease and hydrogenase enzymes

Canopy and leaf area management

Source : Export photoassimilates

·        Ex :Leaves, stipules, fruit wall, young stem, pedicel, awns, peduncle, calyx, bract etc

Sink  : Import photoassimilates

·        Ex: Storage organs – Fruit and Seed

Source strength:  Source Size x Source activity

·        Ex : Leaf characters – size, thickness, mesophyll size, compaction, vascular bundle

Sink strength : Sink size x Sink activity

Harvest index(HI)

·        Ye = Yb x h

·        HI = {Yield (Eco)/ Yield (Biol)} x 100

Sink limitation:

·        Late anthesis (Long Vegetative phase)

·        Indeterminate (Vegetative & Reproductive growth)

·        Vegetative growth at Reproductive phase

·        Less sink number and size

·        Hormonal imbalance

·        Any Stress

Source limitation:

·        Low canopy photosynthesis

·        Low optimum LAI

·        Slow peak LAI (lag vegetative growth)

·        Low LAD at filling

·        Early leaf senescence

·        Stress – nutrients, water

Growth.

It occurs in meristematic regions where the meristematic cell has to pass through the following 3 phases.

·        Cell formation phase

·        Cell elongation phase

·        Cell differentiation (cell maturation)

·        Meristem is active cell region where continuous growth is takes place. Radicle is first emerged out during seed germination.

Growth phases

          ·        ‘S’ shaped curve or sigmoid curve.

·        three well marked regions can be observed,

·        the initial growth stage (lag phase),

·        the grand period of growth (exponential or log phase)

·        steady stage (maturity stage or senescence or stationary phase).

·        Blackman (1919) suggested that the growth of the plants can be represented by equation.

·        W1 = Wo ert

Factors influencing growth

Internal factors

   Light,Temperature,Oxygen,Carbon dioxide,Water, Nutrients and food materials

External factors

o   Growth hormones and their availability

o   Resistance to climatic, edaphic and biological stresses

o   Photosynthetic rate and respiration

o   Assimilate partitioning and nitrogen content

o   Chlorophyll and other pigments

o   Source-sink relationship and enzyme activities

Leaf Area Index (LAI).

Ratio of the leaf of the crop to the ground area over a period of interval of time.

Relative Growth Rate (RGR)

·        The term was coined by Williams (1946).

·        It expresses the total plant dry weight increase in a time interval in relation to the initial weight

·        RGR =  loge W2 – loge W1/t2 – t1

·        CGR=   (W2 –W1) /ρ (t2 – t1)

Harvest index ( HI)  

·        It expressed as the percent ratio between the economic yield and total biological yield

·        suggested by Nichiporovich (1951).

Water relations in plants

Water potential :  

• The potential energy of water in a system, relative to the potential energy of pure water at the same temperature and pressure.

• Water potential is expressed in ψ (psi) and their unit is Bar or Mega Pascal

• Component of water potential are Ψs and Ψp 

• Water potential of pure water is Zero bars (0), sea water is -24 bars, at field capacity is 0.33 bars and  at permanent wilting point is -15 bars

• The highest value of ψ is Zero (0).

Cohesion

·        Attraction between same molecules

Adhesion

·        Attraction between different molecule

·        The cohesive force of water is due to the Hydrogen bonds (H-Bonds)

Water movement

Imbibition

 Absorption of water by dry seed

Diffusion:

      The movement of water or gases from higher concentration to lower concentration

Osmosis

      The movement of water from higher concentration to lower concentration through  semi permeable membrane

Passive transport

• The movement of substances across membranes in favor of their concentration gradient, from a more concentrated region to a less concentrated region.

• No energy is used in passive transport

Active transport:  

• • The transport of substances across membranes against their concentration gradient

  •  From a less concentrated to a more concentrated region.

  • Active transport, requires energy (work) to occur.

  • Active transport works to maintain or increase the concentration

  • Passive transport works to reduce the concentration gradient.

  • Cellulose polymer has highest imbibing capacity when compare to protein, fats and pectin compounds.

Water potential

       The difference between the free energy of water molecules in pure water and energy of water in any other system 

Turgor Pressure

            In a cell the pressure exerted outwards on the cell wall against the entry of solvent inside is termed as Turgor pressure.

Plasmolysis

             It is the shrinkage of the protoplasm of the cell away from its cellulose wall when placed in hypertonic solutions,

De-plasmolysis

               When a plasmolysed cell exposed to normal solvent system, the cell becomes turgid and get their original shape

DPD (Diffusion Pressure Deficit): When a solvent is separated from a solution, the solvent molecule being higher in concentration will diffuse towards the solution under pressure.

                    In hypotonic environment a cell attained a fully turgid condition, DPD (Diffusion Pressure Deficit)  is Zero.

o   Hypotonic: When a solution is less concentrated than other solution.

o   Hypertonic: the solution is more concentrated than other solution.

o   Isotonic: When two solutions have the same concentration.

o   Osmotic pressure is highest in halophytes (salt living plants)

Water holding capacity: The amount of water retained in the soil is called as water holding capacity.

Permanent Wilting Point:

· A point at which, the plant not regained their normal cell turgid condition, after attained a severe water scarcity even if, we provide 24 hrs water.

· Temporary wilting in plants occurs when the rate of absorption is less than the transpiration

· The main cause of permanent witling is poor availability of soil water

· Wilting coefficient: Percentage of water left in the soil when a plant wilts is known as wilting coefficient.