7. plant growth and minerals Nutrition

7.1 Plant Growth

1. Growth

Growth is a characteristic feature of living organisms. It has two aspects: quantitative and qualitative.

A. Quantitative Aspect

Refers to an increase in length, breadth, size, volume, body mass, dry weight and number of cells.

It is a quantitative change (change that can be measured).

Growth is the final end product of successful metabolism (chemical reactions occurring inside living organisms).

B. Qualitative Aspect

Refers to changes in the nature of growth.

Development: An ordered change or progress in an organism.

Differentiation: A process through which cells become specialised, leading to a higher and more complex state.

2. Definition of Growth

Growth is a permanent, irreversible increase in the bulk of an organism, accompanied by a change in form.

Permanent: Lasting.

Irreversible: Cannot be reversed.

Bulk: Overall size or mass of an organism.

3. Growth in Plants

In multicellular vascular plants, growth is indeterminate (continues throughout life).

Growth occurs in specific regions called meristems.

Meristems continuously produce new cells through cell division.

4. Types of Meristems

Based on their location, meristems are of three types:

1. Apical meristem

2. Intercalary meristem

3. Lateral meristem

Types of Meristems Based on Location

1. Apical Meristem

Location: Tips (apices) of roots and shoots.

Function: Increases the length/height of the plant and helps in cell differentiation (specialisation of cells).

Growth: Responsible for primary growth (increase in length).

2. Intercalary Meristem

Location: At the nodes or base of internodes of the stem.

Functions:

Increases the length of internodes.

Helps form leaf primordia (early developing leaves) and lateral buds.

3. Lateral Meristem

Location: Along the sides of the plant axis, mainly in dicotyledons and gymnosperms.

In dicot stems, it occurs as strips within vascular bundles and is called vascular cambium.

Function: Increases the girth (thickness) of the stem by adding secondary vascular tissues.

Growth: Responsible for secondary growth (increase in thickness).

7.2 Phases of Growth

Meristematic cells undergo three stages: cell division, cell enlargement and differentiation. Accordingly, plant growth occurs in three phases.

A. Phase of Cell Division / Formation

Cells: Thin-walled, non-vacuolated, with a prominent nucleus and granular cytoplasm.

Process: Meristematic cells (especially cambial cells) undergo mitosis to form two new cells.

One cell remains meristematic, while the other undergoes enlargement and differentiation.

Growth rate: Slow; called the Lag Phase.

B. Phase of Cell Enlargement / Elongation

Newly formed cells become vacuolated (develop vacuoles) and osmotically active.

They absorb water and become turgid (swollen and firm due to water pressure).

Turgidity causes cells to enlarge in both length and breadth.

New cell-wall materials and other substances are synthesised to support enlargement.

Growth rate: Rapid and accelerated; called the Exponential or Log Phase.

C. Phase of Cell Maturation / Differentiation

Enlarged cells become specialised to perform specific functions.

Cells attain morphological maturity (mature structure/form) and physiological maturity (ability to perform specific functions).

Growth rate: Slows down and reaches a steady state called the Stationary Phase.

7.3 Conditions for Growth

Plant growth depends on various environmental and physiological conditions.

Essential Conditions

1. Water

Essential component of protoplasm (living content of the cell).

Maintains cell turgidity (firmness due to water pressure).

Provides an aqueous medium (water-based environment) for biochemical reactions.

2. Nutrients

Macroelements: Nutrients required in large amounts.

Microelements: Nutrients required in small amounts.

Both are essential for proper plant growth.

3. Temperature

Optimum temperature for plant growth: 25–35°C.

4. Oxygen

Essential for respiration and energy release.

5. Light

Essential for photosynthesis.

Also influences seed germination.

6. Gravitational Force

Determines the direction of root and shoot growth.

7. Other Conditions

Carbon/nitrogen ratio.

Growth hormones (plant hormones that regulate growth and development).

7.4 Growth Rate and Types of Growth

A. Growth Rate

Growth rate: Increase in growth per unit time; also called the efficiency index.

It can be measured by an increase in the size and area of plant organs such as leaves, flowers and fruits.

Absolute Growth Rate (AGR):

The change in cell number (dn) per unit time (dt).

It measures and compares the total growth per unit time.

B. Measurements of Plant Growth

Plant growth can be measured by:

1. Increase in cell number: One maize root apical meristem can produce more than 17,500 new cells/hour.

2. Increase in leaf surface area: Example – growth of a dorsiventral leaf (leaf with distinct upper and lower surfaces).

3. Increase in length: Example – growth of a pollen tube.

4. Increase in fruit volume: In watermelon, the ovary may increase in size up to 3,50,000 times after fertilisation.

5. Increase in shoot girth: Increase in the thickness of the shoot.

6. Increase in dry weight: Increase in the dry mass of a plant organ.

C. Methods to Measure Linear Growth

Linear growth means an increase in length, such as the length of a stem or radicle (embryonic root).

1. Direct Method

Measured using an ordinary measuring scale.

Simple method.

2. Horizontal Microscope

Used to measure plant growth in fields.

3. Auxanometer

Used for precise measurement of the linear growth of a shoot.

Types:

Arc auxanometer

Pfeffer’s auxanometer

4. Crescograph

Records primary growth very accurately.

Magnifies growth up to 10,000 times.

Measures growth per second.

Developed by Sir J. C. Bose.

7.4 Growth Rate and Types of Growth (Continued)

1. Absolute Growth Rate (AGR)

AGR is the total growth per unit time.

AGR}=dn/dt

dn = Change in the number of cells

dt = Time interval

2. Relative Growth Rate (RGR)

RGR is obtained by dividing AGR by the total number of cells initially present.

It expresses growth per unit time on a common basis.

It is the ratio of growth during a given time to the initial growth or size.

RGR=AGR/n 

Here, n = Initial number of cells.

Importance: AGR and RGR help describe the dynamics (changes over time) of cell growth in culture.

Types of Growth

There are two types of growth:

1. Arithmetic growth

2. Geometric growth

A. Arithmetic Growth

The growth rate remains constant.

Growth occurs in arithmetic progression, e.g., 2, 4, 6, 8 cm.

After mitosis (cell division):

One daughter cell continues to divide.

The other daughter cell undergoes differentiation and maturation.

Example: Elongation of a root at a constant rate.

A graph of length against time produces a linear (straight-line) curve.

Mathematical expression:

Lt=L0+rt

Where:

Lt= Length at time t

Lo= Length at time zero (initial length)

r = Growth rate

t = Time of growth

B. Geometric Growth

In geometric growth, a cell divides by mitosis into two daughter cells, and both daughter cells continue to divide repeatedly.

Growth is initially slow but later becomes rapid at an exponential rate.

Mathematical expression:

Wt = W0 e^{rt}

Where:

Wt = Final size

Wo = Initial size

r = Growth rate

t = Time of growth

e = Base of natural logarithm (approximately 2.718)

AGR and RGR (Quantitative Comparison)

Growth of a living system can be compared in two ways:

Absolute Growth Rate (AGR): Measurement and comparison of total growth per unit time.

Relative Growth Rate (RGR): Growth per unit time expressed on a common basis, per unit of the initial parameter (e.g., initial size or cell number).

7.5 Growth Curve

Growth curve: A graphical representation of total growth against time.

Types of Growth Curves

1. Linear curve: Represents arithmetic growth.

2. Exponential curve: Represents geometric growth.

3. Sigmoid curve: An S-shaped curve representing changes in growth rate through three phases.

Phases of the Growth Curve

1. Lag Phase- Growth rate is slow.

2. Exponential (Log) Phase - Growth rate increases rapidly and reaches its maximum.

3. Stationary Phase - Growth rate gradually slows down.

When the rate of growth is plotted against time across these three phases, an S-shaped (sigmoid) curve is obtained.

Grand Period of Growth

The total time required for all phases of growth to occur is called the Grand Period of Growth.

7.6 Differentiation, Dedifferentiation and Redifferentiation

A. Differentiation

Definition: Permanent change in the structure and function of cells, leading to their maturation and specialisation.

It occurs in cells derived from the apical meristems of roots and shoots.

Cells undergo anatomical (structural) and physiological (functional) changes.

Maturation occurs at the cost of the cell's capacity to divide and redivide.

Example:

In hydrophytes (aquatic plants), parenchyma develops large schizogenous intercellular spaces (spaces formed between cells).

These spaces help in buoyancy, aeration and mechanical support.

B. Dedifferentiation

Definition: The process in which a living, differentiated cell that has lost its capacity to divide regains this capacity when required.

The mature cell becomes meristematic again and starts dividing.

Examples:

Interfascicular cambium: Formed from parenchyma cells between vascular bundles.

Cork cambium: Formed from cells of the innermost layer of the cortex.

C. Redifferentiation

Definition: The process in which cells formed by dedifferentiation lose their capacity to divide again and mature to perform specific functions.

Example:

Secondary xylem and secondary phloem develop from the dedifferentiated cambium present in vascular bundles.

7.7 Development

Definition: Ordered or progressive changes in the shape, form and degree of complexity of a plant.

It includes all changes occurring in sequence from seed germination to senescence or death during the plant's life cycle.

Development includes:

1. Growth – increase in size.

2. Morphogenesis – development of shape and form.

3. Maturation – attainment of a mature state.

4. Senescence – ageing and gradual deterioration.

7.8 Plasticity

Definition: Plasticity is the ability of plants to change their form and develop different structures in response to environmental (external) or internal stimuli during different stages of life.

Heterophylly

Heterophylly: Presence of different types of leaves on the same plant during different growth phases or under different environmental conditions.

Types of Heterophylly:

1. Developmental (Intrinsic) Heterophylly:

Different leaf forms appear during juvenile and mature stages of the same plant.

Examples: Cotton and coriander.

2. Environmental Heterophylly:

Leaf forms differ according to environmental conditions.

Example: Ranunculus flabellaris (buttercup).

7.9 Growth Hormones

The term hormone was first coined by Starling (1906) in animal physiology.

Internal factors that influence plant growth are called growth hormones or growth regulators.

They may promote, inhibit or modify plant growth.

Types of Growth Regulators

1. Growth Promoters

Auxins

Gibberellins (GA)

Cytokinins (CK)

2. Growth Inhibitors

Ethylene

Abscisic acid (ABA)

Definition of Plant Hormones

According to Thimann and Pincus (1948):

Plant hormones are naturally produced organic substances in higher plants that affect growth or other physiological functions at a site distant from where they are produced and are active in very minute (optimum) amounts.

Transport: According to Phillips (1971), hormones are transported through the phloem parenchyma.

7.9 Growth Hormones

A. Auxins (Auxien = to grow)

Discovery and Characteristics

F. W. Went (1931): First used the term auxin.

Kögl and H. Smith (1931): Isolated auxin from the urine of a person suffering from pellagra.

Auxin is synthesised in growing tips and meristematic regions and transported to other plant parts.

Most common natural auxin: Indole-3-acetic acid (IAA).

Precursor of IAA: Tryptophan.

Auxin was the first plant hormone discovered and primarily promotes cell elongation.

It shows polar transport (transport in a specific direction); in stems, transport is basipetal (from the tip towards the base).

Synthetic Auxins

IBA – Indole-3-butyric acid

NAA – Naphthalene acetic acid

2,4-D – 2,4-Dichlorophenoxyacetic acid

Physiological Effects and Applications of Auxin

1. Cell enlargement: Its primary effect is cell enlargement and elongation.

2. Apical dominance: The growing apical bud inhibits the growth of lateral buds.

3. Stem and root growth: Auxin stimulates the growth of stems and roots.

4. Tissue culture: Promotes cell multiplication and is used to produce a mass of undifferentiated cells called callus.

5. Root formation: Stimulates lateral and adventitious root formation and promotes early rooting in plant propagation by cuttings.

6. Weed control: Synthetic auxins such as 2,4-D act as herbicides. They kill dicot weeds without significantly affecting monocot crop plants when used appropriately.

7. Seedless fruit formation: Auxin application can induce parthenocarpy (fruit development without fertilisation), producing seedless fruits such as oranges, lemons, grapes and bananas.

8. Vascular tissue development: Promotes cell division in the cambium and early differentiation of xylem and phloem.

9. Concentration-dependent effects: Low auxin concentrations promote root elongation, while higher concentrations can promote shoot growth.

10. Flowering: Foliar sprays of NAA and 2,4-D are used to induce flowering in litchi and pineapple.

11. Prevention of fruit drop: Prevents premature fruit drop in apples, pears and oranges by delaying the formation of the abscission layer (layer responsible for separation of plant parts).

12. Respiration: Auxin can increase the rate of respiration.

13. Seed germination: Auxin can help break seed dormancy and promote germination.

B. Gibberellins (GA)

1. Discovery and Characteristics

Gibberellins are growth-promoting plant hormones.

Abundant in root tips and developing seeds.

Show non-polar transport through vascular tissues (transport without a specific direction).

Kurosawa (1926): First isolated gibberellins from the fungus Gibberella fujikuroi.

He observed that infected rice plants showed excessive stem elongation, called Bakanae disease.

Yabuta and Sumiki (1938): Isolated crystalline gibberellins from fungal culture and named them gibberellin.

Synthesised in young leaves, seeds, roots and stem tips.

Synthesised from mevalonic acid.

More than 150 chemical types are known.

GA₃ (Gibberellic acid) is the most common and biologically active form.

Chemically, it contains a gibberellane ring .

2. Physiological Effects and Applications

1. Breaks dormancy: Breaks bud and seed dormancy, helping growth and germination begin.

2. Seed germination: Promotes germination in cereals such as barley and wheat by stimulating the production of amylase (an enzyme that breaks down starch into sugars).

3. Stem elongation: Increases internode length and can convert genetically dwarf plants into taller plants.

4. Bolting: Promotes rapid elongation of internodes just before flowering in rosette plants, e.g. beet and cabbage.

Rosette habit: Leaves remain close to the ground with very short internodes.

5. Parthenocarpy: Induces fruit development without fertilisation, e.g. tomato, apple and pear.

6. Flowering: Promotes flowering in long-day plants.

7. Grape production: Increases fruit size and bunch length in grapes.

8. Dwarf plants: Application to genetically dwarf maize causes rapid stem elongation, allowing plants to reach the height of normal tall varieties.

9. Vernalization: Can overcome the requirement for vernalization (exposure to low temperature to promote flowering).

10. Root growth: Usually inhibits root growth.

11. Senescence: Delays senescence (ageing).

12. Abscission: Prevents abscission (shedding of leaves, flowers or fruits).

13. Sex expression: Application can promote the production of male flowers on female plants.

C. Cytokinins (CK)

1. Discovery and Characteristics

Function: Plant growth hormones that mainly promote cell division.

Letham coined the term cytokinin.

Skoog and Miller (1954): Discovered the first cytokinin, kinetin, while studying tobacco (Nicotiana tabacum) callus culture.

Callus (an undifferentiated mass of cells) grew rapidly when the nutrient medium contained coconut milk and degraded DNA from herring sperm.

Kinetin: A cytokinin; chemically, a purine derivative.

Zeatin: First natural cytokinin, obtained from unripe maize grains by Letham et al.

6-benzyl adenine: Synthetic cytokinin.

Seven types of cytokinins have been recorded in plants.

Natural cytokinins are also found in banana flowers, apple and tomato fruits, and coconut milk.

2. Physiological Effects and Applications

1. Cell division and enlargement: Stimulates cell multiplication and cell expansion.

2. Shoot formation: High cytokinin levels promote shoot development.

3. Root and shoot differentiation: The ratio of cytokinin to auxin determines development:

Low cytokinin : auxin ratio → Root formation.

High cytokinin : auxin ratio → Bud and shoot formation.

Their interaction controls morphogenesis.

4. Lateral bud growth: Promotes the growth of lateral buds and reverses apical dominance 

5. Delays ageing: Delays senescence (ageing) and abscission (shedding of plant parts); reported by Richmond and Lang (1957).

6. Cambium formation: Promotes the formation of interfascicular cambium (cambium developing between vascular bundles).

7. Dormancy and germination: Helps break dormancy and promotes seed germination.

8. RNA synthesis: Stimulates the synthesis of RNA, which supports cellular activities.

D. Ethylene

1. Discovery and Characteristics

Ethylene is the only gaseous plant growth regulator.

It is a simple gaseous hydrocarbon that plays an important role in fruit ripening.

Denny (1924): Reported ethylene's effect on fruit ripening.

Crocker (1930): Proposed that ethylene is the plant hormone responsible for fruit ripening.

Gane (1934): Established that plants naturally produce ethylene.

Synthesis: Produced in roots, shoot apical meristems (growing shoot tips) and ripening fruits.

Ethephon: A widely used compound that releases ethylene in plants.

D. Ethylene — Physiological Effects and Applications

1. Fruit ripening: Promotes ripening of bananas, apples and mangoes.

2. Root formation: Stimulates the initiation of lateral roots (side roots).

3. Dormancy: Helps break bud and seed dormancy.

4. Abscission: Promotes the formation of an abscission layer (separation layer), causing leaves, flowers and fruits to fall.

5. Lateral buds: Inhibits lateral bud growth and promotes apical dominance (suppression of side buds by the shoot tip).

6. Flowering: Inhibits flowering in most plants but promotes it in pineapple.

7. Senescence: Accelerates ageing of plant organs.

8. Epinasty: Causes downward bending or drooping of leaves and flowers.

9. Degreening: Increases chlorophyllase activity (an enzyme that breaks down chlorophyll), removing green colour from bananas and citrus fruits.

E. Abscisic Acid (ABA)

1. Discovery and Characteristics

ABA is a natural growth-inhibiting hormone.

Carns and Addicott (1961–65): Identified abscisin I and II, substances associated with the shedding of cotton bolls.

Wareing (1963): Isolated dormin from Acer buds; it induced bud dormancy.

Abscisin and dormin were found to be chemically identical and given the common name abscisic acid.

Synthesis: Produced in leaves, fruits, roots and seeds.

Chemical nature: A 15-carbon sesquiterpenoid (a type of organic compound) synthesised from mevalonic acid.

2. Physiological Effects and Applications

1. Abscission: Promotes shedding of leaves.

2. Dormancy: Induces and maintains dormancy in buds and seeds by suppressing growth.

3. Senescence: Accelerates ageing of leaves, flowers and fruits.

4. Cell growth: Inhibits and delays cell division and cell elongation.

5. Cambium activity: Suppresses mitosis (cell division) in vascular cambium, reducing its activity.

6. Stomatal closure: Causes the release of K⁺ ions from guard cells, leading to stomatal closure and reducing water loss.

Antitranspirant: A substance that reduces transpiration (water loss from plants).

7. Stress hormone: Helps plants respond to unfavourable environmental conditions, such as water stress.

8. Flowering: Inhibits flowering in long-day plants but stimulates flowering in short-day plants.

7.10 Photoperiodism

1. Basic Concept

Flowering: Transition from vegetative growth (growth of roots, stems and leaves) to reproductive growth (formation of flowers).

Flowering is influenced by environmental and nutritional factors, especially:

Light duration → Photoperiodism

Temperature → Vernalization (induction of flowering by low-temperature exposure).

Light has three aspects: quality, intensity and duration.

The duration of light has a major influence on flowering in higher plants.

2. Discovery

Garner and Allard (1920): Coined the term photoperiodism.

Studied flowering in:

Soybean

Maryland Mammoth variety of tobacco

Observations:

Soybean flowered in late summer.

Maryland Mammoth tobacco flowered in winter.

Flowering occurred according to specific light-duration requirements, regardless of when plants were sown.

Different temperatures, nutrition and soil moisture did not explain the flowering pattern.

Experiments using artificial light showed that the relative length of day and night controls flowering.

 Definitions : 

Photoperiodism: Response of plants to the relative duration of light and darkness, especially in relation to flowering.

4. Types of Plants Based on Photoperiodic Response

1. Short-Day Plants (SDP)

2. Long-Day Plants (LDP)

3. Day-Neutral Plants (DNP)

5. Reception and Transmission of Photoperiodic Stimulus

Receptor organ — Leaf: The main organ that receives the photoperiodic stimulus. Demonstrated by Knoff (1934).

Defoliated plants - fail to flower even under the correct photoperiod.

Florigen: A flowering stimulus of hormonal nature, produced in response to suitable photoperiod and transported through the phloem to the flowering region.

Photoreceptors — Phytochromes: Light-sensitive pigments present in cell membranes that detect light signals.

Blue light: Blue wavelengths of light can influence flowering.

Types of Plants Based on Photoperiodism

A. Short-Day Plants (SDP)

Flowering condition: Day length is shorter than the critical photoperiod (critical photoperiod 

is that length of photoperiod above or below 

which flowering occurs).

Require a long, uninterrupted night; hence, also called long-night plants.

A brief flash of light during the dark period inhibits flowering.

Examples: Dahlia, Aster, Tobacco, Chrysanthemum, Soybean (Glycine max), Cocklebur (Xanthium).

B. Long-Day Plants (LDP)

Flowering condition: Day length exceeds the critical photoperiod.

Require a short night; hence, also called short-night plants.

A brief light flash during a long night can promote flowering.

Examples: Pea, Radish, Sugar beet, Cabbage, Spinach, Wheat, Poppy.

C. Day-Neutral Plants (DNP)

Flowering is independent of photoperiod (day/night duration).ì

Do not require a specific day length to flower.

Examples: Cucumber, Tomato, Cotton, Sunflower, Maize, Balsam.

Phytochrome

Hendricks and Borthwick (1952): Studied the effect of red and far-red light on flowering.

In short-day plants, a flash of red light (660 nm) during the night inhibits flowering.

If followed immediately by far-red light (730 nm), flowering can occur.

3. Role in Flowering

During daytime: Pfr accumulates.

SDP → Inhibits flowering.

LDP → Promotes flowering.

During darkness: Pfr gradually converts into Pr.

SDP → Pr promotes flowering.

LDP → Pr inhibits flowering.

7.11 Vernalization (Yarovization)

Vernalization: Induction or acceleration of flowering by exposing plants or their seeds/seedlings to low temperature (chilling).

Temperature affects plant growth and reproduction.

Three cardinal temperatures: minimum, optimum and maximum (lowest, best and highest temperatures for growth).

Klippart (1918): Reported the effect of low temperature on early flowering.

T. D. Lysenko (1928): Coined the term vernalization.

Chouard (1960): Defined it as acceleration of flowering ability through chilling treatment.

2. Process and Site

Required by many plants, especially cereals and crucifers (mustard family).

Seeds or seedlings are treated at 1–6°C for 1–1.5 months to induce early flowering.

Site: Shoot apical meristem (growing tip of the shoot).

In annual plants, vernalization is generally effective at the seed stage.

Vernalin: Proposed chemical stimulus associated with vernalization; its effect can be transferred through grafting (Melchers, 1939).

Vernalization — Experiment on Wheat

 

Klippart (as stated in textbook: 1957)

Winter wheat

Sown in winter → Flowers in the following summer.

Sown in spring → Does not flower that summer; flowers next summer.

Reason → Requires prior low-temperature treatment.

Spring wheat

Sown in spring → Flowers in the same summer.

Experiment:

Germinated seeds of winter wheat treated with low temperature → Behave like spring wheat.

Conclusion: Winter wheat requires chilling before flowering; this process is called vernalization.

3. Advantages

1. Early flowering: Shortens the juvenile phase

2. Wider cultivation: Allows crops to be cultivated in regions where they do not naturally grow easily.

7.12 Mineral Nutrition

1. Mineral Nutrition — Basic Points

Mineral nutrition: Absorption and use of minerals by plants.

Sources: Soil, air and water.

Plant ash analysis: Confirms the presence of mineral elements in plants.

Elements used: Approximately 36–40 elements.

Absorption: Mainly through roots, in ionic form (dissolved charged particles).

Examples of ions: PO₄³⁻ (phosphate), CO₃²⁻ (carbonate), SO₄²⁻ (sulphate).

2. Sources of Elements

Element Source

Carbon (C) Atmospheric CO₂

Hydrogen (H) Water

Oxygen (O) Air + Water

Nitrogen (N) Mainly soil

C, H, O: Not mineral in origin ; obtained mainly from CO₂ and water.

Classification of Minerals (Older classification)

A. Essential elements

Necessary to complete the plant life cycle.

Functions → Structural + Physiological.

Deficiency → Characteristic symptoms.

Examples → C, H, O, N, P.

B. Non-essential elements

Not indispensable for completing the life cycle.

Absence → No specific deficiency symptoms.

 Important: This classification is obsolete. 

Classification Based on Quantity Requirement (Older classification)

Microelements Macroelements

Required in trace amounts Required in large amounts

Mainly catalytic role (help enzymes function) Mainly nutritive + structural roles

Act as enzyme cofactors Support growth and plant structure

Examples: Zn, Cu, Al, Si, Mn, B, Mo, Cl Examples: C, H, O, P, Mg, N, K, S, Ca, Fe

C, H, O: Major elements but non-mineral in origin.

This quantity-based classification is not accepted now.

5. Criteria for Essentiality of Elements

Proposed by Arnon and Stout (1939)

An element is essential only if it fulfils all three criteria:

1. Necessary: Required for normal growth and reproduction.

2. Non-replaceable: Its specific function cannot be performed by another element.

3. Direct role: Must be directly involved in plant Nutrition.

6. Liebig’s Law of Minimum

Proposed by: Liebig.

Law: Crop yield depends on the essential element available in the minimum quantity.

Even if other nutrients are sufficient, deficiency of one essential element limits plant growth.

Modern classification: Minerals are classified according to their biochemical functions.

7. Symptoms of Mineral Deficiency

Deficiency symptom / Hunger sign: Visible abnormality in plant structure or function due to nutrient deficiency.

Critical concentration: Minimum concentration of an essential element required for normal plant growth.

Deficient element: An essential element present below its critical concentration.

B. Location of Deficiency Symptoms

1. Immobile Elements

Cannot move easily from older tissues to younger tissues.

Deficiency symptoms appear first in young tissues.

Examples: Sulphur (S), Calcium (Ca).

2. Mobile Elements

Move from older leaves to young, developing tissues.

Deficiency symptoms appear first in older leaves (senescent leaves).

Examples: Nitrogen (N), Magnesium (Mg), Potassium (K).

C. Major Deficiency Symptoms

1. Stunting: Retarded growth; stem becomes short and condensed.

2. Chlorosis: Yellowing of leaves due to loss or failure of chlorophyll development.

3. Necrosis: Death of localised tissue, especially in leaves.

4. Mottling: Appearance of irregular green and non-green patches on leaves.

5. Abscission: Premature falling of leaves, flowers or fruits.