Revision: 12th Std >> Plant Growth and Mineral Nutrition MAH-MHT CET (PCM/PCB) Plant Growth and Mineral Nutrition

Increase in growth per unit time is called growth rate or efficiency index.

• Relative growth rate is the growth per unit initial parameter.
• RGR refers to the growth of a particular system per unit time, expressed on a common basis or it is the ratio of growth in the given time over initial growth.

Growth rate can be defined as increased plant growth per unit time.

• The ratio of change in the cell number (dn) over the time interval (dt) is called Absolute growth rate (AGR).
• Absolute growth rate is the measurement of total growth per unit time.

Re-differentiation is the process by which dedifferentiated cells regain specialised characteristics and functionality.

The living differentiated cells, that have lost the capacity to divide, can regain the capacity of division under certain conditions. This phenomenon is termed as dedifferentiation.

Differentiation is the process by which unspecialised cells become specialised and take on specific functions.

Development refers to the ordered or progressive changes in shape, form and degree of complexity.

Plasticity is the ability of plants to form different kinds of structures (i.e. change) in response to different environmental (external) or internal stimuli, in various phases of life.

Responses are the actions or movements shown by an organism as a result of the stimuli.

Plant hormones are known as phytohormones. These are organic molecules that serve as mediators, similar to animal hormones, to facilitate the coordination of a variety of cellular activities within a plant.

Stimuli are the changes in the external or internal environment of an organism that trigger a response or movement.

A hormone is a chemical messenger produced in one part of an organism and transported to other parts, where it regulates growth, development, and responses.

Apical dominance is the phenomenon where the apical (terminal) bud of a plant suppresses the growth of lateral buds. This is mainly due to the action of auxins produced in the apical bud.

Parthenocarpic fruits are those that develop without fertilization. Auxins can induce such fruit formation in plants like apples, tomatoes, and bananas.

Parthenocarpy is the process by which fruits develop naturally or artificially without the necessity of fertilizing ovules, resulting in seedless fruits.

In the majority of vascular plants, apical buds dominate over lateral buds. The growth of lateral buds occurs only after the removal of the apical buds. This phenomenon of the suppression of the growth of lateral buds by apical buds is called apical dominance.

Parthenocarpy is the process of fruit development without fertilization.

Gibberellin is a plant growth hormone found mainly in meristematic tissues like stem apex, root apex, buds, and seeds. It promotes internode elongation, breaks seed dormancy, and supports fruit development. Gibberellic acid (GA₃) is the most studied form of gibberellin.

Cytokinin is a plant hormone discovered in the 1950s by Skoog and Miller. It is primarily produced in root tips and transported via xylem. Cytokinin promotes plant growth by stimulating cell division, especially in germinating seeds, developing fruits, and embryos.

Abscisic acid is a plant hormone found in chloroplasts of leaves and in high amounts in fruits and seeds. It acts as a growth inhibitor and is present in angiosperms, gymnosperms, pteridophytes, and some mosses.

Abscission is the process by which plants shed or detach various portions that are no longer necessary for them, such as leaves, fruits, flowers, and seeds.

The response of plants to the relative length of light and dark periods with reference to the initiation of flowering is called photoperiodism.

The effects of photoperiods or daily duration of light hours (and dark periods) on the growth and development of plants, especially flowering, is called photoperiodism.

Photochemical receptors in the leaves are the biliproteins (pigments) located in the cell membrane and are called phytochromes.

Plants that flower usually during summer are called long-day plants.

The low-temperature treatment or chilling treatment of germinating seeds or seedlings to promote early flowering in plants is called vernalization. It was evidenced by Klippart (1918).

The reversion of vernalization by high temperature treatment is called devernalization.

The effects of photoperiods or daily duration of light hours (and dark periods) on the growth and development of plants, especially flowering, is called photoperiodism.

The response of plants to the relative length of light and dark periods with reference to the initiation of flowering is called photoperiodism.

Plants that flower usually during summer are called long-day plants.

Photochemical receptors in the leaves are the biliproteins (pigments) located in the cell membrane and are called phytochromes.

The movement of mineral ions into root cells as a result of diffusion is without expenditure of energy is called passive absorption.

Chemoautotrophs are those who make their own food by deriving the energy from chemical reactions. e.g., Nitrifying bacteria, Iron bacteria etc.

\[\mathrm{AGR}=\frac{\mathrm{dn}}{\mathrm{dt}}\]

\[\mathrm{RGR}=\frac{\mathrm{AGR}}{\mathrm{n}}\]

\[\mathbf{W}_{1}=\mathbf{W}_{0}\mathbf{e}^{n}\]

Where
W₁ = Final size
W₀ = initial size
r = growth rate
t = time of growth
e = base of natural logarithm

Lt = Lo + rt

Where
Lt = Length at time ‘t’
Lo = Length at time ‘Zero’
r = Growth rate
t = Time of growth

• Growth - Permanent, irreversible increase in size, mass, or number of cells. Has two aspects: quantitative (measurable increase) and qualitative (differentiation into complex forms).
• Indeterminate vs Determinate - Vascular plants grow throughout their life (indeterminate). Organs like leaves, flowers, and fruits grow up to a fixed genetic size (determinate).
• Meristems - Special regions where new cells are constantly produced. Three types: Apical, Intercalary, and Lateral.
• Apical Meristem - At root and shoot tips; responsible for growth in length (primary growth).
• Intercalary Meristem - At nodes/base of internodes; increases internode length and forms leaf primordia and lateral buds.
• Lateral Meristem - Along the axis of dicots/gymnosperms; vascular cambium adds secondary vascular tissue → increases girth of stem (secondary growth).

• Three Phases - Cell Division (Lag) → Cell Elongation (Log) → Cell Maturation (Stationary).
• Cell Division (Lag Phase) - Thin-walled, non-vacuolated cells with a prominent nucleus undergo mitosis. Growth is slow.
• Cell Elongation (Log Phase) - Cells absorb water, become vacuolated and turgid, and enlarge in length and breadth. Growth is fastest.
• Cell Maturation (Stationary Phase) - Cells attain morphological and physiological maturity and become specialised. Growth stops.
• Growth Curve - These three phases together form an S-shaped (sigmoid) curve.

• Necessary conditions for plant growth are water, oxygen, and nutrients (both macro and micro essential elements).
• Water is crucial for cell enlargement, turgidity, extension growth, and acts as a medium for enzymatic activities.
• Oxygen helps in releasing metabolic energy, which is required for growth activities.
• Each plant has an optimal temperature range for growth - deviations (too high or too low) can be harmful.
• Environmental signals like light and gravity also influence certain phases and stages of plant growth.

• Growth Rate - Increase in growth per unit time; also called efficiency index. Measured by increase in size, length, volume, or dry weight.
• Absolute Growth Rate (AGR) - Total growth per unit time; ratio of change in cell number over a given time interval.
• Relative Growth Rate (RGR) - Growth per unit initial parameter; obtained by dividing AGR by the total number of cells.
• Measurement of Growth - Measured by increase in cell number, leaf area, length, volume, or dry weight. (e.g. Watermelon ovary increases 3,50,000 times after fertilisation.)
• Instruments - Direct method (scale), Horizontal microscope (field), Auxanometer (linear shoot growth), Crescograph (by Sir J.C. Bose; magnifies up to 10,000 times).

• Two Types - Plant growth is of two types: Arithmetic and Geometric, differing in rate and pattern of cell division.
• Arithmetic Growth - Rate of growth is constant. One daughter cell divides, the other differentiates. Example: root elongation. Produces a linear curve.
• Geometric Growth - Both daughter cells keep dividing repeatedly. Growth is slow initially but later becomes rapid and exponential. Produces a sigmoid (S-shaped) curve.
• Sigmoid Curve Phases - Lag phase (slow) → Log phase (rapid/exponential) → Stationary phase (growth stops). This is the characteristic curve of most living organisms.

• Growth Curve — Graphical representation of total growth against time. Three types: Linear (arithmetic), Exponential (geometric), and Sigmoid (S-shaped).
• Sigmoid Curve — Characteristic growth curve of living organisms in a natural environment; obtained by plotting growth rate against time for all three phases.
• Three Phases — Lag phase (slow — root/shoot apex) → Log phase (very rapid — fruiting region) → Stationary phase (growth stops — mature tissues).
• Growth Rates — Lag = slow; Log = fastest; Stationary = gradually stops.
• Grand Period of Growth — Total time required for all three phases to complete.

• Meaning - Development refers to ordered, progressive changes in shape, form, and complexity of an organism from seed germination to senescence (death).
• Components of Development - Development includes four processes: Growth, Morphogenesis, Maturation, and Senescence.
• Stages of Development - Seed Germination → Meristem → Cell Division → Plasmatic Growth → Cell Elongation → Cell Maturation → Mature Cell → Senescence → Death.

• Meaning — The ability of plants to form different structures in response to different environmental or internal stimuli during various life phases.
• Heterophylly — Same plant bears different forms of leaves in juvenile and mature stages. e.g. Cotton, Coriander, Larkspur (Delphinium).
• Environmental Heterophylly — Leaf form changes due to external/environmental conditions. e.g. Ranunculus flabellaris (buttercup).
• Intrinsic Plasticity — Leaf form changes due to internal stimuli, e.g. Coriander and Cotton.

• Meaning — Phytohormones regulate growth and physiological functions at a site remote from production. Coined by Thimann & Pincus (1948).
• Types — Promoters: Auxin, Gibberellin, Cytokinin. Inhibitors: ABA, Ethylene.
• Discovery — Darwin (1880) observed growth stimulus at the coleoptile tip, causing bending towards light. Auxin = first plant hormone discovered.
• Key Scientists — Boysen-Jensen (1910): chemical stimulus; Paal (1919): bending in dark; Went (1928): isolated auxin from Avena using agar blocks.
• Transport — Active in minute amounts; transported through phloem parenchyma (Phillips, 1971).

• Discovery — First discovered by Kurosawa (1926) from Gibberella fujikuroi, causing Bakanae disease in rice. Crystalline form isolated by Yabuta & Sumiki (1938). Active form: GA₃ (Gibberellic acid).
• Synthesis & Transport — Synthesised in young leaves, seeds, roots, and stem tips from mevalonic acid. Transport is non-polar. Over 150 types are known.
• Stem Elongation & Bolting — Elongates internodes; converts dwarf plants to tall (e.g. maize, pea). Causes bolting in rosette plants (beet, cabbage).
• Seed Germination & Dormancy — Breaks seed and bud dormancy; stimulates amylase and protease for germination in cereals.
• Parthenocarpy & Fruit Growth — Produces seedless fruits (tomato, apple, pear); increases grape bunch length. Pomalin (GA₄ + GA₇ + cytokinin) = apple enlarger.
• Other Effects — Causes maleness in some plants; delays citrus ripening; inhibits root growth; delays senescence; prevents abscission.
• Vernalization Substitute — Overcomes the need for cold treatment and induces flowering in long-day plants.

• Discovery — Term coined by Letham. The first cytokinin (kinetin) was discovered by Skoog & Miller (1954) from tobacco callus culture. First natural cytokinin: Zeatin (from unripe maize by Letham, 1963). Synthetic: 6-benzyl adenine.
• Nature & Site — Purine (adenine) derivative. Found at root apices and immature fruits. Coconut milk is a rich source.
• Cell Division & Growth — Promotes cell division and enlargement. High cytokinin: auxin ratio → shoots; low ratio → roots.
• Lateral Bud & Apical Dominance — Promotes lateral bud growth and reverses apical dominance.
• Senescence & Dormancy — Delays senescence and abscission (Richmond & Lang, 1957). Breaks seed dormancy and promotes germination.
• Other Effects — Induces flowering (Lemna, Wolffia); promotes chloroplast development; causes femaleness; favours phloem transport.
• Cytokinin + Auxin — Balanced combination induces organogenesis and controls morphogenic differentiation.

• Introduction — Only a gaseous phytohormone. Denny (1924) reported the fruit ripening role; Gane (1934) confirmed natural synthesis. Produced from methionine. Source: Ethephon.
• Site of Synthesis — Roots, shoot apical meristems, ripening fruits. Acts as both a promoter and an inhibitor.
• Fruit Ripening — Promotes the ripening of bananas, apples, mangoes, and tomatoes. Increases respiratory climacteric. Used in post-harvest technology.
• Abscission & Senescence — Enhances abscission of leaves, flowers, and fruits. Promotes senescence. Causes degreening in bananas and citrus.
• Dormancy & Roots — Breaks seed dormancy. In low concentrations, it promotes root growth and lateral root initiation.
• Inhibitory Effects — Inhibits flowering (except pineapple); causes apical dominance; causes epinasty (drooping of leaves/flowers).
• Other Effects — Promotes horizontal seedling growth; may cause ageotropism (roots lose sensitivity to gravity).

• Discovery — Carns & Addicott (1961–65): abscisin from cotton; Wareing (1963): dormin from Acer buds. Both are named ABA. Chemically: 15-carbon sesquiterpenoid from mevalonic acid.
• Nature & Transport — Natural growth inhibitor and stress hormone. Also called anti-gibberellin. Transport is non-polar. Found in leaves, fruits, roots, and seeds.
• Stomatal Closure — Causes K⁺ efflux from guard cells → stomata close during stress. Known as an antitranspirant.
• Dormancy & Abscission — Induces bud and seed dormancy. Causes abscission of leaves, flowers, and fruits. Regulates fruit drop.
• Senescence & Stress — Accelerates senescence. Helps plants tolerate drought, salinity, cold and frost (stress hormone).
• Inhibitory Effects — Inhibits cell division, elongation, and cambium activity. Inhibits flowering in long-day plants; promotes in short-day plants.
• Other Effects — Induces carotenoid synthesis; turns green oranges yellow; causes geotropism when applied to roots.

• Definition — Effect of duration of light on flowering. Coined by Garner & Allard (1920). Leaf is the chief organ for receiving stimulus (Knoff, 1934).
• Three Types — SDP: day < critical photoperiod (Dahlia, Tobacco, Xanthium); LDP: day > critical photoperiod (Wheat, Pea, Spinach); DNP: independent of photoperiod (Tomato, Maize, Sunflower).
• Critical Dark Period — SDP = long night plants (uninterrupted dark needed; light flash stops flowering). LDP = short night plants (light flash during dark promotes flowering).
• Florigen — Hormonal chemical stimulus transported through phloem from leaves to the flowering site.
• Phytochrome — Proteinaceous pigment in leaf cell membranes (Hendricks & Borthwick, 1952). Two forms: Pr (660 nm, red) and Pfr (730 nm, far red).
• Phytochrome Action — Day: Pfr accumulates → inhibits SDP, promotes LDP. Dark: Pfr → Pr → promotes SDP, inhibits LDP.
• Photomorphogenesis — Control of plant development by light and phytochrome.

• Meaning — Low temperature (chilling) treatment that induces early flowering. Coined by T.D. Lysenko (1928) was defined by Chouard (1960).
• Discovery — Klippart (1857) observed that winter wheat treated with low temperature behaves like spring wheat and flowers earlier.
• Process — Seeds/seedlings treated at 1–6°C for 1–1.5 months. Site: shoot apical meristem. Effective at seed stage in annual plants.
• Vernalin — Chemical stimulus of vernalization called vernalin; can be transferred through grafting (Melcher, 1939).
• Devernalization — Reversal of vernalization by high temperature is called devernalization.
• Advantages — Shortens juvenile phase for early flowering; allows crops to grow in regions where they don't grow naturally. e.g. cereals and crucifers.

• Definition — Effect of duration of light on flowering. Coined by Garner & Allard (1920). Leaf is the chief organ for receiving stimulus (Knoff, 1934).
• Three Types — SDP: day < critical photoperiod (Dahlia, Tobacco, Xanthium); LDP: day > critical photoperiod (Wheat, Pea, Spinach); DNP: independent of photoperiod (Tomato, Maize, Sunflower).
• Critical Dark Period — SDP = long night plants (uninterrupted dark needed; light flash stops flowering). LDP = short night plants (light flash during dark promotes flowering).
• Florigen — Hormonal chemical stimulus transported through phloem from leaves to the flowering site.
• Phytochrome — Proteinaceous pigment in leaf cell membranes (Hendricks & Borthwick, 1952). Two forms: Pr (660 nm, red) and Pfr (730 nm, far red).
• Phytochrome Action — Day: Pfr accumulates → inhibits SDP, promotes LDP. Dark: Pfr → Pr → promotes SDP, inhibits LDP.
• Photomorphogenesis — Control of plant development by light and phytochrome.

• Mineral Nutrients: Inorganic substances from soil, air & water absorbed by plants in ionic form (PO₄, SO₄) through roots. ~36–40 elements are used.
• Sources: C → CO₂ | H, O → water & air | N → soil. C, H, O are NOT minerals in origin.
• Classification: Macroelements (C, H, O, N, P, K, S, Ca, Mg, Fe) are needed in large amounts for structural/nutritive roles. Microelements (Zn, Cu, Mn, Mo, B, Cl) are needed in traces and act as co-factors in catalytic roles.
•  Arnon & Stout (1939): An element is essential if it's necessary for growth, has a specific non-replaceable function, and is directly involved in plant nutrition.
• 5. Liebig's Law of Minimum: Yield is limited by the essential element available in the least quantity.
• 6. Mobility Rule: Immobile elements (S, Ca) → deficiency in young tissues. Mobile elements (N, Mg, K) → deficiency in old/senescent leaves.
• 7. Deficiency Symptoms: Chlorosis (yellowing), Necrosis (tissue death), Stunting (short stem), Mottling (patchy leaves), Abscission (premature leaf/flower/fruit fall).

• Most minerals are charged ions and can't pass membranes freely, so they need active absorption using ATP energy.
• Passive Absorption — Ions move from high → low concentration (diffusion), no energy needed. Occurs via ion-exchange, mass flow, or Donnan equilibrium.
• Donnan Equilibrium — Fixed anions inside the cell attract extra cations from outside to maintain electrical balance; a special type of passive absorption.
• Active Absorption — Ions move against the concentration gradient using ATP. If O₂ is cut off from the roots, active absorption drops immediately. Path: Root hair → Cortex → Xylem.
• Transport & Carriers — Minerals travel via xylem with water (transpiration stream) to all parts, redistributed through phloem. Carrier proteins (Hoagland & Davis, 1923) actively pump ions into root cells.

• Nitrogen Fixation — N₂ → Ammonia/nitrogenous salts. Types: Biological (microbes) and Non-biological (lightning/industrial).
• Nitrification — NH₃ → NO₂⁻ (by Nitrosomonas) → NO₃⁻ (by Nitrobacter). Nitrates absorbed by plants.
• Ammonification — Dead organic matter → NH₃, by ammonifying bacteria in soil.
• Denitrification — NO₃⁻ → N₂ (back to atmosphere), by Bacillus, Paracoccus, Pseudomonas denitrificans.
• Cyclic Flow — Atmosphere → Soil → Plants → Animals → Soil → Atmosphere. Nitrogen is continuously recycled.

• Organisms called Nitrogen Fixers / Diazotrophs fix atmospheric N₂. They are of two types: Symbiotic and Free-living.
• Symbiotic Fixers — Rhizobium forms root nodules in plants of the family Fabaceae (legumes) and fixes nitrogen in association with the plant.
• Free-living Fixers — Azotobacter and Azospirillum fix nitrogen independently in the soil without forming any association.
• Energy Required — Nitrogen fixation needs 16 ATP molecules per N₂ molecule fixed. N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pᵢ