Revision: Plant Growth and Mineral Nutrition Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

• 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.

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

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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.

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

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.

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

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 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.

Plants that flower independent of the duration of light are called day neutral plants.

Plants that flower when the day length is longer than the critical photoperiod and require a short dark period are called long day plants.

The ratio of change in the number of cells (dn) over a given time interval (dt), representing total growth per unit time, is called absolute growth rate.

Plants that flower when the day length is shorter than the critical photoperiod and require a long uninterrupted dark period are called short day plants.

The induction or acceleration of flowering in plants by low temperature (chilling) treatment is called vernalization.

The response of plants to the duration of light that influences flowering is called photoperiodism.

The reversal of the effect of vernalization by high temperature treatment is called devernalization.

The hypothetical chemical stimulus produced during vernalization and responsible for flowering is called vernalin.

The permanent change in structure and function of meristematic cells leading to their maturation and specialization, with loss of capacity to divide, is called differentiation.

The process by which living, mature, differentiated cells regain the capacity to divide and become meristematic again, is called de-differentiation.

The process by which cells produced after de-differentiation once again lose the capacity to divide and become specialized to perform specific functions, is called re-differentiation.

The increase in growth per unit time, indicating the efficiency of growth in plant organs, is called growth rate.

The measure that expresses the rate of increase in growth per unit time is called efficiency index.

The light-sensitive proteinaceous pigment present in leaves that perceives photoperiodic stimulus and regulates flowering is called phytochrome.

The growth rate of a system per unit time expressed on a common basis, obtained by dividing AGR by the total number of cells present, is called relative growth rate.

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

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

Lt = Lo + rt

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

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

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

• 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.

• Water — Maintains cell turgidity and acts as a medium for biochemical reactions.
• Nutrients — Macroelements and microelements are required for proper growth and metabolism.
• Temperature & Oxygen — Optimum temperature is 25–35°C; oxygen is needed for respiration and energy release.
• Light — Essential for seed germination and photosynthesis.
• Gravity & Growth Hormones — Gravity decides the direction of growth (shoots upward, roots downward); hormones regulate and coordinate 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.

• 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.

• Mineral Nutrition
  The process by which plants absorb and utilize inorganic mineral elements from soil, water, and air for growth and development is called mineral nutrition.
• Source and Absorption
  Plants absorb minerals mainly from the soil in dissolved ionic form (e.g. PO₄³⁻, SO₄²⁻, CO₃²⁻) through roots.
• Essential Elements
  Plants require about 36–40 elements, of which some are essential for completing the life cycle and performing structural and physiological functions.
• Classification of Minerals
  Based on quantity required, minerals are classified into macroelements (required in large amounts) and microelements (required in trace amounts).
• Deficiency and Critical Concentration
  The minimum concentration of an essential element below which plant growth is affected is called critical concentration, and its shortage leads to deficiency.
• Deficiency Symptoms
  Common mineral deficiency symptoms include chlorosis, necrosis, stunting, mottling, and abscission, depending on the mobility of the element in plants.

1. Plant Growth
   Growth is a characteristic feature of living organisms and is defined as a permanent and irreversible increase in size, mass, or number of cells.
2. Aspects of Growth
   Growth has two aspects: quantitative (increase in size, length, volume, dry weight, and cell number) and qualitative (development and differentiation).
3. Growth and Development
   Development is an ordered and progressive change leading to higher complexity, while differentiation leads to specialized structures.
4. Nature of Growth in Plants
   In multicellular vascular plants, growth is indeterminate and continues throughout life.
5. Meristematic Regions
   Plant growth is restricted to specific regions called meristems, where cells divide continuously.
6. Types of Meristems
   Meristems are of three types based on location: apical (increase in length), intercalary (elongation of internodes), and lateral (increase in girth).