Revision: Class 11 >> Plant Growth and Development NEET (UG) Plant Growth and Development

Growth is characterised as an essential process that results in permanent and irreversible changes to the shape, size, form, weight, and volume of a cell, organ, or entire organism, along with an increase in dry matter.

• 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.

• 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.

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

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

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

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.

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

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.

Parthenocarpy is the process of fruit development without fertilization.

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.

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.

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

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

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

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

• Plant growth is seen in roots, stems, leaves, flowers, fruits, and seeds.
• Life stages: seed → seedling → plantlet → mature plant.
• Trees grow continuously, but leaves, flowers, and fruits have limited growth.
• Development includes growth and differentiation from a zygote.
• Germination starts growth in favourable conditions and depends on internal and external factors.

• Plant growth is indeterminate because it continues throughout life due to meristems.
• Meristematic cells keep dividing, and their daughter cells later differentiate to form plant parts.
• Continuous formation of new cells by meristems is called open growth.
• Apical meristems in roots and shoots cause primary growth (increase in length).
• Lateral meristems (in dicots and gymnosperms) cause secondary growth (increase in girth).

• 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).

• Plant growth is measurable as it involves an increase in protoplasm.
• It is measured indirectly using parameters like length, area, volume, fresh weight, dry weight, and cell number.
• Growth can occur by an increase in cell number (e.g., maize root) or cell size (e.g., watermelon).
• Different plant parts use different measures, such as length (pollen tube) and surface area (leaf).
• Growth rate can be very high, e.g., maize roots produce thousands of new cells per hour.

• 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.

• 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.

• 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.

• 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.

• PGRs (phytohormones) are classified into two groups - growth promoters (auxins, gibberellins, cytokinins) and growth inhibitors (abscisic acid, ethylene).
• Growth promoters help in cell division, enlargement, flowering, fruiting and seed formation; growth inhibitors respond to stress and control dormancy and abscission.
• Auxin - discovered by Charles & Francis Darwin (phototropism in coleoptiles), isolated by F.W. Went. Gibberellins - discovered from "bakanae" disease in rice by E. Kurosawa.
• Cytokinins - discovered by F. Skoog through tobacco stem callus, identified as kinetin. ABA - identified in the mid-1960s from inhibitors by independent researchers.
• Ethylene - discovered by H.H. Cousins, the only gaseous PGR

• 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.

• PGRs play critical roles in various growth phases, often with complementary or antagonistic effects.
• Examples include interactions in dormancy, abscission, senescence and apical dominance.
• PGRs mediate responses to extrinsic factors like temperature and light, affecting processes like vernalisation, flowering, dormancy, seed germination and plant movements.