Plant Growth and Development — Revision Notes

⚡ 30-Second Revision

Growth: — Irreversible increase in size/mass.
Development: — Entire life cycle changes.
PGRs: — Auxins, Gibberellins, Cytokinins (Promoters); ABA, Ethylene (Inhibitors).
Auxin: — Apical dominance, rooting, parthenocarpy, 2,4-D (herbicide).
Gibberellin: — Stem elongation (bolting), breaks dormancy, fruit size ($GA_3$).
Cytokinin: — Cell division, lateral bud growth, delays senescence.
ABA: — Stress hormone, stomatal closure, seed dormancy, abscission.
Ethylene: — Gaseous, fruit ripening, senescence, abscission.
Photoperiodism: — Response to day/night length (SDP, LDP, DNP).
Vernalization: — Cold treatment for flowering.
Growth Curves: — Arithmetic (linear), Geometric (sigmoid).
Plasticity: — Developmental changes due to environment (e.g., heterophylly).

2-Minute Revision

Plant growth is an irreversible increase in size, while development encompasses all life cycle changes. These processes are regulated by Plant Growth Regulators (PGRs) and environmental factors. PGRs include promoters (Auxins, Gibberellins, Cytokinins) and inhibitors (Abscisic Acid, Ethylene).

Auxins promote cell elongation, apical dominance, and root initiation. Gibberellins cause stem elongation (bolting) and break seed dormancy. Cytokinins stimulate cell division and delay senescence. Abscisic Acid (ABA) is the 'stress hormone,' inducing dormancy and stomatal closure.

Ethylene, a gaseous hormone, promotes fruit ripening and senescence. Environmental factors like light duration (photoperiodism) and cold exposure (vernalization) critically influence flowering. Photoperiodism classifies plants into short-day, long-day, and day-neutral types.

Vernalization ensures flowering at appropriate times. Growth can follow arithmetic (linear) or geometric (S-shaped) patterns. Plants also exhibit developmental plasticity, adapting their form to environmental cues.

5-Minute Revision

Plant growth, an irreversible increase in size, occurs through three phases: meristematic (cell division), elongation (cell enlargement), and maturation (differentiation). Development is the broader sum of all changes from germination to senescence.

Growth rates can be arithmetic (linear, constant rate) or geometric (S-shaped curve with lag, log, and stationary phases). These processes are intricately controlled by Plant Growth Regulators (PGRs).

Auxins, like IAA, promote cell elongation, apical dominance, root initiation, and parthenocarpy; synthetic auxins (e.g., 2,4-D) are herbicides. Gibberellins (e.g., GA3) are known for stem elongation (bolting), breaking seed dormancy, and increasing fruit size.

Cytokinins (e.g., kinetin, zeatin) promote cell division, lateral bud growth, and delay senescence. Abscisic Acid (ABA) is a growth inhibitor, often called the 'stress hormone,' inducing seed dormancy, stomatal closure, and abscission.

Ethylene, a gaseous hormone, promotes fruit ripening, senescence, and abscission. Environmental factors like light duration (photoperiodism) and cold treatment (vernalization) are crucial. Photoperiodism categorizes plants into Short-Day Plants (SDPs), Long-Day Plants (LDPs), and Day-Neutral Plants (DNPs) based on their flowering response to critical day/night lengths, mediated by phytochrome.

Vernalization is the requirement of a cold period for flowering, preventing premature reproduction. Plants also display plasticity, altering their development (e.g., heterophylly) in response to their environment.

Understanding the specific roles, interactions, and applications of these PGRs and environmental cues is paramount for NEET.

Prelims Revision Notes

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Growth: — Irreversible increase in size/mass. Development: All changes from germination to senescence.
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Phases of Growth: — Meristematic (cell division), Elongation (cell enlargement), Maturation (differentiation).
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Growth Curves:

* Arithmetic: $L_t = L_0 + rt$ (linear graph). One daughter cell divides, other differentiates. * Geometric: $W_1 = W_0 e^{rt}$ (S-shaped/Sigmoid graph). Both daughter cells divide. Phases: Lag, Log (exponential), Stationary.

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Conditions for Growth: — Water, Oxygen, Nutrients, Temperature, Light.
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Differentiation: — Cells specialize. Dedifferentiation: Specialized cells regain division capacity. Redifferentiation: Dedifferentiated cells re-specialize.
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Plasticity: — Ability to change developmental pathway based on environment (e.g., heterophylly in cotton, larkspur, buttercup).
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Plant Growth Regulators (PGRs) / Phytohormones:

* Auxins (Promoter): IAA, IBA (natural); NAA, 2,4-D (synthetic). Produced in shoot/root apices. Functions: Apical dominance, cell elongation, root initiation in cuttings, parthenocarpy, 2,4-D as herbicide.

* Gibberellins (Promoter): $GA_3$ most common. Produced in young leaves, seeds. Functions: Stem elongation (bolting in rosette plants), breaks seed dormancy, fruit enlargement (grapes), malting. * Cytokinins (Promoter): Kinetin, Zeatin.

Produced in root apices, developing buds. Functions: Cell division, lateral shoot growth, delays senescence, morphogenesis in tissue culture. * Abscisic Acid (Inhibitor): ABA. Produced in chloroplasts of mature leaves.

Functions: 'Stress hormone', induces seed/bud dormancy, stomatal closure, promotes abscission. * Ethylene (Inhibitor/Promoter): Gaseous hormone ($C_2H_4$). Produced in ripening fruits, senescing tissues.

Functions: Fruit ripening (climacteric fruits), senescence, abscission, root growth/hair formation, apical hook formation.

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Photoperiodism: — Response to day/night length for flowering. Mediated by phytochrome.

* Short-Day Plants (SDPs): Flower when day length < critical photoperiod (e.g., Xanthium, tobacco, chrysanthemum). Require long uninterrupted dark period. * Long-Day Plants (LDPs): Flower when day length > critical photoperiod (e.g., spinach, radish, wheat). * Day-Neutral Plants (DNPs): Flower irrespective of photoperiod (e.g., tomato, corn, cucumber).

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Vernalization: — Cold treatment required for flowering (e.g., winter wheat, biennials). Perceived by apical meristem.
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Seed Dormancy: — Failure to germinate under favorable conditions. Causes: impermeable seed coat, chemical inhibitors (ABA), immature embryo. Overcome by: scarification, stratification, GAs, nitrates.

Vyyuha Quick Recall

All Good Crops Always Emerge:

Auxins: Apical dominance, Allongation, Adventitious roots.
Gibberellins: Giant growth (stem elongation), Germination, Grape enlargement.
Cytokinins: Cell division, Cytokinesis, Counteract apical dominance.
Abscisic Acid (ABA): Absent growth (dormancy), Absolute stress (stomatal closure), Abscission.
Ethylene: Excellent ripening, Early senescence, Elongation (deep water rice).