Plant Growth and Development

Plant Growth and Development NEET Notes | High-Yield Biology PYQs

Growth and Its Measurement

Growth: An irreversible, permanent increase in the size, weight, or volume of an organ, its parts, or an individual cell. It is accompanied by metabolic processes (both anabolic and catabolic).

Plant growth is unique because plants retain the capacity for unlimited growth throughout their life due to the presence of meristems at specific locations.

Measurement of Growth

Growth is measured by a variety of parameters like increase in fresh weight, dry weight, length, area, volume, and cell number.

Example: A single maize root apical meristem can give rise to more than 17,500 new cells per hour.
The growth of plants can be measured by an instrument called an Auxanometer AIPMT 2000.

Phases of Growth

Meristematic phase: Constantly dividing cells at the root and shoot apex.
Elongation phase: Proximal to the meristematic zone; cells show increased vacuolation, cell enlargement, and new cell wall deposition.
Maturation phase: Proximal to the elongation zone; cells attain maximum size in terms of wall thickening and protoplasmic modifications.

Growth Rates and Sigmoid Curve

Arithmetic Growth: Only one daughter cell continues to divide while the other differentiates and matures. Mathematically expressed as L_t = L₀ + rt (forms a linear curve).
Geometric Growth: Both progeny cells retain the ability to divide. Mathematically expressed as W₁ = W₀ e^rt.
Sigmoid (S-shaped) Curve: A plot of growth against time in geometric growth produces an S-shaped curve. It consists of a lag phase, a log (exponential) phase, and a stationary phase.
Crucial detail: The Sigmoid curve is a characteristic feature of living organisms growing in a natural environment AIPMT 1999, 2004.

Differentiation, Dedifferentiation, and Redifferentiation

Differentiation: Cells derived from root/shoot apical meristems and cambium mature to perform specific functions. They undergo major structural changes (e.g., losing protoplasm and developing strong, elastic lignocellulosic secondary walls to form tracheary elements).
Dedifferentiation: Living differentiated cells that had lost the capacity to divide can regain the capacity of division under certain conditions.
Examples: Formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
Redifferentiation: Dedifferentiated cells (meristems) divide and produce cells that once again lose the capacity to divide but mature to perform specific functions.
Examples: Secondary xylem, secondary phloem, cork.

Development and Plasticity

Development includes all changes that an organism goes through during its life cycle, from germination of the seed to senescence.

Plasticity: Plants follow different developmental pathways in response to the environment or phases of life to form different kinds of structures NEET 2021.
Heterophylly: A classical example of plasticity where different shapes of leaves are present on the same plant.
- Developmental Heterophylly: Difference in shapes of leaves produced in juvenile versus mature/adult phases. Seen in cotton, coriander, and larkspur NEET 2022, 2024.
- Environmental Heterophylly: Difference in shapes of leaves produced in air versus those produced in water. Seen in buttercup (Ranunculus) NEET 2020, 2024.

Plant Growth Regulators (PGRs)

Chemical Nature and Discovery of PGRs

PGR Category	Chemical Nature	Key Discovery Facts & Bioassays
Auxins	Indole compounds (e.g., Indole-3-acetic acid / IAA)	Charles & Francis Darwin observed phototropism in canary grass (Phalaris) coleoptiles NEET 2024. F.W. Went successfully isolated auxin from the tips of oat (Avena sativa) coleoptiles AIPMT 2014, 2015, NEET 2018, 2024.
Gibberellins	Terpenes (e.g., Gibberellic acid / GA₃)	E. Kurosawa (1926) found the 'bakanae' (foolish seedling) disease of rice was caused by the fungal pathogen Gibberella fujikuroi NEET 2024.
Cytokinins	Adenine derivatives (Purines) (e.g., Kinetin, Zeatin)	Skoog and Miller crystallized Kinetin from herring sperm DNA. Letham isolated Zeatin from corn kernels and coconut milk NEET 2024.
Ethylene	Gas (C₂H₄) AIPMT 2005	Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of unripened bananas.
Abscisic Acid (ABA)	Derivatives of Carotenoids AIPMT 2012	Researchers purified three inhibitors (Inhibitor-B, Abscission II, Dormin) which proved chemically identical as ABA.

Physiological Effects of Auxins

Types: Natural (IAA, IBA isolated from plants) and Synthetic (NAA, 2,4-D).
Precursor & Mineral: Synthesized from the amino acid Tryptophan. Zinc (Zn) is essential for auxin synthesis AIPMT 2003.
Apical Dominance: The growing apical bud inhibits the growth of lateral (axillary) buds. Decapitation (removal of shoot tips) allows lateral buds to grow, widely applied in tea plantations and hedge-making AIPMT 2012, NEET 2020.
Root Initiation: Widely used to initiate rooting in stem cuttings for plant propagation AIPMT 2001, 2011.
Flowering & Fruiting: Promotes flowering (e.g., in pineapples). Induces parthenocarpy (seedless fruits) in tomatoes AIPMT 2015.
Abscission Control: Prevents fruit and leaf drop at early stages but promotes the abscission of older, mature leaves and fruits NEET 2017.
Herbicides/Weedicides: 2,4-D is widely used to kill dicotyledonous weeds; it does not affect mature monocotyledonous plants NEET 2024.
Tissue functions: Controls xylem differentiation and helps in cell division NEET 2020. Responsible for phototropism and geotropism in stems/roots AIPMT 2007.

Physiological Effects of Gibberellins (GAs)

Over 100 GAs are known, all of them are acidic (GA₁, GA₂, GA₃, etc.). GA₃ was one of the first discovered and remains the most intensely studied.

Axis/Stem Elongation: Increases the length of axes (e.g., grape stalks). Sprayed on sugarcane crops, it increases the length of the stem, increasing the yield by as much as 20 tonnes per acre NEET 2020, 2024.
Bolting: Promotes rapid internode elongation just prior to flowering in rosette plants like beet, cabbages, and many plants with rosette habit NEET 2017, 2024.
Brewing Industry: Speeds up the malting process in brewing industries AIPMT 2014, NEET 2020.
Seed Germination: Promotes seed germination by mobilizing stored food reserves, inducing the synthesis of α-amylase in barley and cereal grains AIPMT 1990, 2005, NEET 2024.
Delay of Senescence: Allows fruits to be left on the tree longer to extend the market period.
Crucial detail: GAs act as an antagonist to Abscisic Acid (ABA) regarding seed dormancy and germination NEET 2024.

Physiological Effects of Cytokinins

Synthesized in regions where rapid cell division occurs (root apices, developing shoot buds, young fruits).
Overcoming Apical Dominance: Cytokinins promote the growth of lateral shoots, actively counteracting the apical dominance induced by auxins NEET 2020.
Delay of Leaf Senescence: Promotes nutrient mobilization which helps in the delay of leaf senescence (known as the Richmond-Lang effect).
Tissue Culture (Morphogenesis): The ratio of Auxin to Cytokinin controls organ differentiation. High cytokinin-to-auxin ratio promotes shoot differentiation, while high auxin-to-cytokinin ratio promotes root differentiation AIPMT 2003, 2004.

Physiological Effects of Ethylene

A simple gaseous PGR synthesized in large amounts by tissues undergoing senescence and ripening fruits. Ethephon is the most widely used source in agriculture.

Fruit Ripening: Highly effective in fruit ripening. It enhances the respiration rate during ripening, a phenomenon called climacteric respiration (respiratory climacteric) AIPMT 1998, NEET 2019, 2020.
Flowering & Fruit Set: Highly effective in inducing flowering and synchronizing fruit set in pineapples and mangoes NEET 2019, 2020, 2024.
Deep Water Rice Plants: Promotes rapid internode/petiole elongation, helping the leaves/upper parts of the shoot remain above water NEET 2021.
Dormancy Breaking: Breaks seed and bud dormancy; initiates germination in peanut seeds and sprouting of potato tubers NEET 2024.
Root Growth: Promotes root growth and root hair formation, which vastly increases the absorption surface of the plant AIPMT 2007, NEET 2024.
Abscission & Thinning: Promotes senescence and abscission of plant organs (leaves and flowers). Used as a thinning agent in cotton, cherry, and walnut AIPMT 2001, NEET 2017.

Physiological Effects of Abscisic Acid (ABA)

Acts as a general plant growth inhibitor and an inhibitor of plant metabolism.

Stress Hormone: Stimulates the closure of stomata during severe drought or water stress, increasing the tolerance of plants to various kinds of stresses AIPMT 2001, NEET 2024.
Seed Dormancy: Plays a major role in inducing seed dormancy (inhibits seed germination), helping seeds withstand desiccation and other factors unfavorable for growth NEET 2020.
Acts as an antagonist to GAs (Gibberellins) NEET 2024.

Protected Content