PLANT GROWTH AND DEVELOPMENT
NCERT Masterclass & NEET Biology Study Module
1. Plant Growth: An Open Process
Growth is defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability is due to the presence of meristems at certain locations.
Because new cells are always being added to the plant body by the activity of the meristem, this form of growth is called the open form of growth.
- Cell Number: A single maize root apical meristem can give rise to more than 17,500 new cells per hour.
- Cell Size: Cells in a watermelon may increase in size by up to 3,50,000 times.
- Length: Growth of a pollen tube.
- Surface Area: Growth in a dorsiventral leaf.
2. Phases & Rates of Growth
The period of growth is generally divided into three phases: Meristematic, Elongation, and Maturation.
Note: Absolute growth rate is the measurement and comparison of total growth per unit time. Relative growth rate is the growth of the given system per unit time expressed on a common basis (e.g., per unit initial parameter).
3. Differentiation, Dedifferentiation and Redifferentiation
- Differentiation: Cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. (e.g., losing protoplasm to form a tracheary element).
- Dedifferentiation: Living differentiated cells can regain the capacity to divide under certain conditions. (e.g., formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells).
- Redifferentiation: Meristems formed by dedifferentiation divide and produce cells that once again lose the capacity to divide but mature to perform specific functions. (e.g., secondary xylem/phloem).
4. Development & Plasticity
Development is a term that includes all changes that an organism goes through during its life cycle from germination of the seed to senescence. Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called Plasticity.
- Developmental Heterophylly: Leaves of juvenile plants are different in shape from those in mature plants (e.g., Cotton, Coriander, Larkspur).
- Environmental Heterophylly: Leaves in air are different from leaves in water (e.g., Buttercup / Ranunculus).
5. Plant Growth Regulators (PGRs)
PGRs are small, simple molecules of diverse chemical composition. They are broadly divided into two groups: Plant growth promoters (Auxins, Gibberellins, Cytokinins) and Plant growth inhibitors (Abscisic acid). Ethylene is largely an inhibitor but fits in either group.
A. Auxins (IAA, IBA, NAA, 2,4-D)
First isolated from human urine. Produced by growing apices of stems and roots.
- Initiate rooting in stem cuttings.
- Promote flowering (e.g., in pineapples).
- Prevent early fruit and leaf drop, but promote abscission of older leaves.
- Apical Dominance: Growing apical bud inhibits the growth of lateral (axillary) buds. Removal of shoot tips (decapitation) usually results in the growth of lateral buds (used in tea plantations, hedge-making).
- Induce parthenocarpy (e.g., in tomatoes).
- 2,4-D: Widely used as a herbicide to kill dicotyledonous weeds. Does not affect mature monocot plants.
B. Gibberellins ($GA_3$)
All GAs are acidic. There are more than 100 known gibberellins.
- Produce an increase in length of axis (used to increase length of grapes stalks).
- Elongate and improve shape of fruits like apple.
- Delay senescence, allowing fruits to be left on the tree longer.
- Speed up the malting process in the brewing industry.
- Sugarcane: Spraying sugarcane with gibberellins increases the length of the stem, increasing yield by as much as 20 tonnes per acre.
- Bolting: Promote internode elongation just prior to flowering in rosette plants (e.g., beet, cabbages).
C. Cytokinins (Kinetin, Zeatin)
Discovered as kinetin (a modified form of adenine) from autoclaved herring sperm DNA. Zeatin was isolated from corn kernels and coconut milk. Synthesized in regions of rapid cell division (root apices, developing shoot buds, young fruits).
- Help overcome apical dominance.
- Promote nutrient mobilisation which helps in the delay of leaf senescence.
- Produce new leaves, chloroplasts in leaves, lateral shoot growth, and adventitious shoot formation.
D. Ethylene
A simple gaseous PGR. Synthesized in large amounts by tissues undergoing senescence and ripening fruits.
- Promotes transverse growth, swelling of axis, and apical hook formation in dicot seedlings.
- Highly effective in fruit ripening (enhances respiration rate, called respiratory climacteric).
- Breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers.
- Promotes rapid internode/petiole elongation in deep water rice plants.
- Promotes root growth and root hair formation, increasing absorption surface.
- Used commonly as Ethephon (hastens fruit ripening in tomatoes/apples and accelerates abscission in flowers/fruits like thinning of cotton, cherry, walnut).
E. Abscisic Acid (ABA)
Discovered for its role in regulating abscission and dormancy. Acts as a general plant growth inhibitor and inhibitor of plant metabolism.
- Inhibits seed germination.
- Stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Hence, it is called the stress hormone.
- Important for seed development, maturation, and dormancy (helps seeds withstand desiccation).
- Acts as an antagonist to GAs.
🚀 NEET PLANT GROWTH & DEVELOPMENT MEGA QUIZ (100 MCQ)
Solve the 5 parts below to master Growth Rates, Plasticity, and all 5 PGRs.
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