Plant Growth and Development
[PREMIUM NCERT MODULE • PLANT PHYSIOLOGY]
Plant growth is Indeterminate due to meristems (Root Apical, Shoot Apical, and Lateral meristems).
- Primary Growth: Elongation of axis (Apical meristems).
- Secondary Growth: Increase in girth (Lateral meristems: Vascular cambium, Cork cambium).
- Phases: Meristematic -> Elongation -> Maturation.
- Differentiation: Cells specialize for function (e.g., Tracheary elements).
- Dedifferentiation: Differentiated cells regain ability to divide (e.g., formation of interfascicular cambium).
- Redifferentiation: Dedifferentiated cells produce cells that lose division capacity and mature again.
| PGR | Main Functions | Discovery Context |
|---|---|---|
| Auxins | Apical dominance, Cell elongation, Herbicide (2,4-D). | Coleoptile (Charles & Francis Darwin). |
| Gibberellins | Internode elongation (Sugarcane), Bolting, Delay senescence. | Bakanae (Foolish Seedling) disease. |
| Cytokinins | Cell division, Overcome apical dominance, Delay leaf senescence. | Kinetin from herring sperm DNA. |
| Ethylene | Fruit ripening, Respiratory Climacteric, Horizontal growth. | Gaseous hormone; H.H. Cousins. |
| Abscisic Acid | Stress hormone, Stomatal closure, Seed dormancy. | Promoter of abscission/dormancy. |
- Photoperiodism: Response to duration of light/dark. Short-day, Long-day, and Day-neutral plants. Site of perception = Leaves.
- Vernalisation: Promotion of flowering by a period of low temperature. Prevents precocious reproductive development.
Inability of seed to germinate due to internal factors. Caused by:
- Impermeable/Hard seed coat.
- Chemical inhibitors (ABA, Phenolic acids, Para-ascorbic acid).
- Immature embryos.
Plant Growth HOTS Questions
[ HORMONE LOGIC & GROWTH MATH ]
ABA helps plants tolerate various environmental stresses. For example, it stimulates the closure of stomata during water scarcity (water stress) to prevent transpiration. It also promotes seed dormancy, helping the plant "wait out" unfavorable conditions.
Auxins are used to prevent premature fruit and leaf drop at early stages. However, later in the cycle, other hormones like Ethylene promote abscission. Additionally, Auxins are used for rooting in stem cuttings and to induce parthenocarpy (seedless fruit) in tomatoes.
Removal of shoot tips (decapitation) eliminates the source of Auxins, which are responsible for Apical Dominance. Once the apical bud is gone, the inhibition on lateral buds is removed, allowing the plant to become bushy and spread out laterally.
Geometrical growth (where both daughter cells divide) requires unlimited nutrients and space. In nature, competition and limited resources eventually slow down growth, leading to a Sigmoid (S) curve rather than an infinite exponential rise.
It is an example of Environmental Plasticity. The plant produces broadly lobed leaves in air but finely dissected leaves under water. This adaptation helps the plant survive different habitats (aquatic vs terrestrial) by optimizing photosynthesis and reducing water drag.
Gibberellins; to increase stem length (and sugar yield).
The leaves (which then transmit "Florigen" to the apex).
Development = Growth + Differentiation.
Sharp rise in respiration rate during fruit ripening caused by Ethylene.
Synthetic auxin used as a weedicide to kill broad-leaved dicot weeds.
Apical meristems of shoot and root.
Sudden internode elongation just before flowering (e.g., Cabbage).
IAA (Indole-3-Acetic Acid).
Skoog and Miller isolated 'Kinetin' from herring sperm DNA.
Swelling of axis and apical hook formation.
GAs cause cell elongation and axis lengthening (e.g., Grapes).
Dormancy: Internal factors. Quiescence: Lack of water/environment.
Vascular cambium and Cork cambium.
Corn kernels and coconut milk (Natural cytokinin).
Lt = L0 + rt.
Plant Growth: 50 Mastery Facts
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