13Class 11 Biology
Plant Growth and Development short notes
Short notes on growth, differentiation and the five main plant growth regulators. Most questions ask which hormone does what.
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Growth
- Growth is irreversible and permanent, and it's measurable.
- Phases: meristematic, elongation and maturation.
- Arithmetic growth forms a straight line, and geometric growth an S-shaped (sigmoid) curve.
- Differentiation, dedifferentiation and redifferentiation change what cells can do.
Growth regulators
- Auxins (IAA, IBA, NAA, 2,4-D): apical dominance, root initiation, parthenocarpy, and weedicide use.
- Gibberellins: stem elongation, bolting, and longer sugarcane stems and grape stalks. They also speed up malting.
- Cytokinins (zeatin, kinetin): cell division, new leaf and shoot growth, and they delay ageing.
- Ethylene: a gas that ripens fruit and promotes ageing, and causes the triple response.
- Abscisic acid: the stress hormone. It closes stomata, keeps seeds dormant, and inhibits growth.
Discoveries
- Darwin observed phototropism in canary grass, and Went isolated auxin from oat tips.
- Gibberellins were discovered through foolish seedling disease of rice, from the fungus Gibberella fujikuroi.
- Kinetin was first found in autoclaved herring sperm DNA.
- Cousins found the ripening gas, later identified as ethylene.
Where students slip up
- Saying auxin always promotes growth. High doses inhibit, and this is how 2,4-D kills weeds.
- Mixing up ABA and ethylene, since both are linked with ageing and stress.
- Treating ethylene as a liquid hormone. It's a gas.
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