what is coleoptile? it is a tough protective sheath that encloses/protects the tip of a shoot.
- refer to page 22-24, chapter 3.
- Plant hormone is a chemical substance produced by plant, which is able to influence the growth & development of the plant.
- Plant hormones can be used to produce fruit without seeds, to ripen fruits, to increase the size of fruit, in growing cuttings, controlling weeds, etc.
- There are 5 classes of plant hormone:
1. Auxins (Indole Acetic Acid) - promotes cell elongation at stem tip, roots initiation, fruit development.
2. Cytokinins - promotes cell division
3. Gibberellins - promotes cell elongation
4. Abscisic acid - promotes seed dormancy
5. Ethylene (ethene) - a gas hormone which is used to ripen fruits, etc - Auxin is a plant hormone that controls tropic response.
- It is produced in the apical meristem of the shoot & root.
- Auxin diffuse from zone of cell division into zone of cell elongation where it stimulates the cell to elongate.
- Diagram below shows:
1. concentration of auxins needed for maximum growth of shoots will inhibit roots growth.
2. at low concentration of auxin hormone, this promotes roots growth but cannot stimulate shoots growth.
Below are some experiments related to Auxins hormone:
1. The coleoptile tip is removed (just above the zone of cell elongation). Gelatin/agar block which contains auxin hormone is then placed at one side of the coleoptile.
Results: coleoptile bending towards the side without the agar block.
2. Role of auxins in controlling phototropism: (unilateral light)
3. Shoot placed horizontally. Auxin hormone will accumulate at the lower side.
4. Diagram below showing geotropism on shoot & root of seedling when the seedlings are placed in different positions. The shoot shows negative geotropism while the root shows positive geotropism.
