⚡ Quick Revision: Plant Hormones (Phytohormones)
🔹 Introduction to Phytohormones
- ✔ Definition: Chemical messengers that regulate growth, development, and responses to stimuli.
- ✔ Transport: Produced in minute quantities and transported through phloem or cell-to-cell diffusion.
🔹 Major Plant Growth Regulators
| Hormone | Primary Function | Site of Synthesis |
|---|---|---|
| Auxins | Cell elongation, Apical dominance | Shoot tips, Root tips |
| Gibberellins | Stem elongation, Breaking seed dormancy | Young leaves, Seeds |
| Cytokinins | Cell division, Delaying senescence | Rapidly dividing cells |
| Ethylene | Fruit ripening (Gaseous hormone) | Ripening fruits |
| Abscisic Acid (ABA) | Growth inhibition, Closing of stomata | Chloroplasts of leaves |
Apical Dominance: The phenomenon where the main central stem grows more strongly than other side stems because the terminal bud suppresses the growth of lateral buds.
Gibberellins with Auxins. Both promote growth, but Gibberellins specifically cause internodal elongation (making the stem longer between nodes), while Auxins focus on the tip.
⚡ Quick Revision: Plant Movements
🔹 Tropic Movements (Tropisms)
Growth movements where the direction is determined by the direction of the stimulus.
- ✔ Phototropism: Response to light. (Shoots are +ve, Roots are -ve).
- ✔ Geotropism: Response to gravity. (Roots are +ve, Shoots are -ve).
- ✔ Hydrotropism: Response to water. (Roots are +ve).
- ✔ Thigmotropism: Response to contact (e.g., tendrils of climbers).
🔹 Nastic Movements
Movements where the direction is independent of the stimulus direction.
- ✔ Seismonastic: Response to touch (e.g., Mimosa pudica / Touch-me-not).
- ✔ Nyctinasty: Sleep movements (e.g., leaves folding at night).
Pulvinus: The swollen base of a petiole or leaflet in plants like Mimosa pudica, which undergoes turgor changes to cause movement.
Chemotropism with other tropisms. Remember: Chemotropism is specifically the growth of the pollen tube towards the ovule in response to chemical secretions.