1. Regulation of Plant Growth

2. What is a tropism?
A change in the direction of growth or movement of a plant in response to a stimulus.
Occurs slowly/not noticeable
2 types of plant responses:
Tropisms
Nastic movements

3. Tropisms cont’d Phototropism: a growth response to light.
Plants are able to bend towards areas of higher light in an attempt to maximize the amount of light they receive.
The cells on the side away from the light elongate, bending it towards the light.

4. Experiments:

5. Positive phototropism: towards light
Negative phototropism: away from light

6. Tropisms cont’d
Gravitropism: regardless of the orientation of the plant, the root will always grow downward (in the direction of gravity) (positive gravitropism). Also called ‘geotropism’.
Positive:
Negative:
The shoot will always grow away from gravity (negative gravitropism).
Germination of corn seeds occurs regardless of the seed orientation.

7. Tropisms cont’d Thigmotropism: slow response to touch/pressure
i.e. Vines have positive thigmotropisms
Mimosa plant:
Chemotropisms: response to chemicals
Example: Roots grow toward useful minerals in the soil but away from acids. (positive chemotropism)
Hydrotropism: response to water
Example: Roots exhibit positive hydrotropism.

9. Nastic Movements
Turgor Responses / Nastic Movements: Response is quick (within fractions of a second).
Does not depend on the direction of the stimulus.
Movement is reversible.
Turgid cells filled with water quickly lose water and pressure.
i.e. Venus fly traps and Mimosa plants
(3:30 min)

10. Hormones
Most tropisms and growth within a plant is controlled by hormones.
There are growth promoters and growth inhibitors.

11. Hormones cont’d: Growth Promoters
Auxins:
- Often produced in apical meristems (shoot)
- Promotes cell elongation
- Suppresses the growth of lateral branches (ie. ‘apical dominance’)
- Suppresses leaf drop in spring, promote leaf drop in fall

12. Growth promoters cont’d
Gibberellins:
- Promote cell enlargement
- Increase stem length (ie. taller)
- Promotes seed germination
- Increase fruit size

13. Growth Promoters cont’d
Cytokinins:
- Promotes cell division and differentiation
- Stimulate adventitious buds (buds that develop from other places besides shoot apical meristem)
- Delays ageing in leaves

14. Growth Inhibitors Abscisic Acid:
- Synthesized in mature leaves, fruits and roots
- Inhibits intake of CO2 in leaves.
- Inhibits seed germination
- Inhibits bud growth
- Promotes abscission layer in leaves and fruits

15. Growth Inhibitors cont’d
Ethylene:
- Fruits ripen
- Tissues age