Controlling Plant Growth
- Plants can respond to stimuli in their environment.
- They make plant hormones that control/coordinate:
→ Growth of shoots and roots
→ Flowering
→ Ripening of fruits
- Plant hormones don't work like animal hormones. Therefore, plants don't have blood or endocrine glands. This means hormones cannot travel in the blood.
Tropisms:
- Tropisms are directional growth responses to stimuli.
- Plant responding to light → Phototropism
- Plant responding to gravity → Gravitropism/geotropism
- Plant growth response to electing water → Hydrotropism
- Plant shoots are positively phototropic and negatively geotropic.
- In contrast, plant roots are positively hydrotropic.
- Advantage of shoots being positively phototropic → so they can absorb more light and positively gravitropic → photosynthesise.
- An advantage of roots being positively gravitropic is that they can absorb more water and minerals (e.g., nitrogen), which helps anchor the plant well.
How Shoots Bend?
- The tip of the shoots produces plant hormones known as auxins.
- These move through a plant in solution.
- Auxins make cells divide by mitosis/elongate.
- Uneven light sends more auxin to the shaded side.
- The cells elongate more on the shady side, and the shoots bend toward the light.
- Auxin travels to the lower side of the root or shoots during gravitropism.
- Low auxin levels stimulate growth in roots (opposite of shoots).
Using Auxin:
- Meristem cells are the plant equivalent of stem cells.
→ They can be grown into new plants that are clones of the original plant.
Tissue Culture (a use of auxin):
- We take a few meristem cells and grow them in a culture medium into many new plants.
- The culture medium contains auxins (water, O₂, sucrose, glucose, etc.) that stimulate mitosis.
Weed Killers:
- Auxins can be used in high concentrations to kill weeds.
- They're called selective weedkillers → broad-leaved plants are much more sensitive than narrow-leaved plants.
- They can eradicate all weeds without harming the crops.
- However, weed killers can still impact the food chain and biodiversity in an area, as many insects feed on broad-leaved plants.
Taking Cuttings:
- Rooting powders can also contain auxins.
→ Dip the end of the plant in auxin to create a new plant and aid in root growth.
Experiments
Clinostat:
- It functions as a turntable, gradually rotating seeds to ensure they receive light from every angle.
- If you cut off the tip of a shoot, it stops growing. Repositioning it to the cut stem initiates its growth again.
→ This happens as the tip of the shoot contains/makes auxin, which helps the plant grow.
- Gelatin allows soluble substances, e.g., auxin, through.
- Mica doesn't allow this.
Gibberellins
- Plant hormones produced by seeds such as barley.
- Gibberellins initiate seed germination.
- Seed Germination: The process by which a seed, spore, or other reproductive body sprouts after a period of dormancy.
- While some seeds, like barley, germinate immediately, others are considered dormant.
- Dormant seed: The inability of a viable seed to germinate under conditions favourable for germination.
- You can spray gibberellins on seeds to speed up germination.
- Some plants can also benefit from the spraying of gibberellins.
- Encourage growth (e.g., apple trees).
- Increase size
→ Cherries cause the stem to elongate, leaving more space for the fruit to grow.
→ Grapes cause the stem to elongate, leaving more space for fruit to grow (seedless).
Ethene:
- Ethene is a hydrocarbon gas (C₂H₄).
- Many plants release ethene as the fruits begin to ripen.
→ e.g., bananas.
- Ethene is also involved in other processes in plants, like cell division and leaf drop.
- Many people don't store bananas in the same bowl as other fruits. Therefore, ripening bananas emit a lot of ethylene gas, which causes other fruits to spoil quickly.
- It's possible to pick and transport many fruits before they're ripe. Once they arrive at their destination, they can undergo ripening. This process uses ethylene to help the fruits have a longer shelf life.
- Ethrel: A commercial chemical that releases ethene gas.
- Ethylene Inhibitors: Applied to fruits to delay or prevent over-ripening.