Revision notes · Homeostasis and response
Plant hormones
Control and coordination4.5.4.1
Definition: Tropism is a plant's growth response to a directional stimulus, such as light or gravity.
Plants produce hormones to coordinate and control their growth towards or away from light and gravity.
- •Auxin is a plant hormone produced in the tips of shoots and roots that controls growth by affecting cell elongation.
- •An uneven distribution of auxin causes an unequal growth rate on different sides of the plant tissue, resulting in bending.
| Shoots (phototropism) | Roots (gravitropism) | |
|---|---|---|
| Auxin accumulates on the... | Shaded side | Lower side |
| Effect on cell elongation | Stimulates — grows faster, bends towards light | Inhibits — grows slower, bends downwards |
⚠️ Common mistake: Thinking auxin promotes growth everywhere; auxin stimulates cell elongation in shoots, but inhibits cell elongation in roots.
🧠 Remember: Shoots are positively phototropic (grow towards light); roots are positively gravitropic (grow towards gravity).
Use of plant hormones4.5.4.2
Definition: Plant hormones are chemical substances used commercially in agriculture and horticulture to alter plant growth and development.
Plant hormones can be extracted or synthetically produced to control crop growth, fruit ripening, and weed destruction.
| Hormone | Commercial uses |
|---|---|
| Auxins | Selective weedkillers (broad-leaved plants), rooting powder for cuttings, tissue culture (cloning) |
| Ethene | Controls fruit ripening during storage and transport |
| Gibberellins | Ends seed dormancy (starts germination), promotes flowering, increases fruit size |
⚠️ Common mistake: Thinking ethene prevents fruit from ripening; ethene actually speeds up ripening so fruit can be transported unripened and ripened on demand.
🧠 Remember: Auxin grows roots and kills weeds, Ethene ripens fruit, and Gibberellins germinate seeds and enlarge fruit.
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