1. Plant growth regulators
Plants have no nervous system. They coordinate growth with plant growth regulators (phytohormones): chemicals made in small amounts in one part of the plant that move to other parts and control growth and development.
| Group | Regulators | General effect |
|---|---|---|
| Growth promoters | Auxins, gibberellins, cytokinins | Speed up cell division, cell elongation, germination, fruit growth |
| Growth inhibitors | Abscisic acid (ABA), ethylene | Slow growth; bring about dormancy, ripening, ageing and leaf fall |
Hormones often act together or against each other: gibberellin breaks dormancy while abscisic acid maintains it; auxin from the tip suppresses side buds while cytokinin helps them grow.
Key terms
- Plant growth regulator
A chemical made in small amounts in one part of a plant that controls growth elsewhere.
- Growth promoter
A regulator that speeds up growth: auxin, gibberellin, cytokinin.
- Growth inhibitor
A regulator that slows growth: abscisic acid, ethylene.
2. Effects of each regulator
| Regulator | Main effects | Everyday use |
|---|---|---|
| Auxin (made at shoot tips) | Cell elongation just below the tip; apical dominance (lateral buds stay dormant); root formation; fruit growth | Rooting powder for cuttings; seedless fruits; prevents early fruit drop; synthetic 2,4-D kills broad-leaved weeds |
| Gibberellin | Lengthens internodes (dwarf plants grow tall); breaks dormancy of seeds and buds; bolting of rosette plants | Larger grapes; faster germination of barley |
| Cytokinin (made in root tips; rich in coconut water) | Promotes cell division; delays ageing of leaves; helps lateral buds grow | Keeps cut leaves and flowers fresh; tissue culture |
| Abscisic acid — the stress hormone | Closes stomata when water is short; keeps seeds and buds dormant; promotes leaf fall | Helps crops survive drought |
| Ethylene — a gas | Ripens fruits; hastens ageing and fall of leaves, flowers and fruits | Ripening of bananas and mangoes in closed rooms |
Apical dominance. The shoot tip makes auxin, which moves down and keeps lateral buds dormant. Removing the tip (pinching, pruning, plucking tea) removes the auxin, so side shoots grow and the plant becomes bushy.
| Hormone | One key effect |
|---|---|
| Auxin | Apical dominance |
| Gibberellin | Longer internodes |
| Cytokinin | Cell division |
| Abscisic acid | Closes stomata |
| Ethylene | Fruit ripening |
Key terms
- Apical dominance
Suppression of lateral buds by auxin from the shoot tip.
- Bolting
Sudden lengthening of the stem of a rosette plant before flowering, caused by gibberellin.
- Senescence
Ageing of a leaf or other part; delayed by cytokinins.
- Parthenocarpy
Formation of fruit without fertilisation, giving seedless fruits.
3. Tropic movements
A tropism (tropic movement) is the growth of a plant part towards (positive) or away from (negative) a stimulus coming from one direction. It is slow and permanent because it results from unequal growth on the two sides of the organ.
| Tropism | Stimulus | Positive | Negative |
|---|---|---|---|
| Phototropism | Light | Shoot | Root |
| Geotropism | Gravity | Root | Shoot |
| Hydrotropism | Water | Root | — |
| Thigmotropism | Touch / contact | Tendrils of pea, gourd, grapevine coil round a support | — |
| Chemotropism | Chemicals | Pollen tube grows towards the ovule | — |
The folding of Mimosa (touch-me-not) leaflets when touched is not a tropism: it is not a growth movement and does not depend on the direction of the touch.
Experiments
- Geotropism: a seedling laid flat curves its root down and its shoot up. On a slowly rotating clinostat, gravity acts on all sides in turn and the seedling grows straight.
- Hydrotropism: seeds in a tilted sieve of moist sawdust: roots grow down out of the holes, then curve back into the moist sawdust.
Key terms
- Tropism
Growth movement of a plant part in response to a one-sided stimulus.
- Positive tropism
Growth towards the stimulus.
- Clinostat
An instrument that rotates a plant slowly so that gravity acts equally on all sides.
4. Phototropism and auxin
How a shoot bends towards light
- The tip of the shoot senses light coming from one side.
- Auxin moves to the shaded side and travels down the stem.
- Cells on the shaded side elongate more than those on the lit side.
- The shaded side becomes longer, so the shoot curves towards the light. The bending happens a short distance below the tip, in the zone of elongation.
Coleoptile experiments (oat seedlings lit from one side): an intact coleoptile bends; one with its tip cut off does not; one with an opaque cap over the tip does not; one with an opaque collar below the tip still bends. So the tip perceives light and a chemical messenger (auxin) passes down to the region that bends.
Phototropism experiment: grow a potted seedling in a box with an opening on one side. After a few days the shoot curves towards the opening. A control seedling lit from above grows straight.
In roots, auxin collecting on the lower side slows elongation, so the upper side grows more and the root curves downwards.
Key terms
- Phototropism
Growth movement in response to one-sided light.
- Coleoptile
The sheath covering the young shoot of a grass seedling such as oat.
- Auxin
Growth promoter made at the shoot tip; causes cell elongation.
Exam tips
Common mistakes
- Writing that auxin collects on the lit side: it collects on the shaded side.
- Calling ethylene a growth promoter: it is an inhibitor (and a gas).
- Saying abscisic acid opens stomata: it closes them.
- Naming a tropism without positive or negative.
- Saying the tip bends: the tip senses; the region below it bends.
How marks are usually awarded
- 1 mark: a hormone or tropism named, a biological term, an odd one out with its category.
- 2 marks: an effect with its explanation, or two uses.
- 3 marks: an experiment (set-up, observation, conclusion) or a labelled diagram of phototropism.