Learning objectives
- Name the five main plant hormones and state one function of each
- Explain phototropism and geotropism in terms of auxin distribution
- Give two agricultural uses of plant growth substances
1. The five hormones you must know
Auxin (IAA) causes cell elongation in shoots and controls apical dominance. Gibberellin breaks seed dormancy and causes stem elongation. Cytokinin promotes cell division and delays leaf ageing. Ethylene is a gas that ripens fruit and causes leaf fall. Abscisic acid (ABA) closes stomata under drought and enforces dormancy.
- Auxin — cell elongation, apical dominance, rooting of cuttings
- Gibberellin — germination, stem elongation, malting in brewing
- Cytokinin — cell division, delays senescence
- Ethylene — fruit ripening, abscission
- Abscisic acid — stomatal closure, drought and dormancy
2. Tropisms explained by auxin
A tropism is a directional growth response to a directional stimulus. In phototropism, auxin moves to the shaded side of the shoot, cells there elongate more, and the shoot bends towards the light — a positive phototropic response.
In geotropism (gravitropism), auxin collects on the lower side. In roots a high auxin concentration inhibits elongation, so the upper side grows faster and the root bends downwards (positive geotropism); in shoots the same concentration stimulates growth, so the shoot bends upwards (negative geotropism). The classic answer is: the same hormone, opposite effects, because roots and shoots have different sensitivities.
3. Practical uses in agriculture
Rooting powders contain synthetic auxin (NAA, IBA) to root stem cuttings. Selective herbicides such as 2,4-D are auxin analogues that overstimulate growth in broad-leaved weeds while cereals survive. Ethylene (as ethephon) ripens bananas and mangoes uniformly after transport, and gibberellin increases berry size in seedless grapes.
Key definitions
- Tropism
- A directional growth movement of a plant part in response to a directional external stimulus.
- Apical dominance
- Suppression of lateral bud growth by auxin produced at the shoot tip.
- Etiolation
- Pale, spindly growth with long internodes produced when a plant grows in darkness.
Exam tips
- Always name the stimulus and the direction: 'positive phototropism — shoot grows towards light'.
- For the coleoptile experiments, state the control clearly; marks are given for the comparison, not the drawing.
Practice exam questions & model answers
Q1.Define tropism and give two examples with their stimuli.
A tropism is a directional growth response of a plant part to a directional stimulus. Examples: phototropism — response to light, e.g. shoots grow towards light; geotropism — response to gravity, e.g. roots grow downwards.
Q2.Explain why removing the shoot tip makes a plant bushy.
The shoot tip produces auxin, which suppresses the growth of lateral buds (apical dominance). Removing the tip removes the auxin source, so the lateral buds are released from inhibition and grow out, making the plant bushy.
Q3.State two commercial applications of plant hormones.
Synthetic auxin in rooting powder to induce roots on stem cuttings, and ethylene (ethephon) used to ripen harvested fruit such as banana uniformly; auxin analogues like 2,4-D are also used as selective weed killers.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Put the theory to work — diagnose a real leaf photo or browse the disease library.