OCR A Level Biology A: Plant responses and plant hormones (H420)

This guide covers learning outcomes 5.1.5(a) to 5.1.5(f) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. They sit in module 5, Communication, homeostasis and energy. Use the course hub and the printable checklist to track progress, then test yourself with the revision notes, the practice questions and the free diagnostics. Related pages: exchange and transport for stomata and water movement, and development of practical skills.

What this unit covers

Outcome	What you must be able to do
5.1.5(a)(i)	Describe types of plant response: abiotic stress, herbivory (alkaloids, pheromones, folding on touch) and the range of tropisms
5.1.5(a)(ii)	Describe practical investigations into phototropism and geotropism
5.1.5(b)	State the roles of plant hormones, including leaf loss in deciduous plants, seed germination and stomatal closure
5.1.5(c)	Interpret experimental evidence for auxin in apical dominance, including effects of concentration
5.1.5(d)	Interpret experimental evidence for gibberellin in stem elongation and seed germination, including effects of concentration
5.1.5(e)	Describe and analyse practical investigations into hormone effects on growth (serial dilution is an opportunity)
5.1.5(f)	Describe commercial uses: controlling ripening, rooting powders, hormonal weedkillers

For (c) and (d) the specification says you apply your understanding to experiments you have not seen. You are not asked to recall named experiments.

Types of plant response (5.1.5(a)(i))

Plants cannot move away from danger, so they change how they grow or change their chemistry. Hormones coordinate most responses.

A tropism is a directional growth response to a directional stimulus. Growth towards the stimulus is positive and growth away is negative.

Tropism	Stimulus	Typical response
Phototropism	Light	Shoots positive, many roots negative
Geotropism (gravitropism)	Gravity	Roots positive, shoots negative
Hydrotropism	Water	Roots grow towards moist soil
Thigmotropism	Touch or contact	Tendrils coil round a support
Chemotropism	Chemicals	Pollen tubes grow towards the ovule

A nastic response is not directional: the movement does not depend on the direction of the stimulus.

Abiotic stress is stress from the non-living environment, such as drought, cold or low light. Closing stomata under water shortage and shedding leaves in winter are the two examples the specification points to, and both are explained below.

Herbivory is feeding by animals. Plants use three responses the specification names.

1. Alkaloids. These bitter, nitrogen-containing chemicals are toxic or deterrent to many herbivores. Caffeine, nicotine and morphine are examples.
2. Pheromones. Damaged plants can release airborne chemical signals. Neighbouring plants that detect them can switch on their own defences before they are attacked. Some signals also attract animals that feed on the herbivore.
3. Folding on touch. In the sensitive plant, Mimosa pudica, touching a leaf makes the leaflets fold within seconds. Cells at the base of each leaflet lose turgor: ions leave, water follows by osmosis, and the leaflet droops. Folding may deter herbivores. It is nastic because the folding does not depend on the direction of the touch.

Tropisms and auxin (5.1.5(a)(ii))

The main hormone in tropisms is auxin, mainly indole-3-acetic acid (IAA). It is made in shoot tips and root tips and moves to where cells elongate. Unequal auxin distribution gives unequal growth, and the organ bends.

Phototropism in a shoot. Light from one side makes auxin move to the shaded side. Auxin there stimulates cell elongation, so the shaded side grows faster and the shoot bends towards the light. The usual explanation is that auxin makes cells pump hydrogen ions into the cell wall. The wall becomes acidic, bonds between cellulose fibres loosen, and turgor stretches the cell.

Geotropism. Auxin collects on the lower side of a horizontal organ. In a shoot that stimulates elongation on the lower side, so the shoot curves upwards (negative geotropism). In a root the same side receives a concentration that inhibits elongation. The upper side grows faster and the root curves downwards (positive geotropism). Roots are more sensitive to auxin than shoots, so the same hormone gives opposite effects.

Practical: phototropism and geotropism (PAG11)

Use seedlings that grow quickly, such as cress, mustard or oat seedlings.

1. Grow identical seedlings in the dark.
2. For phototropism, illuminate a set from one side only. Keep a control set in the dark or with light from all sides.
3. For geotropism, lay seedlings on their sides in the dark. Rotating some slowly on a clinostat cancels the one-sided effect of gravity, so they act as a control.
4. After a fixed time, measure the angle of curvature with a protractor or from a photograph with the same scale.
5. Use at least ten seedlings per treatment, calculate the mean angle and compare treatments.

Control species, age, temperature, light intensity and distance, and humidity. Wash hands after handling compost. Measuring an angle by eye is subjective, so photograph every seedling at a fixed distance.

Roles of plant hormones (5.1.5(b))

Hormone	Main roles in this unit
Auxin	Cell elongation, tropisms, apical dominance, delays leaf fall
Gibberellin	Stem elongation, starts germination
Abscisic acid (ABA)	Closes stomata, keeps seeds dormant
Ethene	Leaf fall, fruit ripening

Stomatal closure. Under water stress, leaves release more ABA. ABA binds to receptors on guard cells, and calcium ions enter the cytoplasm as a signal. Channels open and potassium ions leave. The guard cells’ water potential becomes higher than that of their surroundings, water leaves by osmosis, turgor falls and the stomata close. Less water vapour is lost, but less carbon dioxide enters.

Leaf loss in deciduous plants. As days shorten and temperature falls, auxin from the leaf declines. Ethene then promotes formation of an abscission zone at the base of the leaf stalk. Cells in the separation layer make enzymes that digest cell walls, and the leaf falls. A protective layer on the stem side seals the scar against infection. Losing leaves reduces water loss when water is hard to absorb.

Seed germination. ABA helps keep a seed dormant. When the seed takes up water, the embryo makes gibberellin. Gibberellin diffuses to the aleurone layer and stimulates transcription of the gene for amylase. Amylase hydrolyses stored starch to maltose, which is converted to glucose. The glucose supplies respiration and growth in the embryo until it can photosynthesise.

Auxin and apical dominance (5.1.5(c))

In apical dominance, the apical bud suppresses growth of lateral buds. Auxin from the apical bud passes down the stem and inhibits lateral buds. Removing the apical bud lets laterals grow. Pinching out shoot tips therefore gives a bushier plant.

Worked example. Pea seedlings were treated, and the mean length of the lateral shoot was measured 8 days later.

Treatment	Lateral shoot length / mm
Apex intact	3
Apex removed, plain lanolin on stump	40
Apex removed, lanolin with 0.01% IAA	29
Apex removed, lanolin with 0.1% IAA	12
Apex removed, lanolin with 1.0% IAA	4

1. Plain lanolin is the control for the IAA treatments, so compare each with 40 mm.
2. Percentage inhibition at 0.01% IAA = (40 − 29) ÷ 40 × 100 = 27.5%. At 0.1% it is (40 − 12) ÷ 40 × 100 = 70%. At 1.0% it is (40 − 4) ÷ 40 × 100 = 90%.
3. Conclusion: the more auxin applied to the stump, the greater the inhibition, so auxin replaces the effect of the apex. At 1.0% the lateral shoot is almost as short as with the apex intact (4 mm and 3 mm).

Gibberellin and stem elongation (5.1.5(d))

Gibberellins stimulate cell elongation and division in stems. Dwarf varieties often lack a working gibberellin pathway. Two different faults give a dwarf plant, and experiments separate them: if applying gibberellin restores height, the plant is short because it makes too little; if there is no response, the plant cannot respond.

Worked example. Dwarf pea plants were sprayed with gibberellin (GA) solutions. Mean stem growth over 10 days is shown. Tall peas grew 52 mm with no treatment.

GA concentration / mg dm⁻³	0	0.001	0.01	0.1	1.0
Mean growth / mm	12	15	27	46	49

1. Percentage increase at 0.01 mg dm⁻³ = (27 − 12) ÷ 12 × 100 = 125%. At 0.1 mg dm⁻³ it is (46 − 12) ÷ 12 × 100 = 283%.
2. Between 0.1 and 1.0 mg dm⁻³ growth rises by only 3 mm, so the response is levelling off. Something else, such as the number of receptors, now limits growth.
3. At 1.0 mg dm⁻³ the dwarf plants reach 49 ÷ 52 × 100 = 94% of the tall plants. They respond, so they make too little gibberellin rather than failing to respond.

For germination, the standard evidence is the barley half-seed. Seeds without an embryo do not break down their starch. Adding gibberellin to an embryo-less half-seed on starch agar produces a clear zone after iodine staining, showing amylase is made. Abscisic acid reduces the clear zone.

Practical investigations into hormone effects (5.1.5(e))

A typical investigation grows cress seedlings on filter paper in solutions of different hormone concentration. Make the concentrations by serial dilution, which gives a series of equal dilution factors.

Method for a 1 in 10 series. Start with a 1.0 g dm⁻³ stock. Transfer 1 cm³ of stock to 9 cm³ of distilled water and mix: 0.1 g dm⁻³. Take 1 cm³ of that into another 9 cm³ of water: 0.01 g dm⁻³. Repeat to reach 0.001 g dm⁻³. Use a fresh pipette for each transfer and discard 1 cm³ from the last tube so all volumes are equal. Include 0 (distilled water only) as the control.

Because concentrations differ by factors of ten, plot them on a logarithmic axis.

Worked example. Root length in mm after 5 days for 10 cress seedlings each: control 31, 28, 34, 30, 33, 29, 32, 30, 31, 32; IAA at 0.01 mg dm⁻³ 24, 27, 22, 26, 25, 23, 28, 24, 26, 25.

1. Control mean = 310 ÷ 10 = 31 mm. IAA mean = 250 ÷ 10 = 25 mm.
2. The sample standard deviation is 1.83 mm for each group. The ranges barely overlap.
3. The mean root length is 19.4% shorter with IAA: (31 − 25) ÷ 31 × 100.
4. A Student’s t-test compares two means. The null hypothesis is that there is no difference in mean root length. Here t = 7.35 with 18 degrees of freedom (10 + 10 − 2). The critical value at p = 0.05 is 2.10, so reject the null hypothesis: IAA at this concentration reduced root growth significantly.
5. To test for a relationship across many concentrations, use a correlation test instead.

Commercial uses (5.1.5(f))

Controlling ripening. Ethene stimulates ripening: starch converts to sugar and cell walls soften. Ripening fruit make more ethene, a positive feedback. Bananas are picked unripe because they bruise less, then exposed to ethene when they arrive. Delay ripening with cold, low oxygen and removal of ethene from the air.

Rooting powders. Cuttings dipped in auxin powder grow roots from the cut stem more quickly. This gives many genetically identical plants (clones).

Hormonal weedkillers. Synthetic auxins such as 2,4-D are not broken down by plants as natural IAA is. They persist and cause fast, uncontrolled growth in broad-leaved weeds, which then die. They are selective because narrow-leaved crops such as cereals and lawn grasses are far less affected. Evaluate by balancing higher yields against run-off and harm to non-target plants.

Common errors

• Writing that auxin “causes” the shoot to bend towards light without saying it moves to the shaded side.
• Saying roots and shoots respond to auxin in the same way. High auxin inhibits root elongation.
• Describing ABA as closing stomata by “making guard cells shrink” without saying water leaves by osmosis.
• Comparing treatments with the wrong control.
• Saying gibberellin “makes seeds grow” without linking it to amylase and starch.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025: learning outcomes 5.1.5(a) to 5.1.5(f).