Plant Hormones

Plants cannot move from place to place — but they can grow in a direction in response to their environment. The chemicals that control this growth are called plant hormones (sometimes 'plant growth regulators').

The main plant hormones you need on the AQA spec:

• Auxin — the central one. Controls growth responses to light and gravity.
• Gibberellins — promote seed germination.
• Ethene (also written 'ethylene') — promotes fruit ripening and is involved in cell division.

Where is auxin made?

• In the tips of shoots and roots (the meristem regions where cells are actively dividing).

What does auxin do?

• In shoots: auxin causes cells to elongate (get longer). High auxin = more elongation = faster growth on that side.
• In roots: the OPPOSITE — auxin INHIBITS cell elongation. High auxin = slower growth on that side.

This contrast is important because it explains why shoots and roots bend in opposite directions in response to the same stimulus.

Phototropism = a plant's growth response to light.

Shoots are positively phototropic — they grow TOWARDS light. Useful because they need light for photosynthesis.

How it works (the mechanism):

1. Auxin is made at the tip of the shoot.
2. When light comes from ONE side, auxin moves to the shaded side of the tip.
3. Auxin causes cells on the shaded side to elongate more than cells on the lit side.
4. The shaded side grows longer than the lit side → shoot bends towards the light.

Roots are negatively phototropic — they grow AWAY from light (i.e. down into the soil).

Gravitropism (also called geotropism) = a plant's growth response to gravity.

Roots are positively gravitropic — they grow DOWNWARDS, into the soil. Useful because they need water and anchorage.

Shoots are negatively gravitropic — they grow UPWARDS, away from gravity. Useful so they can reach light for photosynthesis.

How it works:

• If a seedling is laid on its side, auxin sinks under gravity to the lower side of both the shoot and the root.

In a horizontal shoot:

• Auxin on the lower side → cells there elongate more → shoot curves UPWARDS.

In a horizontal root:

• Auxin on the lower side → INHIBITS cell elongation → lower side grows LESS → root curves DOWNWARDS.

Required Practical 8: investigating phototropism / gravitropism.

Typical setup:

1. Germinate cress (or mustard) seeds on damp filter paper in Petri dishes.
2. Set up control (all-round light) and test groups (one-sided light using box with slit; or upside-down dishes for gravity).
3. Leave for several days.
4. Observe / measure direction and amount of growth.

Expected results:

• All-round light: seedlings grow straight up.
• One-sided light: shoots bend towards the light source.
• Upside-down: roots curve back down (positive gravitropism); shoots curve back up (negative gravitropism).

Always include a control (e.g. all-round light) so you can tell if the difference is caused by the variable you're testing.

Although auxin is the star of AQA 4.5.4.1, you should also know two other plant hormones for the basic spec content:

Gibberellins.

• Promote seed germination — they trigger the breakdown of starch stored in the seed into glucose, providing energy for the growing embryo.
• Also promote stem elongation and flowering.

Ethene (ethylene).

• A gas given off naturally by fruits.
• Controls cell division and the ripening of fruits.
• That's why a ripe banana speeds up the ripening of nearby fruit in a fruit bowl.

These hormones are covered in much more detail (and their commercial uses) in spec 4.5.4.2 'Use of plant hormones'.

Auxins are the most widely used plant hormones in farming and gardening.

1. Auxins as selective weedkillers.

• A weed in a wheat field or a lawn is usually a broad-leaved plant (dandelion, dock, daisy) growing among narrow-leaved crops (wheat) or grass.
• Synthetic auxins (like 2,4-D) at high concentration cause uncontrolled growth in broad-leaved plants — their stems grow so fast they twist, distort and use up all their resources, then die.
• Grasses and cereals (narrow-leaved) are far less sensitive to these doses and survive.
• So a sprayed lawn kills the dandelions but the grass continues to grow normally.

Advantages: higher crop yields (no competition for water/nutrients/light from weeds); less labour than hand weeding.

Disadvantages: can harm non-target plants if spray drifts; environmental build-up; some weeds may evolve resistance.

2. Auxins in rooting powder.

• A 'cutting' is a small piece of stem or leaf cut off a parent plant. To make it grow into a new plant, it needs to develop roots.
• Dipping the cut end of a stem cutting in rooting powder (which contains auxin) stimulates root growth quickly and reliably.
• This is vegetative propagation — it produces genetic clones of the parent plant, so all the new plants have the desired qualities (flower colour, fruit flavour).

3. Auxins in tissue culture.

• A few cells from a parent plant can be grown into many whole plants in sterile gel containing auxins, sugars and nutrients.
• Used to mass-produce identical plants quickly — especially valuable for rare or endangered species and elite crop varieties.

Gibberellins are a group of plant hormones used commercially for three main purposes.

1. Ending seed dormancy.

• Many seeds will not germinate immediately after being shed — they 'wait' until conditions are right (warmth, moisture). This is called dormancy.
• Soaking seeds in gibberellin breaks the dormancy and triggers immediate germination.
• Useful for uniform crop emergence (all seedlings appear together, easier to harvest) and for growing seeds out of season.

2. Inducing flowering.

• Some plants need specific day length or temperature to flower.
• Applying gibberellin can induce flowering in plants that wouldn't normally flower yet — useful in horticulture (e.g. for cut flowers) and for breeding programmes.

3. Increasing fruit size.

• Spraying gibberellin onto developing fruit makes the fruit grow larger (cell elongation).
• Famously used on seedless grapes — without gibberellin spray, seedless grapes are small. With it, supermarkets get the large, plump grapes consumers expect.

Benefits: more reliable yields, can grow out-of-season produce, supports valuable seedless varieties.

Drawbacks: cost of hormone treatment; some consumers prefer 'natural' produce.

Ethene (also called ethylene) is a gaseous plant hormone that controls cell division and fruit ripening.

Why is ethene important commercially?

• Ripe fruit (like bananas, tomatoes, avocados) is soft and damages easily during transport.
• If shipped fully ripe, much would be bruised, mouldy and unsellable.
• Solution: pick fruit unripe (firm and green), ship it in cooled, ethene-free conditions (slows ripening), then expose to ethene in a controlled atmosphere room when ready for sale — the fruit ripens within days, ready for the supermarket shelves.

The natural reason: ripe fruit gives off ethene gas. This is why one ripe banana speeds up ripening of every fruit in a bowl (ethene diffuses to neighbours). You can use this at home: put a green banana with a ripe one to ripen it faster.

Benefits: drastically reduces food waste in transit; lets supermarkets supply fruit out-of-season from other continents (bananas from Ecuador, kiwis from New Zealand); fruit reaches consumers at peak ripeness.

Concerns: very long supply chains have a carbon footprint; some 'super-ripened' fruit may have inferior flavour.