Plant Hormones and Tropisms

Plants cannot move from place to place, but they can change how they grow to suit their surroundings. They do this using chemical messengers called plant hormones, which coordinate and control growth and responses to light and gravity.

A growth response in the direction of a stimulus is called a tropism. The two you need for AQA are:

• Phototropism — a growth response to light.
• Gravitropism (also written geotropism) — a growth response to gravity.

A response is positive if the plant grows towards the stimulus and negative if it grows away from it.

A tropism is a directional growth response to an external stimulus. The direction of growth — not just whether it grows — is what matters.

Because these responses come from changes in growth rate rather than muscle movement, they are slow and permanent. A shoot that has bent towards a window stays bent; it does not straighten when the light moves.

Shoots and roots respond to the same two stimuli in opposite directions, because each organ is suited to a different job. Shoots need to reach light for photosynthesis; roots need to grow down into the soil for water and anchorage.

Organ	Light (phototropism)	Gravity (gravitropism)
Shoot	Positive — grows towards light	Negative — grows up, away from gravity
Root	Negative — grows away from light	Positive — grows down, towards gravity

So a shoot is positively phototropic and negatively gravitropic; a root is the reverse — negatively phototropic and positively gravitropic.

A simple way to remember it: the part of the plant that needs light (the shoot) grows towards light and upwards; the part that needs water and grip (the root) grows downwards into the dark soil.

Shoots grow up and towards light. Roots grow down and away from light. Mixing up which organ is positive or negative for each stimulus is the single most common error on this topic.

The hormone responsible for both phototropism and gravitropism is auxin. Auxin is made in the tips of shoots and roots and moves back along the organ.

The key idea is unequal distribution: when auxin spreads evenly, the organ grows straight. When a stimulus pushes auxin to one side, that side gets a different amount of auxin from the other, the two sides grow at different rates, and the organ bends.

The catch — and the reason shoots and roots bend in opposite directions — is that auxin has opposite effects in the two organs:

• In shoots, more auxin speeds up growth (the high-auxin side grows faster and longer).
• In roots, more auxin slows down growth (the high-auxin side grows slower and shorter).

Auxin is made in the tip and causes unequal growth when it builds up on one side. It is the same hormone in shoots and roots — but it promotes growth in shoots and inhibits growth in roots.

(Higher Tier only — exactly how auxin produces this unequal growth, by being redistributed to the shaded or lower side, is examined in detail on the Higher Tier paper; see the next slide.)

Picture a young shoot lit from one side by a window. Follow the cause and effect step by step.

1. Auxin is produced in the shoot tip.
2. Light shining from one side causes auxin to move to the shaded side of the shoot. The shaded side now has more auxin than the lit side.
3. In a shoot, more auxin means faster growth, so the cells on the shaded side grow longer than those on the lit side.
4. Uneven growth makes the shoot bend towards the light — a positive phototropic response.

(Higher Tier only — the statement that auxin accumulates on the shaded side and redistributes in response to the stimulus is the Higher Tier explanation of phototropism.)

Now apply the same logic to gravity. Lay a shoot on its side in the dark. Auxin gathers on the lower side. In a shoot, the extra auxin on the lower side makes those cells grow faster, so the shoot curves upwards — a negative gravitropic response. In a root lying on its side, auxin also gathers on the lower side, but because auxin inhibits root growth, the lower side grows slower, the upper side grows faster, and the root curves downwards — a positive gravitropic response.

Required practical activity 8 investigates the effect of light or gravity on the growth of newly germinated seedlings (commonly cress or mustard seeds).

Investigating light (phototropism):

1. Grow seedlings in several dishes on damp cotton wool until shoots emerge.
2. Set up groups in different light conditions — for example, all-round (even) light, light from one side only, and darkness.
3. Keep everything else the same: same seed type, same number of seeds, same temperature, same water.
4. After a set number of days, measure the length of the shoots with a ruler and record the direction they grew. Record the results as both length measurements and careful labelled biological drawings, as the spec requires.

Expected result: shoots in one-sided light bend towards the light; shoots in even light grow straight up; shoots in darkness grow tall, pale and spindly with no particular direction.

Control variables keep the test fair: seed type and number, temperature, volume of water, and time grown. The comparison group (even light, or all conditions kept normal) shows what growth looks like without the stimulus, so any bending can be attributed to the variable you changed.

To investigate gravity instead, germinate seedlings and then turn some dishes so the seedlings lie on their side or are inverted; compare the direction of root and shoot growth against seedlings left upright.

(Higher Tier only — this entire slide, covering the commercial uses of auxins, ethene and gibberellins, is Higher Tier and Biology-only content.)

People extract or make plant hormones and use them in agriculture and horticulture to control how plants grow. You need to know what each one is used for — not the underlying mechanism.

Hormone	Uses	Why it is useful
Auxins	Weed killers; rooting powders; promoting growth in tissue culture	As selective weed killers they make broad-leaved weeds grow abnormally fast and die, leaving narrow-leaved crops; as rooting powder they make cuttings grow roots quickly; in tissue culture they promote growth of new plants from small samples
Ethene	Controlling ripening of fruit during storage and transport	Fruit can be picked unripe (firm, less easily damaged), shipped, then exposed to ethene to ripen on arrival
Gibberellins	Ending seed dormancy; promoting flowering; increasing fruit size	Used to make seeds germinate when wanted, to bring plants into flower, and to grow larger fruit

(Higher Tier only — alongside their commercial uses, the spec names two further roles these hormones play in the plant itself. Ethene controls cell division as well as the ripening of fruits. Gibberellins are important in initiating seed germination. Note that this germination role is a natural control function, separate from the commercial use of gibberellins to end seed dormancy listed above.)

(Higher Tier only) The AQA spec states that the mechanisms of how gibberellins and ethene work are not required — you only need their uses.

Selective weed killers based on auxins can reduce biodiversity: removing broad-leaved "weeds" also removes food and habitat for insects and other organisms that depend on them.

1. Mixing up positive and negative for each organ

A shoot is positively phototropic but negatively gravitropic; a root is the reverse. Learn the four combinations as a block (shoot: towards light, away from gravity; root: away from light, towards gravity) rather than guessing from the word "positive".

2. Saying auxin does the same thing in roots as in shoots

It is the same hormone, but it promotes growth in shoots and inhibits growth in roots. That single difference is why a horizontal shoot bends up while a horizontal root bends down, even though auxin collects on the lower side of both.

3. Forgetting that auxin moves to the shaded or lower side

The high-auxin side is the shaded side (for light) or the lower side (for gravity). On Higher Tier, marks come from stating where auxin accumulates and that this causes unequal growth rates, not just that the plant "bends".

4. Vague answers in the Required Practical

"It grew towards the light" is not enough on its own. Reference the measurements taken, the control variables kept constant, and a comparison group (even light or darkness) so the response is shown to be due to the stimulus you changed.

5. Explaining the mechanism of ethene or gibberellins

The spec asks only for the uses of these hormones. Describing how they work at a cellular level wastes time and is not credited — name the use (e.g. ethene controls fruit ripening) and stop there.