Plant hormones

Biology · Homeostasis and response. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Plants use hormones to control and coordinate growth. A growth response to light is called phototropism. A growth response to gravity is called gravitropism (or geotropism). Shoots grow towards light and away from gravity. Roots grow towards gravity.

Both responses are caused by an unequal distribution of auxin. Auxin is made in the tips of shoots and roots. It moves to the shaded side of a shoot, and to the lower side of a shoot or root lying on its side. The two sides then grow at different rates, so the organ bends.

Two other hormones matter: gibberellins start seed germination, and ethene controls cell division and the ripening of fruit.

Rules to remember

In shoots, more auxin → cells elongate more. The side with more auxin grows faster, so the shoot bends away from that side.
In roots, more auxin → cells elongate less. The lower side grows more slowly, so the root bends downwards.
Shoot towards light = positive phototropism. Root downwards = positive gravitropism. Shoot upwards = negative gravitropism.
The tip of the shoot detects the light. If the tip is removed or covered, the shoot does not bend towards light.
Uses of auxins: selective weedkillers (broad-leaved weeds grow out of control and die), rooting powders for cuttings, promoting growth in tissue culture.
Uses of gibberellins: end seed dormancy, promote flowering, increase fruit size.
Use of ethene: ripening fruit during storage and transport, so it is ripe when it reaches the shops.

Common mistake

Writing that auxin moves towards the light, or that the lit side grows faster. Auxin collects on the shaded side, and it is the shaded side of the shoot that elongates more.

Worked example

A young shoot is laid flat on its side in a dark box. After a day the tip has curved upwards. Explain why.

There is no light, so the response must be to gravity.
Gravity makes auxin collect on the lower side of the shoot.
In shoots, auxin makes cells elongate more, so the lower side grows faster than the upper side.

Answer: The lower side grows more, so the shoot bends upwards. This is negative gravitropism.

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