Auxin and cytokinin interactions, and ethylene in fruit ripeningSpec C3.1.22, C3.1.23
In short
Root tips produce cytokinin, which is transported to shoots, and shoot tips produce auxin, which is transported to roots; interactions between them integrate root and shoot growth. Ethylene stimulates the changes in fruit ripening, and ripening stimulates more ethylene production. This positive feedback makes ripening rapid and synchronized.
Auxin and cytokinin
Root tips produce cytokinin, which is transported to the shoots in the xylem. Shoot tips produce auxin, which is transported to the roots (from cell to cell by efflux carriers, and in the phloem). Each hormone signals the state of the other part of the plant, and the two hormones often have opposite effects in the same tissue:
- In the roots, auxin arriving from the shoots promotes the formation of new lateral roots, while cytokinin inhibits it. More auxin from a growing shoot system therefore favours root growth.
- In the shoots, auxin from the shoot tip inhibits the growth of side buds (apical dominance), while cytokinin arriving from the roots promotes cell division and side-bud growth. More cytokinin from a growing root system therefore favours shoot growth.
- So it is the balance (ratio) of auxin to cytokinin that matters: a high auxin to cytokinin ratio favours root growth, and a low ratio favours shoot growth. In plant tissue culture, changing this ratio makes a callus form roots or shoots.
These interactions help to ensure that root and shoot growth are integrated: the roots can supply enough water and mineral ions for the leaves, and the leaves can supply enough sugars for the roots.
Ethylene and fruit ripening
Ethylene (IUPAC name: ethene) is a gaseous phytohormone. It stimulates the changes that occur in fruits during ripening, such as conversion of starch to sugars, softening of the flesh and change of colour. Ripening also stimulates increased production of ethylene. This is positive feedback: the response increases the stimulus. It happens in fruits such as bananas, apples and tomatoes, which show a burst of ethylene production as they ripen.
- A fruit starts to produce ethylene.
- Ethylene stimulates ripening changes in the fruit.
- Ripening stimulates the fruit to produce even more ethylene.
- Ethylene, a gas, also diffuses to neighbouring fruits and triggers their ripening.
The benefit is that fruit ripening is rapid and synchronized: all the fruits on a plant ripen at about the same time, which makes them attractive to seed-dispersing animals together.
Linking question: positive feedback amplifies change and moves a system away from its starting state, so it suits one-off events such as ripening. Contrast it with the negative feedback that keeps heart rate and blood pH stable.
Quick check
What is meant by an emergent property, and what example does the IB guide use?
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A property of the whole organism that arises from integration of its parts; a cheetah becomes an effective predator through integration of its body systems.
Which part of the nervous system is an integrating centre for unconscious processes?
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The spinal cord.
Where are baroreceptors located?
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In the walls of the aorta (aortic arch) and the carotid arteries.
Which gland secretes melatonin, and when is secretion highest?
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The pineal gland; secretion is highest at night, in darkness.
HL only How does auxin make cells elongate?
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It promotes H⁺ secretion into the cell wall; the acidified wall has loosened cross links between cellulose molecules, so the cell can elongate.
Written and checked against the IB Biology HL specification · Updated October 2026