Auxin and phototropisms

are caused by an unequal distribution of . This makes the plant grow either towards or away from the light, depending on which part of the plant receives the light.

In a stem, the cells on the shaded side contain more auxin and grow longer than the cells on the light side. This causes the stem to grow towards the light. It is vital to note that the plant does NOT bend towards the light.

A visual to show four stages of positive geotropism. — Figure caption,
In the shoot, auxin moves to the shady side and promotes cell elongation

Auxins have the opposite effect in roots. This is negative phototropism. In a root the shaded side contains more auxin but grows less. This causes the root to bend away from the light.

Auxin experiments

The role of auxin in phototropism can be proven by carrying out a range of experiments. Below is one example:

Three sets of plants. A: furthest from light, pointing up, have grown about 2cm. B: grow straight, have grown twice as high as A. C: have grown highest, but are bending towards light. — Figure caption,
Typical results by oat seedlings grown in a box with a light source

Three groups of seeds are grown in a cardboard box.

A - when the tips are removed, no auxin is made so the stems do not grow
B – when the tips are covered by foil, auxin moves to all parts of the stem causing all parts to grow
C - when the tips are lit from one side only auxin accumulates on the shaded side causing it to grow more than the illuminated side

	Seedling A	Seedling B	Seedling C
Treatment	The tips have been removed	No light reaches the tips due to foil caps placed on top	More light reaches one side of the tips
Effect of auxin concentration	No auxin is produced	Equal concentration of auxin on both sides	Greater concentration of auxin on shaded side
Result	The stems do not grow longer	The stems grow evenly and longer on both sides	The cells on the darker side of the stems grow longer

Treatment
Seedling A	The tips have been removed
Seedling B	No light reaches the tips due to foil caps placed on top
Seedling C	More light reaches one side of the tips

Effect of auxin concentration
Seedling A	No auxin is produced
Seedling B	Equal concentration of auxin on both sides
Seedling C	Greater concentration of auxin on shaded side

Result
Seedling A	The stems do not grow longer
Seedling B	The stems grow evenly and longer on both sides
Seedling C	The cells on the darker side of the stems grow longer

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How is growth controlled in plants? - OCR 21st CenturyAuxin and phototropisms