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During phototropism, where does indole acetic acid accumulate and what does it do there?
AOn the shaded side, where it makes cells elongate
BOn the shaded side, where it stops cells elongating
COn the lit side, where it makes the cells elongate
DOn the lit side, where it stops the cells elongating
Answer & Solution
Correct answer: A. On the shaded side, where it makes cells elongate
1. Light passing through a stem is diffracted and activates phototropin across the stem, with most activation on the lit side.
2. This causes indole acetic acid to accumulate on the shaded side.
3. Stem cells elongate under the influence of that hormone.
4. Faster elongation on the shaded side than on the illuminated side tips the shoot towards the light.
5. Putting the hormone on the lit side would bend the shoot away from light, which is not what a positively phototropic stem does.
6. Inhibition of elongation by the same hormone is what happens in roots, not in stems.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.6 Plant Sensory Systems and Responses_
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