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A leaf shaded by another plant receives far red enriched light. What follows?
APhytochrome shifts to Pfr and growth speeds up
BPhytochrome shifts to Pr and growth is slowed
CPhytochrome breaks down and the leaf is shed
DPhytochrome shifts to Pfr and the leaf is shed
Answer & Solution
Correct answer: B. Phytochrome shifts to Pr and growth is slowed
1. Unfiltered sunlight contains much more red light than far red light.
2. Chlorophyll absorbs strongly in the red region but not in the far red, so shade under another plant is red depleted and far red enriched.
3. That preponderance of far red light converts phytochrome in the shaded leaves to the inactive Pr form.
4. With phytochrome inactive, growth of the shaded leaf slows.
5. Leaves in nearby brighter patches sense red light, form Pfr and grow, so shoots grow away from shade and towards light.
6. Options that make far red light produce Pfr invert the conversion and would have the plant grow harder in deep shade.
_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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