Light under a canopy is sunlight minus the light absorbed by photosynthesis, which is mostly red. This changes the ratio in favor of far-red light, all of which passes through the leaves in the canopy. This means that a phenomenon promoted by phytochome above the leaves will be inhibited below based on the changed ratio.
Work Step by Step
Review past material on photosynthesis to see where the red light goes as light moves through a canopy. Then, simply apply this to the normal roles of phytochrome discussed in this chapter.