[Campbell Biology P.906] In a dense forest, why might a tree exhibit a "shade avoidance" res... | Practice Question

In a dense forest, why might a tree exhibit a "shade avoidance" response, characterized by elongated vertical growth and reduced branching, and how is this related to phytochrome?

  • A: The forest canopy screens out more far-red light than red light, increasing the Pfr/Pr ratio, which promotes vertical growth.
  • B: The forest canopy screens out more red light than far-red light, shifting the phytochrome ratio in favor of Pr, which induces the tree to allocate resources to growing taller.
  • C: The low light intensity in the shade leads to an accumulation of Pfr, which promotes vertical growth for light capture.
  • D: Increased synthesis of Pr in shaded conditions directly stimulates the genes responsible for tall growth.

Explanation

The "Phytochromes and Shade Avoidance" section explains that in dense forests, the canopy screens out more red light than far-red light because chlorophyll absorbs red light. This shifts the phytochrome ratio in favor of the Pr form. The text then states: "The shift in the ratio of red to far-red light induces the tree to allocate more of its resources to growing taller." This "shade avoidance" response is characterized by elongated vertical growth and reduced branching to compete for light.