AP BIOLOGY

Detailed explanation-1: -Exposure to red light converts the chromoprotein to the functional, active form (Pfr), while darkness or exposure to far-red light converts the chromophore to the inactive form (Pr).

Detailed explanation-2: -Absorption of red light by Pr converts the protein to the far-red-absorbing conformation, Pfr. Absorption of far-red light by Pfr converts the conformation back to Pr (Figure 1).

Detailed explanation-3: -1: Phytochrome system: The biologically-inactive form of phytochrome (Pr) is converted to the biologically-active form Pfr under illumination with red light. Far-red light and darkness convert the molecule back to the inactive form. The phytochrome system acts as a biological light switch.

Detailed explanation-4: -This far-red–rich light causes the removal of the active Pfr form of phytochrome and, in plants native to an open canopy, the result is what is known as the “shade-avoidance response.” Shade avoidance involves a dramatic promotion of elongation growth so as to prevent overtopping by neighboring plants (5).

Detailed explanation-5: -Under the canopy, far-red (FR) light represses seed germination by inactivating phytochrome photoreceptors. This elicits a decrease in gibberellins (GA) levels and an increase in abscisic acid (ABA) levels. GA promotes germination by enhancing the proteasome-mediated destruction of DELLA repressors.