8.11—Plant responses: phytochrome, auxin and gibberellins

Phytochrome exists as inactive Pr and active Pfr. Red light converts Pr to Pfr; far-red light converts Pfr back to Pr. The balance changes with day and night length and can switch flowering genes on or off.

In a long-day plant, short nights leave more Pfr, which activates gene expression leading to flowering. A far-red pulse can reverse the conversion and alter the response.

The pigment is a reversible sensor: the plant responds to the light history and duration of darkness, not simply to daytime brightness.

“Long-day” describes short-night flowering, and Pr/Pfr are pigment states rather than hormones. Keep light wavelength, pigment form and gene expression distinct.