The phenotypes of phytochrome-deficient mutants have
been useful in identifying the physiologically active form
of phytochrome. If the phytochrome-induced response to
white light (hypocotyl growth inhibition) is caused by the
absence of Pr, such phytochrome-deficient mutants (which
have neither Pr nor Pfr) should have short hypocotyls in
both darkness and white light. Instead, the opposite occurs;
that is, they have long hypocotyls in both darkness and
white light. It is the absence of Pfr that prevents the
seedlings from responding to white light. In other words,
Pfr brings about the physiological response