Induction of plant gene expression by light. Physiological and molecular studies of light-regulated nuclear genes in higher plants. Light modulation of vegetative development. B. Phytochrome 1. Interconversion - Phytochrome exists in two forms, P r and P fr .
P r is the inactive form of phytochrome.
It has an absorption maximum at 650 nm. When P r is exposed to red light, it shifts to P fr, the active form, which has an absorption maximum at 730 nm. Conversely, when P fr is exposed to far red light, it shifts to the inactive P r form. Consequently, when plants are exposed to alternating red and far-red treatments, the last treatment will govern whether phytochrome responses are activated. Thus, phytochrome responses have some degree of reversibility .