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 . Phytochrome regulation of greening in Pisum . Plant Physiol .
Potentiation is proportional to fluence ie.
Many genes are red-light inducible, but have various regulatory patterns Kaufman, L.S. et al. Plant Physiol . If a FR pulse is given within a few hours of an initial red pulse, phytochrome potentiation can be partially or completely reversed. The longer you wait to give the red light pulse, the less of a reversal will occur. Eventually, no reversability is possible.