Phytochrome Action: A Reappraisal

Stems of fully green plants show at least two types of response to light. In one, P fr inhibits elongation. The second is a promotion of elongation which operates only in light; the effectiveness of red and far‐red wavelengths indicates that this response is also mediated through phytochrome. Studies of the de‐etiolation process also provide evidence for two modes of action of phytochrome; one is a P fr ‐dependent reaction, and the second requires continuous light (or frequent short irradiations). It is proposed that, in addition to reactions which require P fr and proceed in darkness, an important reaction of phytochrome in green plants occurs only in light. We have termed these two modes of action of phytochrome “static” and “dynamic”. The static mode operates after a brief exposure to light which establishes P fr ; the potential responses are largely reversible by far‐red and exhibit reciprocity. The dynamic mode operates only in light and the responses do not show reciprocity. We consider that this mode operates through the transition from one bound form of phytochrome to another. The possible involvement of these two modes of action of phytochrome in photoperiodic mechanisms is discussed.