The Role of Phytochrome in the Natural Environment

Abstract

Almost everything we know about the structure and properties of phytochrome has been derived from studies of dark-grown plants. Inevitably, therefore, all serious attempts to achieve an understanding of the molecular mechanism of phytochrome action have also been directed towards the photoresponses of dark-grown plants. This work has, in general, been very productive, but it has not led to an agreed view, either in mechanistic, or formalistic, terms of the mechanism of action. It is generally assumed that “Pfr is the active form of phytochrome”, but this can only be asserted with any degree of confidence for the very restricted situation of dark-grown plants given single pulses of light within the boundary conditions of reciprocity. Even under these conditions, the relationship between [Pfr] and response is not simple and uniform, exhibiting/ for example, a biphasic form for the regulation of anthocyanin synthesis in mustard¹, whilst the control of lipoxygenase levels, also in mustard, displays an all-or-none threshold relationships. It has long been known³ that even simple pre-irradiation treatments which alter the total amount of phytochrome in subsequent darkness, can destroy the relationship between [Pfr] and response for subsequent light treatments⁴.