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A protein pigment that mediates in photoperiodic responses and certain other photoreactions, e.g. light-stimulated germination and the removal of the symptoms of etiolation. It exists in two interchangeable forms, Pr, which absorbs in the red part of the spectrum (660 nm), and Pfr, which absorbs in the far red (730 nm). Following exposure of a plant to red light Pr changes to Pfr, while after exposure to far-red irradiation Pr is reformed from Pfr. This reversion of Pfr to Pr may also occur in the dark in some plants, a process that is inhibited by low temperatures. Pfr may also be lost by decay. In practice an illuminated plant receives a mixture of red and far-red light and the proportion of Pr to Pfr, and hence the response of the plant, will depend on the relative proportions of far-red and red light. Sunlight has a high proportion of red light as compared to far-red and so promotes formation of Pfr. Fluorescent light is virtually devoid of far-red light and is thus even more effective in promoting Pfr formation. Incandescent lights however have a relatively high proportion of far-red wavelengths, and sunlight that manages to penetrate the shade has had most of the red light absorbed by the chlorophyll in the canopy. Far-red wavelengths consequently predominate and most of the phytochrome of shaded plants will be in the Pr form.
Part of the phytochrome molecule consists of a nonprotein portion, the chromophore. This is similar to the phycobilins of blue-green and red algae. It is thought that the change from Pr to Pfr and vice versa is brought about by a shift of two hydrogen atoms in the chromophore. This property of the molecule explains the observation that red and far-red light are mutually antagonistic in their effects, the response of the plant often depending on the last type of radiation received. If the last exposure is to red light then light-requiring seeds will germinate and etiolated plants will begin normal growth. However if the last exposure is to far-red these responses will not occur. This implies that Pr is the inactive form and Pfr the active form of phytochrome. The reversal effect of far-red light is only seen if the far-red is given soon after the red light. This is especially true in the case of rapid light-mediated responses, e.g. leaf movements in Mimosa pudica. In experiments on short-day plants given a short period of light during the dark period it was found that red light was effective in inhibiting flowering but far-red irradiation reverses this. The inhibitory effectiveness of red light increases when given towards the end of the   dark   period   being   most   effective when applied shortly before completion of the critical number of hours. In long-day plants a short period of red irradiation in the dark period promotes flowering.
Phytochrome is found in very low concentrations but is extremely sensitive even to very short flashes of weak red light. Thus exposure of an etiolated plant to moonlight can induce normal growth. It is uncertain how these responses are brought about though it has been noted that gibberellin and cytokinin levels increase following exposure to red light as do the levels of certain enzymes. The transformation of phytochrome may bring about conformational changes in the molecule that expose an active site in the Pfr form that is able to bind to membranes and change their function.

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