TEMPERATURE AND PH STUDIES ON PHYTOCHROME IN VITRO

Phytochrome from oat seedlings was studied over a range of temperatures from -50 to +15° and in aqueous and mixed aqueous-buffered solutions of pH 4.7-8.6. The results show that phytochrome in vitro behaves as a thermochromic as well as a photochromic protein, particularly at lower temperatures and in the pH range 6.0 and below. Furthermore, the rate at which phytochrome undergoes a spontaneous reaction during prolonged dark periods is strongly dependent upon temperature and pH. A model involving three forms of phytochrome is proposed to account for these results. Two of the forms are the well-established species having their first absorption maxima at λ 663 and 727 nm, respectively, whereas the third form exhibits a relatively weak and broad absorption at approximately λ 650 nm. Analysis of the model shows that the rate of the dark reaction is insensitive to temperature at certain specific pH values. It is suggested that this temperature compensation in the phytochrome dark reaction may be physiologically important. It is also suggested that phytochrome may act as a thermoregulator as well as as a photoreceptor in plants. © 1969, American Chemical Society. All rights reserved.