Light maintains High Levels of Phytochrome Intermediates

THE plant photomorphogenetic pigment phytochrome exists in two forms, Pr and Pfr, interconvertible by light, which have peaks of absorption in the red and far-red regions of the spectrum respectively¹. Intermediates between Pr and Pfr have been demonstrated during photoconversion by Linschitz and his coworkers^2,3 using flash photolysis techniques. Low temperatures studies have also proved useful in identifying intermediates^4–8. Briggs and Fork^9,10 detected intermediates in vitro and in vivo in conditions of pigment cycling by mixed red and far-red light, but were restricted to studying the minor peaks of phytochrome absorption in the blue region of the spectrum because of the available instrumentation. In this type of measurement the problem is that actinic light has to be prevented from falling onto the photomultiplier. Briggs and Fork inserted a red cutoff filter, but this precluded measurement at the peaks of absorption of Pr and Pfr in the red and far-red regions of the spectrum. The design and construction of a sensitive quasi-continuous measuring spectrophotometer have now overcome this problem and made possible an investigation in vivo of phytochrome intermediates at any wavelength under conditions of pigment cycling, for example, in high intensity white incandescent light. The instrument can detect intermediates with a half life in excess of 0.2 ms. The longer lived intermediates between Pr and Pfr observed in the in vitro flash studies^2,3 should be readily detectable if they accumulate in conditions of cycling.