The Dose Response Curve in Phytochrome‐mediated Anthocyanin Synthesis in the Mustard Seedling

Abstract—The dose response curve for light (phytochrome)‐induced anthocyanin synthesis was determined in the mustard seedling. The curve gives the amount of anthocyanin (A) synthesized within 24 h as a function of the amount of Pfr* produced by a brief light pulse. The [Pfr] response curve is composed of two linear parts with very different slopes (a1,2) connected by a relatively narrow transient range (curved segment). The [Pfr] response curve extrapolates precisely through zero [Pfr]. The reciprocity law is valid over the whole range investigated (up to 320 s of irradiation). It is concluded that the initial (or primary) reaction of Pfr (Pfr+ X → PfrX) does not involve any significant cooperativity in the case of phytochrome‐mediated anthocyanin synthesis. It is speculated that the linear parts of the [Pfr] response curve truly reflect the mode of phytochrome action (A=a1,2 [Pfr]; X does not come into play since it is not rate limiting) whereas the curved segment represents a transition of the reaction matrix of Pfr. The large difference between a1 and a2 seems to indicate that the physiological effectiveness of a given amount of Pfr (or PfrX) is determined by [Pfr] through a Pfr‐induced change in the reaction matrix.