Phytochrome: First-order phototransformation kinetics in vivo.

The deviation from first order commonly observed in phototransformation kinetics of phytochrome in vivo is due to a light-intensity gradient within the sample. This gradient was measured and was found to approach that predicted by the Kubelka-Munk theory of light scatter in turbid materials. The influence of this gradient is eliminated and first-order phototransformation kinetics are obtained, when either (i) thin samples of translucent (low optical density) material of high phytochrome content are measured directly; or (ii) thin samples of opaque (high optical density) or translucent material are sandwiched between two layers of light-scattering material. This result is consistent with the existence of only one population of photoreversible phytochrome molecules in vivo.