On the kinetics of phytochrome photoconversion in vivo.

Samples for spectrophotometric measurement of phytochrome in vivo are not optically thin. For different cross sections of the sample, the rate constant of a photochemical reaction will, therefore, have different values. We have developed a mathematical model, based on the assumption that the rate of phytochrome phototransformation is proportional to the light intensity and that the light intensity gradient in the sample is exponential. Kinetic curves computed with this model conform closely with the measurements. The simplest explanation of the observed kinetics is that there is only one type of phytochrome and that the light intensity gradient in samples that are not too thin, is close to exponential.