A Kinetic Theory of First Order Cyclical Processes—Phytochrome Controlled Red Light Induced Cereal Leaf Unfolding Compared with Theory

The phytochrome controlled unfolding of cereal leaves was studied as a function of irradiation time and light intensity (narrowband red light) over a wide energy range (5 decades). With different intensities, a family of similarly shaped response curves appear with distinct time-dependent maxima and minima. A theoretical kinetic model based upon a cyclical phytochrome photoconversion scheme has been calculated by us. The theoretical calculations and the experimental findings are in excellent agreement. The same model explains the early photoresponses (first maxima) as an effect of one active phytochrome form, P₂, and the delayed photoresponses as an effect of a second active form P_n. The active transitory form, P₂ (although it may not be the primary product), is formed upon light absorption from P₁. The P₂ decays by a first order dark reaction through several inactive intermediates to P_n (active). The effect of the intermediates is mainly to delay the production of the second active product. It is possible to identify the two active products, P₂ and P_n, as P_fr and P*_fr, respectively. The presented cyclical phytochrome reaction scheme is a special case of a general first order kinetic cycle which includes all possible feed back loops. The latter scheme also has been calculated and programmed since it has a more general application.