PHYTOCHROME‐MEDIATED PHOTOTROPISM IN Adiantum PROTONEMATA‐I. PHOTOTROPISM AS A FUNCTION OF THE LATERAL Pfr GRADIENT*

Abstract— Phytochrome‐mediated phototropism of the protonema of the fern Adiantum capillus‐veneris was studied in view of the hypothesis that phototropism is controlled by the gradient of Pfr (phytochrome in the far‐red‐absorbing form) across the short axis of the protonema. Fluence‐response relationships were investigated using a microbeam irradiation technique that allowed simultaneous stimulation of the two sides of the subapical portion of the protonema with different fluences of red light. Mathematical models describing the tropic response as a function of fluence were derived from the hypothesis in consideration of the minimal phototransformation kinetics of phytochrome. and the fitness of the functions to the experimental data was examined. The analytical results were then evaluated in view of the photochemical properties of phytochrome known from the literature. It is concluded that the extent of the tropic response is determined by the difference in the Pfr concentrations between the two sides of the protonemal cell. It is further suggested that, even if phytochrome exists as a dimer in vivo, the physiological unit of phytochrome is the monomer.