PHOTOMORPHOGENESIS IN PHYCOMYCES - COMPETENCE PERIOD AND STIMULUS-RESPONSE RELATIONSHIPS

The illumination of Phycomyces mycelia (age, 48 h) with blue-light pulses inhibits the future development of dwarf sporangiophores and stimulates the development of giant sporangiophores. Each of these two responses is the sum of two components; the first components of each response have thresholds at about 10-4 J m-2 and saturate at about 10-2 J m-2, and the second components reach threshold and saturation at about 1 and 102 J m-2 respectively. Each of the two components of the two responses follows a sigmoidal function of the form ax/(b + x) where x is the light exposure. The function for photomicrophorogenesis may be explained by a proportionality between the response and the concentration of a chemical with the assumption that light destroys this chemical when it forms a dimer. A slightly modified explanation serves for photomacrophorogenesis. Under our experimental conditions, each culture contains about 105 separate mycelia. The maximal responses are found when a culture is exposed to a light pulse at the age of 48 h; however, the cultures exhibit some competence to respond to light over a period of about 35 h. The competence periods of individual mycelia in the cultures may be determined indirectly; they are about 6 h on average, but often much less. The competence periods for the two components of the same individual mycelium and of different mycelia in the cultures do not always coincide. When the first component is active, its saturation blocks all microphorogenesis, so that the second component can only be detected in mycelia whose first component is inactive at the time of illumination. © 1990.