Responses of Phycomyces indicating optical excitation of the lowest triplet state of riboflavin.

Phototropic and light growth responses of the sporangiophore of Phycomyces have been elicited using tunable laser stimulation from 575 to 630 nm. The growth response shows additional components of the action spectrum with a sharp peak at 595 nm, a sharp cut-off at 585 nm, and a tail extending beyond 630 nm. The integral over the electronic transition (f-value) is 1.5 X 10(-9) times that at 455 nm. These parameters indicate a direct transition from the ground state to the lowest triplet state of riboflavin.