Action Spectra for Light‐induced Elongation in Fern Protonemata

The action spectrum for promotion of elongation of protonemata of Onoclea sensibilis has peaks at 400–420, 580–600 and 640–660 nm. The largest growth increments at saturating light doses are produced by yellow and far‐red light. Elongation induced by yellow and far‐red irradiation persists in old as well as young filaments, while red‐light promotion is found only in young filaments. The growth promotion caused by yellow light is partially reversed by red light down to the level of growth produced by red irradiation alone. Elongation of rhizoids is under reversible red, far‐red control, while yellow light is inactive. A model is proposed and discussed in which the light‐sensitive elongation of filaments is accounted for by the presence of three distinct photoreceptors: phytochrome; a pigment absorbing yellow light. P580; and a pigment absorbing blue light, P420.