High irradiance response promotion of a subsequent light induction response in Sinapis alba L

Relative quantum responsivity curves for inhibition of hypocotyl elongation in Sinapis alba L. seedlings previously grown in white light confirm that a marked "end of day" inhibition response can be induced by a monochromatic light treatment (30 min) at the end of the light period. In dark grown seedlings, however, no growth inhibition can be induced by a 30 min monochromatic light treatment. A prerequisite for an induction response appears to be a pretreatment with continuous light. Far red light is most effective with blue and red light showing a lesser effectiveness. The light pretreatment also shows a marked fluence rate dependency with respect to its ability to allow an induction response to manifest itself. The pretreatment required shows all the characteristics of a classical "HIR" response. The appearance of the effect in plants treated with the herbicide SAN 9789 seems to exclude chlorophyll as being the photoreceptor.