An attempt to analysis of the effect of light on stem elongation and flowering in Hyoscyamus niger L.

The shooting of Hyoscyamus niger was studied as a reaction to radiation with coloured or white light in various combinations of intensity and duration. Research into the reaction to very short photoperiods became possible by continuing treatments with weak light not longer than 6 or 10 days, and then to keep the plants in LD of white light until shooting.

The data presented were interpreted as follows. Hyoscyamus would grow vegetatively if floral induction (an accumulative and autonomous process comprising flower bud initiation and a shooting impulse) were inhibited, also if stem elongation underlay formative inhibition (suppression of etiolation). Thus, vegetative growth may be due either to unsuitable photoperiod, primarily inhibiting floral induction, or to unsuitable light quality, primarily causing formative inhibition of stem elongation.

The formative inhibition was due to the production of an inhibitor precursor, which became active principally in darkness, after a light period. In the dark, the precursor would be gradually converted into an inhibitor which itself had no measurable persistence but took immediate effect as an inhibition. In longer maintained darkness the plant was no longer inhibited, no more than during light. The inhibition by short photoperiods increased with time of radiation as in a formative light action. Near-infrared may be assumed to antagonize inhibition by inactivating the precursor.