Effects of Radiation on Flowering of Hyoscyamus Niger

Flowering and stem‐lengthening responses of the long‐day plant Hyoscyamus niger L. were followed under various photo‐ and dark‐period combinations. Phytochrome action was identified by its photoreversibility, spectral region of response, and requisite levels of irradiance. Flowering control was also affected by another photoreaction having an action maximum at 710–720 nm. This reaction, while observable over appropriate short periods, is best displayed by protracted irradiation. The effectiveness of this reaction has a half‐life of the order of 1–4 min. Responses to it can be observed over an intensity range of more than 100‐fold. The two photoreactions interplay in flowering control depending on light quality during photoperiods. The interplay is a prominent factor affecting the adequacy of fluorescent and incandescent‐filament radiation in plant growth experiments.