A Quantitative and Cumulative Response to Photoperiodic Induction of Hyoscyamus niger, a Qualitative Long Day Plant

Abstract

The photoperiodic induction of the long day plant Hyoscyamus niger has been investigated using white light and light of restricted spectral regions. Illumination with continuous white light revealed a quantitatively augmented response with both prolonged duration and increased irradiance in contrast to the otherwise qualitative photoperiodic nature of Hyoscyamus. From the three light qualities blue, far-red and red only the last was capable of inducing flowering in night breaks. Though the features of a phytochrome mechanism are apparent, complete reversibility and saturation, however, were lacking and reciprocity between duration and irradiance did not hold. Thus the long day plant reacts in a mode different from short day plants. In analogy with continuous light, a quantitative and cumulative response to subinductive pulses was found. Photosynthesis may participate in the night break response, as being tested by the incorporation of 14CO2 and CO2-free atmosphere.