||The present paper is concerned with bolting and morphogenesis of Hyoscyamus niger L. as reactions upon radiation in the visible spectrum.Experiments are described in which Hyoscyamus plants were exposed to light of various well defined spectral regions. The light of these spectral regions was applied at low intensity, supplementary to a short day in various colours, or at high intensity, as exclusive source of light. Special attention was paid to study the influence of these spectral regions given in the middle or at various other points of the 14-hour dark period, in combination with short days in various colours. Moreover, morphogenetic responses caused by sequences of night breaks of different spectral regions during the dark period were tested.In Hyoscyamus , stem elongation and flower bud initiation are linked, so that elongated plants normally have flower buds and rosette plants have not.A-MORPHOGENETIC EFFECTS OF BROAD BAND COLOURED IRRADIATION AT HIGH INTENSITY USED THROUGHOUT THE MAIN PHOTOPERIOD OR AT LOW INTENSITY, SUPPLEMENTARY TO SHORT PHOTOPERIODS IN VARIOUS COLOURS1. Hyoscyamus reacts as a typical long day plant. Artificial white light, increasing the duration of the light period, speeded up shooting, at an approximate critical day length between 10 and 12 hours (fig. 3 and table 1). However, stem formation in Hyoscyamus occurred in short days when 30 or even 10 mins far-red radiation was given after short basic photoperiods in various colours of light (figs. 10, 13).2. Hyoscyamus plants, grown in long days of blue light produced stems while plants in green and red long days remained in the rosette stage (table 2). Blue light was also more elongative than white light. Plants in blue and white light had higher top/root ratios than plants exposed to green and red light (table 2). A large portion of the top/root differences in the various light qualities was due to differences in root growth, blue and white light reducing it, and the other colours stimulating it. Moreover, strong elongation of petioles occurred as a result of exposure to blue light, whereas this elongation was absent in green and red wavelength bands, and in white light.3. Hyoscyamus reacted to supplementary irradiation in the blue and far-red regions with a marked elongation of stems (fig. 4) and petioles (fig. 6). This result indicated that in Hyoscyamus the high energy mechanism of the B-FR reaction exist. Plants grown in red light remained in the rosette stage, while additional far- red annihilated this inhibitive effect of red light (fig. 4). Moreover, 2 hrs blue light given after long days in red light (14 hrs) caused bolting, but obviously more slowly than a far-red supplement. Therefore, blue light resembled far-red in its influence but appeared less effective.4. Red supplementary light, after a short day in white light was more active than far-red under similar conditions (figs. 7, 18). This observation, however, is in contrast to the available literature which indicates that red extensions mostly are inactive. This contradiction may be due to the fact that the white light used for the main photoperiod contains a relatively high fraction of far-red. This result, and also the long-day effect observed when 6 hrs of red light were given after a short day in blue light (fig. 7) suggest that induction in Hyoscyamus requires intermediate P fr -levels. Additionally the fact that plants in long days red light remain vegetative, supports the idea that high concentrations of P fr are inhibitive.5. Antagonism between red and far-red was not observed under the experimental conditions (figs. 18, 19, 20). Generally, in far-red followed by red extensions and in red followed by far-red extensions, the two colours increased each other in producing the effect, when they were given after a short photoperiod in white light. On the other hand, daylength extensions of blue light followed by red caused a long day effect more or less equal to that of extensions by red light alone (figs. 18, 19). In this case, blue irradiation acts as darkness and red light after blue acts as a night break.B-MORPHOGENETIC EFFECTS OF VARIOUS PATTERNS OF NIGHT BREAKS GIVEN IN THE MIDDLE OR AT VARIOUS OTHER POINTS OF THE DARK PERIOD1. The colour of the basic photoperiod in conjunction with a red night interruption exerts an important influence on the photoperiodic and formative responses. Short photoperiods, rich in far-red (figs. 22, 24, 25, 38) or blue irradiation (fig. 38), combined with red night interruptions yielded quick bolting. Increase of the light intensities applied as night interruption resulted in still earlier flowering. At the intensity of red light of 1000 ergs/cm 2/sec, only 120 mins night breaks were found elongative, in combination with a main photoperiod in red light. With the light intensity of the red night breaks increased to 3800 ergs/cm 2/sec, even a 3 mins exposure led to flowering.2. A 10 mins red night break combined with white light rich in far-red (figs. 22, 24, 25) was sufficient to induce elongation and flower bud formation. The longer the duration of the red irradiation, the sooner stem elongation occurred. However, the photoperiodic response was saturated at 3 mins red night breaks in combination with the blue or mixed (R + FR) basic light periods (figs. 38, 39), and, therefore no clear-cut photoperiodic differences existed in the series of red night break duration or intensities. Generally, upon red night interruption, plants under a main light period consisting of a mixture of red and farred radiation showed the highest stems, the red ones the shortest, while the blue ones were intermediate (fig. 39). Upon a red night interruption, there was a similarity between the blue and red + far-red irradiations when used as basic short periods, the former, however, was slower than the latter.3. Far-red night interruptions (figs. 22, 24, 25 40), were promotive in most cases in durations of at least 10 mins, while increasing the duration enhanced the flowering reaction, but the reaction was slower than that upon red night breaks. However, far-red interruptions caused faster shooting than red night breaks in combination with short days in red light. The difference in far-red intensity applied for night interruptions had no important effect.4. It was observed that with increased duration of the night breaks, stem elongation occurred more rapidly, and consequently the number of leaves also were nicely correlated herewith (figs. 22, 23).5. No distinct differences in petiole length occurred as a result of the various durations of red night breaks as compared with their controls, when these night breaks were combined with red or R + FR basic periods. The only difference was that petiole length was greater with the mixed (R + FR) basic periods than with the red one. However, petiole lengths increased markedly owing to far-red night breaks as compared with their controls under the basic periods of red light and of the mixture of red and far-red light, and stronger so with the mixture than with red light. With the blue main light period, it was found that either red or far-red night breaks produced a sharp reduction of the petiole length.6. Red and far-red night breaks showed opposite photoperiodic response curves when applied at various moments during the long dark period (figs. 27, 28, 30, 33, 43, 44, table 3), red light being most active in the second part of the night, far-red in the first part. A mixture of these two radiations showed the red action curve, but on a higher level than the red itself (table 3).7. Long durations of blue light as a night break at high intensity (3800 ergs/cm 2/sec) were elongative mainly as red (fig. 43) when applied at various moments of the night, while the time curve of the action as spread over the night does not exclude the possibility of some far-red activity in the first half of the night. Short periods (10 mins) at high intensity (fig. 45) or long durations (2 hrs) at low intensity (1000 ergs/cm 2/sec) blue light failed to induce shooting (fig. 27).8. It was demonstrated that in combination with mixed light periods, red night interruptions caused rapid shooting when applied at each point of darkness even immediately after or before the basic light period without the requirement of a dark intercalation (figs. 43, 44, 45). In contrast, with red basic light periods sufficiently long dark periods must precede (at least 6 hrs of the 14-hour darkness) or follow (at least 2 hrs) the night interruption. This observation seems in agreement with the data in fig. 27, where two white light sources were used as a short basic light period; one of them contains more far-red (TL/IL) than the other (TL). In the latter case, 2 hrs of red light given either immediately after or before the main photoperiod were ineffective, only red light given later at several points during the dark period was effective for stem elongation and flower bud formation. However, in combination with the mixed white light source, red light was active at each point during the long dark period, even immediately after or before the basic light period. Additional experiments with variable short lengths of dark periods immediately before the main light period, showed that, for bolting in Hyoscyamus , it was necessary to intercalate at least some 30 mins darkness between a red night interruption and the main photoperiod in case the latter is rich in red (the TL-source) (fig. 35).C-PHOTOPERIODIC RESPONSES CAUSED BY SEQUENCES OF NIGHT BREAKS IN THE DARK PERIOD1. The data did not demonstrate antagonism between red and far-red irradiation during the night in combination with artificial white light sources (figs. 30, 31, 34, table 3). Far-red irradiation given after red night breaks enhanced the promotive action of red light, speeding up bolting particularly in the first half of the night. However, the application of red and far-red simultaneously during breaks in the long night had more effect on stem lengthening than far-red after a red night break The explanation seems to be that, since the durations of either red or far-red were long (2 hrs), far-red radiation given after a red night break could no more annihilate the flowering promotion of red light.2. Ten minutes far-red night interruption applied at various points of the night were inactive (fig. 45) with main light periods rich in far-red (R + FR) or not (R). However, 10 mins red night breaks at the same points were promotive for stem elongation in combination with the mixed basic light period (R+ FR); in case of the red basic period, red night breaks were only active at the 8th and 10th hour point of the 14-hour dark period.3. When Hyoscyamus plants were irradiated successively with 10 mins red and 10 mins far-red radiation at various points of the dark period, it seemed that far-red radiation reduced the elongative effect of the red night interruption in combination with the mixed (R + FR) basic period. This seems a partial manifestation of the antagonistic action of far-red against red with respect to the phytochrome system, however, the elongative action of red light observed was not much reduced (fig. 45). That some reduction of the red light effect was observed here, in contradiction to section 1 above, may be due to the fact that light periods were much shorter (viz., 10 mins v. 2 hrs).In connection with the red light basic period, far-red radiation given after various times of darkness was inactive, and red interruptions at some points also However, especially in the first half of the dark period, the succession of 10 mins red and 10 mins far-red led to flowering at all points of the dark period (fig. 45).Thus, reversibility of red action by subsequent far-red radiation in general, viz., except in first part of this section was not found in our experiments.Generally, the whole of our experiments with Hyoscyamus niger L. leads to the following conclusions:1. In general, applications of red and far-red reinforce each other, and the application of their mixture is particularly effective.2. The observed promotive action of blue light throughout the photoperiod or during the night appears to fit well together with the above results.3. The activity of Hyosyamus plants to red and far-red is different throughout the night viz., greater to far-red in the first half and to red in the second half.4. Speculating about the internal mechanism by which the observed reactions can be provisionally understood, it seems that those mentioned under item 1 may be explained in terms of phytochrome reactions by assuming the requirement of intermediate P fr -levels or specific ratios between P fr and P r for optimum induction.The blue light effects (item 2) fit well in this suggestion as both forms of phytochrome have a definite absorption in the blue region.The observation of the changes in sensitivity during the night (item 3) reveals the existence of some 'endogenous rhythm' which possibly might be understood as causing different changes in P r - and P fr -levels. at various points of a long dark period, or bringing about changes in the plant's sensitivity towards phytochrome stimulation.Direct evidence as to phytochrome reactions under the conditions of our experiments so far is not available but appears to form an extremely important object for future research.