||Investigations have been carried out on the photoperiodic control of flower initiation in the annual strain of the long-day plant Hyoscyamus niger L. Plants were grown in soil; precultivation (in short day (SD) fluorescent light) as well as experimental treatments were applied at 20°C. Coloured radiation was obtained by combining special coloured fluorescent lamps with 'plexiglas' filters (subsection 2.2.2).In white light no absolute juvenile phase could be observed, but with increasing age, up to 30 to 40 days, plants were more sensitive to inductive treatment (subsection 2.3.2). For most experiments, the precultivation period was 3 to 4 months.As a parameter of flower initiation 'number of flower primordia' was found to be more generally applicable than 'days to shooting' and 'leaf increment' (sections 2.4. and 3.2.). For 'days to shooting', plants in a mixed radiation of fluorescent and incandescent lamps had a slightly shorter critical daylength than plants in fluorescent light only. For initiation of flower primordia there was no difference in critical daylength between the two light qualities (section 3.2.).In most experiments various daylengths, from zero to 1000 minutes or to continuous light, were applied during a period of treatment days (TD) interrupted at regular intervals by a few SD in high intensity white fluorescent light, for energy supply. After-treatment was given in long days (LD) in a mixed irradiation of fluorescent and incandescent light, in order to ultimately obtain flower initiation in all treatments. Flower primordia for all treatments were counted at the end of the LD after-treatment. None of the components of the experimental scheme (except, of course, TD) was found to affect essentially the type of response curve obtained (section 3.2.).The effect of light intensity in a mixture of fluorescent and incandescent light on daylength dependence of flower initiation was investigated in section 3.4.. Inhibition of flower initiation (SD-effect), as observed at high light intensity (24,000 ergs.cm -2.sec -1< 700 nm) for daylengths between about 300 and 600 minutes, was lost at low light intensities (2400 or 400 ergs.cm -2.sec -1< 700 nm) for all three parameters mentioned above. With photoperiods longer than ca. 800 minutes, promotion of flower initiation (LD-effect), determined as 'number of flower primordia' or 'days to shooting', was weaker at low light intensities. For 'leaf increment', in all three light intensities a similar, weak LD-effect was observed.The daylength response curve for white light could be split into a saturation curve, representing the inhibitive light action, and a curve reaching saturation at much longer photoperiods, representing a light action, promotive for flower initiation. At extremely short photoperiods (below 50 min.), a weak extra inhibition was apparent (cf. section 3.5., fig. 20).In the experiments of Chapter 4, in which coloured light (far-red, red, green, and blue) was applied during the TD, it was observed that the extra effect at short photoperiods ('reaction 1') could be either inhibitive (in red and green) or promotive (in far-red and blue) for flower initiation. With red and green light, inhibition of flower initiation was observed at all daylengths (cf. section 4.2., fig. 21); the postulated light induced promotion attained only a low level in LD (cf. section 4.5., fig. 31). With far-red, promotion of flower initiation was observed at all daylengths (cf. section 4.2., fig. 21); light induced inhibition was not necessarily involved (cf. section 4.5., fig. 33). With blue light, inhibition was only observed at daylengths between 160 and 730 minutes (cf. section 4.2., fig. 21); at other daylengths promotive actions dominated (cf. section 4.5., fig. 32).Inhibition of flower initiation by 420 minutes red light could be annihilated by 3 minutes far-red; this was shown to be a phytochrome effect (section 4.3., fig. 22). The promotive effect, observed with 3 minutes far-red alone, was not obtained when far-red was applied after exposures to red light of the order of 400 minutes (section 4.4., fig. 28).A brief exposure to far-red, preceding 20 minutes red light, enhanced the inhibiting effect of the red, while a brief exposure to red light, preceding 20 minutes far-red enhanced the promoting effect of the far-red (cf. section 4.4.).Leaf position was strongly affected by phytochrome; however, after 420 minutes far-red, no red/far-red reversal effect on leaf position was any more observed (cf. section 4.3., Plates 6 and 7).The above data, together with suggestions from literature, give rise to the following interpretation.The effect occurring at extremely short daylengths ('reaction 1') most likely is a phytochrome effect on cell membranes.The inhibitive effect (light induced inhibition), responsible for the normal SD-effect, is assumed to require some factor from photosynthesis ('system II') for its realization and, besides this, phytochrome in the far-red absorbing formThe promotive effect (light induced promotion), responsible for the normal LD-effect, is assumed to require ATP for its realization (produced via the photosynthetic 'system I' or via dissimilation), and also P fr .It has been demonstrated that it is possible to split the experimental curves into these three components which shows the daylength dependence of these effects separately (cf. section 4.5.).Occasional masking of the phytochrome control of leaf position may well be attributed to a high ATP level, nullifying the action of P fr .In chapter 5, the nature of the substances which play a role in the subsequent phases of the reaction chain leading to inhibition or promotion of flower initiation, has been discussed. Special attention was paid to effects of gibberellins, auxins and abscisic acid.