Erwin Bünning and Wolfgang Engelmann: establishing the involvement of the circadian clock in photoperiodism

Abstract

In 1936, Erwin Bünning published his groundbreaking work that the endogenous clock is used to measure day length for initiating photoperiodic responses. His publication triggered years of controversial debate until it ultimately became the basic axiom of rhythm research and the theoretical pillar of chronobiology. Bünning's thesis is frequently quoted in the articles in this special issue on the subject of "A clock for all seasons". However, nowadays only few people know in detail about Bünning's experiments and almost nobody knows about the contribution of his former doctoral student, Wolfgang Engelmann, to his theory because most work on this topic is published in German. The aim of this review is to give an overview of the most important experiments at that time, including Wolfgang Engelmann's doctoral thesis, in which he demonstrated the importance of the circadian clock for photoperiodic flower induction in the Flaming Katy, Kalanchoë blossfeldiana, but not in the Red Morning Glory, Ipomoea coccinea.

Keywords: Kalanchoë blossfeldiana; Phaseolus multiflorus; Circadian clock; Erwin Bünning; Photoperiodism; Wolfgang Engelmann.

Fig. 1

The experiments of Bünning that let him conclude that the circadian clock is involved in photoperiodism. a Rhythmic leaf movements of Phaseolus multiflorus plants grown in a greenhouse during autumn (October; light-dark cycle (LD)11:13), spring (April, LD13:11) and summer (June, LD18:6). The leaves reached their day-position (arrows up) always 4–6 h after lights-on (orange double-headed arrow) and the night-position (arrows down) 8–10 h later (blue double-headed arrow). This means that a considerable part of the leaves’ evening phase (blue shade) was in darkness (grey shade and black bar on top) in spring and autumn, while the leaves’ evening phase was completely in light in summer. The proportion by which the evening phase was in darkness determined the degree of flower induction. b Advancing or delaying the clock by red light given every day for 1 h either in the morning or the afternoon (red dots) changes the number of hours in which the evening phase is in darkness and consequently the induction of flowering. Please note that in this experiment the leaf movements were only monitored during the last day in the greenhouse under constant darkness after the shift was performed and before they were planted in the field (solid lines), where they experienced the still short days of April. The curves were manually continued (broken lines) to illustrate their putative relationships to the environmental LD cycle. c Rhythmic leaf movements of two Phaseolus plants with long and short morning phases, respectively. The plant with the long morning phase had a short evening phase that was almost completely in darkness under a long day of May, while the other plant had a long evening phase of which only a small portion was in darkness. Consequently, they showed strong and weak flower induction, respectively. Redrawn after Bünning (1936)

Fig. 2

Petal movements of Kalanchoë blossfeldiana and the Phase Response Curve to light. a Inflorescence of the Flaming Katy. b The petal movements can be recorded in individual flowers; here all flowers are in their night state and therefore closed. c In diurnal 12:12 h cycles, the flowers are completely opened always near the middle of the light period and completely closed in the middle of the dark period. d This petal movement continues under constant darkness. Here, two cycles of petal movement are shown. The previous light periods are indicated as grey bars on top of the diagram. The upward arrow points to the completely opened flower in the middle of the first subjective day, whereas the downward arrow indicates the closed flower in the middle of the subjective night. e Phase shifts of the rhythm evoked by 2 h far-red light pulses administered at different times of the subjective 24 h day. The start of the subjective day is at circadian time CT 0, while the beginning of the subjective night is at CT12. Depending on the time of day at which the light pulse was given, the rhythm of petal movement is advanced or delayed (see text). The red part of the curve indicates the conventional Phase Response Curve (PRC) which shows relatively small phase shifts during the subjective day but large phase shifts during the subjective night. a-c: after Engelmann and Antkowiak (2016), d-e: modified after Zimmer (1962)

Fig. 3

The photoreversible phytochrome system. Pr is the inactive form of phytochrome that is converted into the active Pfr form by red light. Far-red light reverses the active Pfr form back into the inactive Pr form. Pfr signals to the photoperiodic timer and the latter inhibits flowering in short-day plants when it receives simultaneously signals from the circadian clock in its scotophilc phase. In long-day plants the photoperiodic timer needs to receive simultaneously signals from the clock in its photophilic phase. In addition to flowering, the active Pfr form promotes seed germination and photomorphogenesis. Seed germination is seasonally regulated and therefore also needs input from the circadian or the circannual clock (Engelmann 2009b) (not shown here). Pfr signals also to the circadian clock (stippled arrow), but it is not the clock’s only photoreceptor. Short red-light pulses that affect the photoperiodic timer cannot phase-shift the clock

Fig. 4

Dependence of flower formation on the time of red-light disturbance (2 min of 660 k erg cm^− 2) during a 62 h dark period with a 72 h cycle that was applied seven times. a Light program for the controls and the 20 different experimental plant groups. The plants came from continuous light (LL) in the greenhouse and were placed back to LL after the seven 72 h cycles. The 2-minute red light pulses are indicated as thin white bars in the 62-hour dark period. b The number of flowers was counted 4 months after the end of the light induction. The blue line indicates the average number of flowers of the controls (± standard error of the mean, SE, as dotted blue lines). The number of flowers counted in the different groups of experimental plants is indicated as filled black dots (± SE) connected by black lines. The rhythm of petal movements is shown in red. The inhibition of flowering by red light roughly coincided with the assumed scotophil phase of the circadian clock (petals closed, see Fig. 2), while promotion of flowering by red light roughly coincided with its predicted photophil phase (petals open)

Fig. 5

Dependence of flower formation on the length of a single dark period in Ipomoea coccinea. The points represent mean values of several (3–22, mostly about 10) plants. In some cases, vertical lines indicate the mean errors. Modified after Engelmann (1960). The picture of I. coccinea was taken by Michael Wolf in August 2008 in the botanical garden of Dresden

Fig. 6

Three generations of chronobiologists at the Botanical Institute of the University of Tübingen (from left to right: Erwin Bünning, Wolfgang Engelmann, Charlotte Helfrich-Förster). These pictures were taken by Richard Helfrich on July 18, 1985, on CHF’s PhD defense