Erwin Bünning

Erwin Bünning (23 January 1906 – 4 October 1990) was a German biologist. His most famous contributions were to the field of chronobiology, where he proposed a model for the endogenous circadian rhythms governing plant photoperiodism. From these contributions, Bünning is considered a co-founder of chronobiology along with Jürgen Aschoff and Colin Pittendrigh.

Academic career In 1930, Bünning took an assistantship under Otto Renner at the University of Jena, then one of Germany's largest botanical institutes. During the rise of the Nazi Party in Germany in the early 1930s, Renner stood in open opposition to the Nazis, publicly defending Jewish scientists. Similarly, Bünning was considered a communist sympathizer, a stance likely influenced by Bünning's father, a Social Democrat opposed to the Nazis. Bünning's political beliefs created tension between him and some of his students. In response to this tension, Bünning left Jena for a lectureship at the East Prussian University of Königsberg. Bünning would later publish a biography on Pfeffer. Bünning, however, proposed that biological clocks are endogenous and synchronize to daily stimuli. Unlike in the hourglass hypothesis, Bünning's hypothesis proposes that circadian rhythmicity derives from an interaction between light and a circadian pacemaker, not external stimuli alone.

Experiments In the early-1930s, Bünning proposed that organisms rely on a circadian rhythm of sensitivity to light to measure photoperiod. He demonstrated that plants open and close their leaves and insects eclose according to circadian rhythms, even in continuous light or darkness. His crossing experiments with bean plants of different periods in 1935 demonstrated that the next generation had periods of intermediate durations, supporting the suggestion that circadian rhythms are heritable. Bünning also demonstrated that an artificial photoperiod can induce flowering at inappropriate times, supporting his model of endogenous oscillators entrained to external stimuli. To create artificial photoperiods, Bünning exposed plants to a light-dark cycle indicative of a specific season, such as the long days and short nights of summer. By entraining the plants to a spring or summer photoperiod, Bünning was able to induce flowering, even if the actual season were fall or winter. From his results, Bünning proposed that biological clocks have sensors for both light and dark, and their relationship aids photoperiodic timekeeping.

His published works provided key synthesis of existing evidence for biological clocks across species, including plants, flies, and fungi. His 1958 work, "The Physiological Clock", was hailed as a milestone in the field of chronobiology, and his later works on plant physiology helped apply his work on photoperiodism to practical uses. In 1935 he demonstrated that plants with different inherent periods would become desynchronized in constant conditions. This work provided evidence for the dismissal of the "Factor X" hypothesis, which held that circadian rhythms were not endogenous, and were instead reliant on an unknown external stimulus, some "Factor X."

Bünning's model Bünning was the pioneer of the "Bünning Hypothesis," later known as the External Coincidence model. This model proposed a circadian rhythm of photoperiodic photosensitivity in plants. As day length increases with the spring and summer seasons, for example, light hits a plant's photosensitive phase, triggering a physiological or behavioral response. The Bünning Hypothesis proposed that light has two functions for an organism:

1. Light is the primary Zeitgeber for the synchronization of biological rhythms.

2. Light triggers photoinductive responses during certain phases of the circadian cycle. The next year, Bünning was elected Corresponding Member of the Botanical Society of America. During his life he was also elected honorary member of the Japanese Botanical Society, the German Botanical Society, the American Society of Plant Physiologists, and the Swiss Botanical Society. for "meritorious work in plant biology." The Universities of Glasgow (1974), Freiburg (1976), and Erlangen (1977) conferred honorary doctorates on him, as did the University of Göttingen

Selected publications Uber die Erblichkeit der Tagesperiodizitat bei den Phaseolus-Blattern (1932) Die endogene Tagesperiodik als Grundlage der photoperiodischen Reaktion (1936) Entwicklungs und Bewegungsphysiologie der Pflanze (1948) In den Waldern Nordsumatras: Reisebuch eines Biologen (In the North Sumatran Forests : Travels of a Biologist, 1949) Die physiologische Uhr ("The Physiological Clock", 1958) Biological clocks; Chairman's Address; Cold Spring Harbor Symposium (1960) Interference of Moonlight with the Photoperiodic Measurement of Time by Plants, and their Adaptive Reaction (1969, With Ilse Moser) Wilhelm Pfeffer: Apotheker, Chemiker, Botaniker, Physiologe 1845–1920 (1975) Pfeffer's views on rhythms (1975, With M K Chandrashekaran) Fifty years of research in the wake of Wilhelm Pfeffer (1977)