Physiology of Photoperiodic Time-Measurement

Abstract

Few seriously doubt that circadian rhythms are involved in photoperiodic time-measurement: the experimental evidence is difficult to interpret in any other way. Two sets of data may make the point. If sparrow are free-run in darkness there is a circadian rhythm of photoinducibility (Follett et al. 1974). Early in the subjective day a single block of 8 h light will not alter gonadotrophin secretion, but during the subjective night it increases the plasma concentration significantly. This rhythm of inductiveness recurs for at least five cycles with a periodicity close to 24 h. In hamsters, gonadal growth can be induced with only 1 h of light each day if the photoperiodic cycle differs slightly from 24 h (Elliott 1976), a finding which seems explicable only in terms of circadian entrainment theory. Given such results, it was inevitable that various models would be proposed as to how circadian rhythms might measure daylength and two classes of model have attracted the most attention. The first (“external coincidence”) proposes that one of the many circadian oscillators is a rhythm of “photosensitivity”, Should light coincidence with this daily peak of “photosensitivity” then induction occurs. The second (“internal coincidence”) is based upon the belief that in a multioscillatory organism phase relationships between the oscillators alter as daylength changes, with the result that at one time of the year two of the oscillators may be in a state where they cause induction, whilst at another the phase relationships are non-inductive (Pittendrigh 1981).