Flowering and Endogenous Rhythms
The last four decades of studies in animals and plant physiology have clearly established the existence of a biological clock mechanism to measure time in all living organisms. Circadian rhythms are one such type that requires 24 hours (diurnal) to complete one full cycle of the rhythm. Leaf movements, opening and closure of flowers, photosynthesis, auxins production and rate of cell division follow this pattern. A rhythm controlled from within is independent of artificial changes in environment once initiated. Only a prolonged change in light and dark conditions can dampen the rhythm. Bunning did pioneering work on the role of rhythms in flowering. We know that interruption of dark period by red light leads to inhibition of flowering. But at what point it does during the dark period is also important. These studies lead to the concept of photophil and skotophil which means there are light loving phases in dark period (exposure to red light is not inhibitory) and dark loving phases (where exposure to red light is inhibitory).