For flowering plants to cross pollinate, they must produce flowers at the same time of year. How do plants that may be different in size or age synchronize their flowering? In the 1930s, James Bonner and Karl Hamner discovered that the key flowering trigger in some plants is the length of night, which explains why a species will flower during a particular season year after year.
For historical reasons, plants are described as short-day or long-day plants, rather than the more appropriate long-night and short-night plants. Day-neutral plants, in which the flowering trigger does not depend on a specific length of darkness, are perhaps more common than short- or long-day plants. In this animation, we will examine flowering experiments performed on cocklebur, a short-day plant.
How do plants measure the length of a dark night? The photoreceptor molecule, phytochrome, appears to play a key role. The active form of phytochrome—Pfr—promotes flowering in long-day plants, but inhibits flowering in short-day plants.
It was once hypothesized that the timing mechanism might simply be the slow conversion of a phytochrome during the night from the Pfr form—produced during the light hours—to the Pr form. Such phytochrome conversion would function as an "hourglass," and the effect of a night would depend simply upon whether all the phytochrome had been converted. However, this suggestion is inconsistent with many experimental observations, such as the fact that when a plant is subjected to a dark period several days in duration, the plant's sensitivity to a light flash during the long night varies on a roughly 24-hour cycle. Such data suggest instead that the phytochrome is only a photoreceptor, and that the timekeeping role is played by a biological clock that is linked to the phytochrome (which sets the clock) and also to the production of flowers.
Textbook Reference: Concept 27.2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State