Signal Detection

Flowering in long day and short day plants is controlled by photoperiod, but which part of the plant detects it? Experiments in the 1930s showed that the leaves detect the light signal, not the shoot tip where the flowers form.If a short day plant is kept in long days but a single leaf is covered so that it receives short days, the plant still flowers. If all the leaves are removed, the plant does not respond to day length at all. So the stimulus is perceived in the leaf and must then travel to the shoot apex.Grafting experiments support this. When an induced plant (one that has received the correct photoperiod) is grafted onto a non-induced plant, the non-induced plant also flowers. This suggested a mobile flowering signal, named florigen, that moves through the phloem. Florigen is now known to be a protein (FT protein) made in the leaves.Further experiments showed that plants actually measure the length of the dark period, not the light period. A short day plant given long nights will not flower if each night is interrupted by a brief flash of light (a night break). Short day plants are really long night plants: they flower only when the night is longer than a critical length.The fact that a short flash of light had such a large effect suggested that the leaves contain a light-absorbing pigment. This pigment is phytochrome.