Physiological approaches to the study of flowering-time control
Mature leaves perceive photoperiod, yet floral evocation occurs in the shoot apex at the SAM. Therefore, long-distance signalling by floral stimuli must occur (Bernier et al., 1993) .
The first demonstration of the floral stimulus was by Knott (1934) , who demonstrated that exposing only spinach leaves to photoperiodic cycles of light and dark would initiate flowering, whereas exposing only the apex to these conditions would not. He therefore concluded that an unknown factor must be transported from the leaves to the shoot apex, where it promotes floral development.
In 1937, Mikhail Chailakhyan suggested that the floral stimulus was a flowering hormone, which he named florigen (Chailakhyan, 1937; Aksenova, 2002) . This hypothesis was extended by the proposal that an anti-florigen molecule exists and has an inhibitory effect upon flowering. This substance was proposed to be expressed in non-induced leaves (Lang et al., 1977) .
Grafting experiments designed to study the photoperiod response have been performed in numerous species. In these experiments single leaves or the lower shoot and root (stock) of a plant are exposed to one photoperiod and grafted to the shoot apex or upper shoot (scion) of a second plant. The effect of the leaves or stock on flowering of the scion can then be measured. This approach has been used in Tobacco, Perilla, pea and more recently Arabidopsis (Zeevaart, 1962; Lang, 1965; Murfet, 1992; Weller et al., 1997; Turnbull et al., 2002; An et al., 2004) .