Grafting Perilla and citrus
Classical grafting experiments using Perilla crispa or Xanthium demonstrated exposure of a leaf or the stock to inductive photoperiods is required for the production of the floral stimulus. In these experiments grafting of a single Perilla leaf induced by exposure to short days onto a non-induced long-day grown Perilla shoot resulted in the axiliary shoots of the non-induced plant producing flowers (Zeevaart, 1962) . The same leaf was sequentially grafted onto seven non-induced stems and induced flowering in all of them indicating that a single leaf is highly effective in floral induction. Furthermore, grafting citrus experiments showed that floral signals could be transmitted between different plant species. Although for this to be effective, the plant species must be taxonomically closely related such as grafting citrus trees (Zeevaart, 1976) . Continued production of the floral stimulus seems to be required, at least in some species. If induced leaves were removed from Impatiens balsamina plants then floral shoots reverted to a vegetative identity (Tooke et al., 1998) .
Florigen
Three main models were proposed to describe the nature of the floral transmissible stimulus. The florigen/anti-florigen model proposes that the promotive and inhibitor stimuli are single universal growth regulators. In contrast, the nutrient diversion model proposes that the balance of source/sink relationships are modified during flowering to enable the apex to receive increased assimilates. Finally, the multifactorial control model suggests that many different nutrients and phytohormones are required to comprise the floral stimulus (Bernier et al., 1981; Sachs and Hackett, 1983; Lang, 1984).
Many micro and macromolecules that are involved in flowering move readily from the leaf to the shoot apex through the symplastic pathway connections of the phloem (Imlau et al., 1999; Oparka and Cruz, 2000; Ruiz-Medrano et al., 2001) . These molecules include sucrose, cytokinins, and gibberellins, all of which appear to have a role in the promotion of flowering of mustard (Wilson et al., 1992; Blazquez et al., 1998; Bonhomme et al., 2000) . Sucrose and cytokinin concentrations increase at the apex during floral evocation (Corbesier et al., 1996). Interestingly, there is a decrease in the size exclusion limit of plasmodesmata at the shoot apex on floral induction (Gisel et al., 1999; Gisel et al., 2002) . This alteration in the aperture of plasmodesmata may restrict trafficking in a way that enhances the impact of the floral stimulus. (Crawford and Zambryski, 1999; Gisel et al., 2002).
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