Sucrose at the shoot apex
Sucrose is the metabolite that has been shown to reach the SAM earliest after floral induction. Although there is a large accumulation of sucrose at the shoot apex during floral induction, there is no immediate effect on mitotic activity. The source of the sucrose is not increased photosynthesis, but is mobilisation of carbohydrates stored in the leaves and stem (Bodson and Outlaw, 1985; Bernier et al., 1993; Corbesier et al., 1998) . Following the surge in sucrose, cytokinin levels increase in the vascular tissues; the source of the cytokinins is likely to be the roots, where it is released following stimulation by sucrose (Bernier et al., 1993; Lejeune, 1994) . The released cytokinins are mainly zeatin riboside and isopentenyladenine riboside. Bernier and colleagues (1993) inhibited the transient root to shoot flux of cytokinins in white mustard (Sinapis alba ) during a long-day and demonstrated this abolished the flowering response.
Induction of flowering by Gibberellins
Gibberellins are a large group of tetracyclic diterpenoid carboxylic acids, which have an ent-gibberellane skeleton. They induce flowering in many plant species. The application of exogenous gibberellins to Arabidopsis or the dual-day length plant Bryophyllum (which flowers only after a sequence of short days followed by long days) grown under short days initiates flowering (Lang, 1965; Zeevaart, 1985) . This effect also occurs when gibberellins are provided to some short-day plants grown under long-day photoperiods (Taiz and Zeiger, 2002) . Application of exogenous gibberellic acid can also promote flowering in non-vernalised Arabidopsis plants and is able to overcome the age related autonomous floral evocation required by some plants (Wilson et al., 1992; Taiz and Zeiger, 2002) . This is in agreement with the observation that exogenous application of inhibitors of gibberellin biosynthesis to Arabidopsis delays flowering (Chandler and Dean, 1994) .
Work on Spinacia oleracia gave new insights into the regulation of gibberellins by photoperiod. This long-day plant contains low levels of gibberellins when grown in short-day photoperiods, and grows vegetatively (Taiz and Zeiger, 2002) . However when spinach is grown under long days there is a marked increase in the levels of gibberellins made by the 13-hydroxylated pathway (GA 53 – GA 44 – GA 19 - GA 20 – GA 1). This results in a five-fold increase of GA 1 and leads to stem elongation (Zeevaart et al., 1993) . Similar increases in GA biosynthesis in response to photoperiod were described in Arabidopsis (Olszewski et al., 2002)
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