A florigenic effect of sucrose in Fuchsia hybrida is blocked by gibberellin-induced assimilate competition

The use of gas chromatography-mass spectrometry-selected ion monitoring alo ng with a C-13 internal standard has allowed sensitive measurements of the sucrose (Suc) content of individual shoot apices of Fuchsia hybrida. With i ntact plants, as the photosynthetic irradiance increased, so did shoot apex Suc content, reaching saturation at about 500 mu mol m(-2) s(-1). These sa me plants flowered at the higher irradiances, remaining vegetative in 10-h short days at an irradiance of 230 mu mol m-2 s(-1). The strong correlation (r = 0.93) in these studies between flowering and shoot apex Suc content i ndicates a role for Suc as a stimulus to flowering in this species. However , Suc is not the long-day (LD) "florigen" of F. hybrida because 2 to 4 LD g iven as a 14-h low-irradiance photoperiod extension (10-15 mu mol m(-2) - s (-1)) induced flowering but without increase in shoot apex Suc content. Flo wering induced by either pathway, the LD- or the Suc- mediated one, was inh ibited by applying gibberellin (GA) to the shoot tip. Such inhibition of fl owering by GA, at least for the LD pathway, was associated with a reduced a pex Suc content, enhanced elongation of subapical stem tissue and a reduced import into the shoot apex of leaf-sourced assimilate. Thus, our findings show how GA inhibits flowering of F. hybrida and confirm the importance of nutrient diversion in regulating flowering.