Plant Physiol, January 2001, Vol. 125, pp. 488-496

A Florigenic Effect of Sucrose in Fuchsia hybrida Is Blocked by Gibberellin-Induced Assimilate Competition

Commonwealth Scientific and Industrial Research Organization, Plant Industry, G.P.O. Box 1600, Canberra, Australian Capital Territory 2601, Australia (R.W.K., Y.B.-T.); and Agricultural Research Organization, The Volcani Centre, P.O. Box 6, Bet Dagan 50250, Israel (Y.B.-T.)

The use of gas chromatography-mass spectrometry-selected ion monitoring along with a ¹³C internal standard has allowed sensitive measurements of the sucrose (Suc) content of individual shoot apices of Fuchsia hybrida. With intact plants, as the photosynthetic irradiance increased, so did shoot apex Suc content, reaching saturation at about 500 µmol m² s¹. These same plants flowered at the higher irradiances, remaining vegetative in 10-h short days at an irradiance of 230 µmol m² s¹. The strong correlation (r = 0.93) in these studies between flowering and shoot apex Suc content indicates 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 given as a 14-h low-irradiance photoperiod extension (10-15 µmol m² s¹) induced flowering but without increase in shoot apex Suc content. Flowering induced by either pathway, the LD- or the Suc-mediated one, was inhibited by applying gibberellin (GA) to the shoot tip. Such inhibition of flowering by GA, at least for the LD pathway, was associated with a reduced apex 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.