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

Abstract

The use of gas chromatography-mass spectrometry-selected ion monitoring along with a (13)C 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 micromol m(-2) s(-1). These same plants flowered at the higher irradiances, remaining vegetative in 10-h short days at an irradiance of 230 micromol m(-2) s(-1). 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 micromol m(-2) s(-1)) 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.

Figure 1

Induction of flowering of F. hybrida either by exposure to LD or by increasing the irradiance to 500 μmol m⁻² s⁻¹ in SD. Control plants maintained at an irradiance of 230 μmol m⁻² s⁻¹ in SD remained vegetative. Values are means ± se (n = 12).

Figure 2

Flowering of F. hybrida in response to a 4-LD exposure combined with a 1-week increase in irradiance during the daily 10-h main light period. The 1-week increase in irradiance (from 230 to 500 μmol m⁻² s⁻¹) was imposed at various times before, during, or after exposure of the plants to the 4 LD. The low-irradiance (230 μmol m⁻² s⁻¹) LD control is shown as a column. Horizontal bars on the data points show the period of high-light exposure with the plot points being for the midpoint of the 7 d. Vertical bars are ±se of the mean (n = 15).

Figure 3

Effect of time of day on the Suc content of the shoot apex of F. hybrida plants growing vegetatively in SD. The black and white boxes, respectively, indicate the daily cycle of dark and light. Bars are ±se of the mean (n = 5–9).

Figure 4

Effect of increasing the irradiance in SD on shoot apex Suc content and on flowering. Apices (n = 10) were harvested for Suc measurements 10 d after commencing the treatment and flowering (n = 20) was determined after 3 weeks. Values are means ± se. The correlation coefficient between Suc content and flowering was r = 0.93.

Figure 5

Relationship between GA₃ dose, stem elongation, and shoot apex Suc content for plants of F. hybrida growing vegetatively in SD. For Suc measurements, apex samples (six–eight per treatment) were taken 4 d after the GA₃ treatment. Stem lengths (n = 13) for each treatment are given as an increment over the starting value. Plants were measured daily but lengths are only shown after 7 d. Values are means ± se.

Figure 6

Effect of GA₃ application before, during, or after induction of flowering of F. hybrida by exposure to 4 LD or to 10 d at high irradiance in SD. The high irradiance was 500 μmol m⁻² s⁻¹, control plants receiving 230 μmol m⁻² s⁻¹. GA₃ was applied at 200 ng/plant in the experiment involving LD-induced flowering and at 100 ng/plant where high irradiance SD were used to induce flowering. Control plants were induced to flower but not treated with GA. Values are means ± se. (n = 12–20).

Figure 7

Change over time following GA treatment in shoot apex Suc content (top) and stem elongation (bottom) of vegetative or florally induced plants of F. hybrida. The synthetic GA 2,2-dimethyl-GA₄ was applied at a dose of 200 ng/plant to the vegetative plants in SD and at 400 ng/plant to florally induced plants. The daily light (white box), dark (black box), and LD treatment (hatched box) is shown on the abscissa. The inset shows further measurements of stem elongation out to 21 d by which time the stem length of the GA-treated LD plants was 275 mm. Errors are shown as ± se or as the least significant difference (P = 0.05).