Enhancement of [¹⁴C]Sucrose Export from Source Leaves of Vicia faba by Gibberellic Acid

The effect of gibberellic acid (GA(3)) on sucrose export from source leaves was studied in broad bean (Vicia faba L.) plants trimmed of all but one source and one sink leaf. GA(3) (10 micromolar) applied to the source leaf, enhanced export of [(14)C]sucrose (generated by (14)CO(2) fixation) to the root and to the sink leaf. Enhanced export was observed with GA treatments as short as 35 minutes. When GA(3) was applied 24 hours prior to the (14)CO(2) pulse, the enhancement of sucrose transport toward the root was abolished but transport toward the upper sink leaf was unchanged. The enhanced sucrose export was not due to increased photosynthetic rate or to changes in the starch/sucrose ratio within the source leaf; rather, GA(3) increased the proportion of sucrose exported. After a 10-min exposure to [(14)C]GA(3), radioactivity was found only in the source leaf. Following a 2 hour exposure to [(14)C]GA(3), radioactivity was distributed along the entire stem and was present in both the roots and sink leaf. Extraction and partitioning of GA metabolites by thin layer chromatography indicated that there was a decline in [(14)C]GA(3) in the lower stem and root, but not in the upper stem. This pattern of metabolism is consistent with the disappearance of the GA(3) effect in the lower stem with time after treatment. We conclude that in the short term, GA(3) enhances assimilate export from source leaves by increasing phloem loading. In the long term (24 hours), the effect of GA(3) is outside the source leaf. GA(3) accumulates in the apical region resulting in enhanced growth and thus greater sink strength. Conversely, GA(3) is rapidly metabolized in the lower stem thus attenuating any GA effect.