Enhancement of [14C]Sucrose Export from Source Leaves of Vicia faba by Gibberellic Acid 1

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The effect of gibberellic acid (GA3) 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. GA3 (10 micromolar) applied to the source leaf, enhanced export of [14C]sucrose (generated by 14CO2 fixation) to the root and to the sink leaf. Enhanced export was observed with GA treatments as short as 35 minutes. When GA3 was applied 24 hours prior to the 14CO2 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, GA3 increased the proportion of sucrose exported. After a 10-min exposure to [14C]GA3, radioactivity was found only in the source leaf. Following a 2 hour exposure to [14C]GA3, 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 [14C]GA3 in the lower stem and root, but not in the upper stem. This pattern of metabolism is consistent with the disappearance of the GA3 effect in the lower stem with time after treatment. We conclude that in the short term, GA3 enhances assimilate export from source leaves by increasing phloem loading. In the long term (24 hours), the effect of GA3 is outside the source leaf. GA3 accumulates in the apical region resulting in enhanced growth and thus greater sink strength. Conversely, GA3 is rapidly metabolized in the lower stem thus attenuating any GA effect.

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