The effect of gibberellic acid (GA3) on sucrose export from source leaves was studied in broad bean (Viciafaba L.) plants trimmed of all but one source and one sink leaf. GA3 (10 micromolar) applied to the source leaf, enhanced export of ['4qsucrose (generated by '4C02 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 '4CO2 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 1'4qGA,, radioactivity was found only in the source leaf. Following a 2 hour exposure to I'4qGA3, 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 I'4CjGA3 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. and more direct short term effects on transport and carbon metabolism.Based on the evidence in the literature, it is believed that GA and IAA enhance and ABA inhibits phloem loading of sucrose (4, 5). Gibberellin and kinetin influence assimilate transport at the site of hormone application (11,(13)(14)(15) With the exception of work by Daie et al. (4), most evidence on the hormonal regulation ofphloem loading has been obtained in vitro. In the present study we investigated the effect of GA3 on movement of 14C assimilates from the source leaf to remote sinks (root and young sink leaves) in vivo. The results indicate that a short-term effect ofGA3 may be enhanced phloem loading.The observation is consistent with our combined in vitro and in vivo studies in celery (5), supporting a direct role for GA and IAA in phloem loading.
MATERIALS AND METHODSHormones are believed to play an important role in the regulation of assimilate partitioning and translocation in plants (2, 11, 12 and references therein). In the source-path-sink continuum, phloem loading of organic solutes is a crucial step subject to regulation by phytohormones (3,13,16). However, the mechanism by which each hormone influences these processes is not well understood. In general, a distinction must be made between the indirect effect ofhormones through their influence on growth Plant Material. Broad bea...