Effect of Gibberellic Acid on the Distribution of Products of Photosynthesis in Sunflower

Single leaves on growing sunflower plants were allowed to assimilate¹⁴CO₂. Gibberellic acid was applied to the same leaf or to theterminal bud or the roots, and the distribution of assimilated¹⁴C was determined at intervals of 1–96 h. Gibberellicacid had no significant effect on initial distribution of ¹⁴Cduring the period of rapid export from the leaf, but enhancedre-export from the roots after translocation from the leaf hadvirtually ceased. Most of the ¹⁴C exported from the roots accumulatedin the shoot tip. The site of application of the hormone wasof relatively minor importance. Wherever it was applied themajor effect was enhancement of movement from the roots to theshoot tip. Application to the terminal bud was most effectivein this respect. There was no evidence that gibberellic aciddirectly affected the transport system, but the data supportthe hypothesis that it increases the strength of the sink inthe shoot tip. Helianthus annuus L., sunflower, assimilate transport, gibberellic acid, phloem transport