How can the fipronil insecticide access phloem?

Seeds of sunflower plants coated with the fipronil (14)C-insecticide were allowed to grow in the greenhouse. The distribution of the (14)C-compounds was studied in each part of the plant after three months. After 83 days of culture small amounts of (14)C-compounds were found in the inflorescence (0.6 per thousand of the seed deposit) which were fipronil itself or its lipophilic or hydrophilic metabolites. The (14)C-compounds were found in each part of the inflorescence (bracts, ray and disk florets containing pollen, akenes). The (14)C-concentration in the xylem sap evaluated at this stage was much too low to explain the accumulated amount in the inflorescence. Under controlled conditions in a culture chamber, it was then demonstrated that a net phloem transfer of (14)C-fipronil occurred from developed leaves to growing organs. This allowed us to suppose that a similar (14)C-fipronil phloem transfer could occur toward the inflorescence during its formation. A quantitative evaluation suggests that most of the labeled compounds at this stage were not coming from the leaves but from the roots and stem where storage compounds were hydrolyzed for sustaining inflorescence development.