Non‐transportable radioactivity in hypocotyls of Helianthus annuus after application of [3H]–IAA: Analysis of diffusion parameters

IAA was applied as a droplet to the apical cut surface of hypocotyl sections of Helianthus annuus L. After collecting auxin moving in the polar transport stream, the distribution of radioactivity remaining within the tissue was kinetically investigated. By use of regression functions of the type y = a exp(‐bx2) it was demonstrated that two independent diffusion profiles characterize the distribution of the residual radioactivity. An estimate of diffusion coefficients as well as amounts of each of the two components allowed conclusions concerning their molecular size and possibly their role in auxin transport. The results suggest that auxin and auxin metabolites are leaving the tissue by diffusion rather than by the activities of different transport systems. These results are discussed in respect to earlier descriptions of multiple auxin transport systems. H is concluded that a single transport system is sufficient to account for biphasic elution curves.