Paclobutrazol distribution following application to two media as determined by bioassay

A broccoli (Brassica oleracea var, botrytis L.) seedling bioassay was used to measure paclobutrazol activity and distribution in two growing media fol lowing drench or subirrigation applications. The bioassay exhibited a satur ation-type response curve for paclobutrazol concentrations up to 1000 mu g. L-1 in solution and 100 mu g.L-1 in the media, The concentration of paclobu trazol required to achieve one-half of the maximum observed bioassay activi ty was 3-fold as high in bark-based commercial potting medium as in a peat- based medium, Less than 2% of applied paclobutrazol leached out during the drench application despite the collection of up to 50 mt of leachate per 12 0 mL of the solution (1000 mu g.L-1) that was applied per 15-cm pot. Immedi ately following drench application, paclobutrazol concentrations in both me dia were highest in the uppermost 2.5 cm and decreased downward, Bg 3 weeks after treatment, drench-applied paclobutrazol had moved into lower depths. Distribution of paclobutrazol was limited to the bottom 2.5 cm of media wh en applied as a subirrigation soak. Chemical name used: (+/-)-(R*,R*)-beta- [(4-chlorophenyl)methyl]-alpha-(1,1-dimethyl)-1H-1,2,4-triazole-1-ethanol ( paclobutrazol).