INFLUENCE OF MEDIA COMPONENTS ON EFFICACY OF PACLOBUTRAZOL IN INHIBITING GROWTH OF BROCCOLI AND PETUNIA

Three experiments were conducted to evaluate media component effects o n paclobutrazol activity, In Expts, 1 and 2, a broccoli (Brassica oler acea van: botrytis L,) seedling bioassay was used to compare the activ ity of paclobutrazol at six concentrations (0-0.32 mg L-1). Results fr om Expt. a indicated that an average of 4-, 5-, and 10-fold higher con centrations mere required in old composted pine bark, fresh pine bark, and composted pine bark samples, respectively, tea achieve the same a ctivity observed ire sphagnum peatmoss (peat) samples, Activity in coi r was similar to that in peal while activity in vermiculite and perlit e was greater than that in peal. Activity in a fibrous peat sample was greater than in two less-fibrous peat samples, Results from Expt, 2 i ndicated that paclobutrazol activity was reduced more in the fine (<2 mm) fraction of fresh and composted bark samples than in medium (2-4 m m) off coarse (>4 mm) fractions. In Expt, 3, petunia (Petunia hybrida Vilm, 'Madness Red') was grown in a mixture of either 60% composted pi ne bark : 0% peal or 0% composted bark : 60% pest. The paclobutrazol c oncentration required to achieve the same size control was 14 times hi gher in the former mixture than in the latter. Thus, media components differ greatly hn their influence on paclobutrazol activity and the bi oassay procedure may serve as a useful tool for predicting media-paclo butrazol interactions, Chemical Flame used: beta-[(4-chlorophenyl)meth yl]-alpha-(1,1-dimethyl) (paclobutrazol).