The Use of Paclobutrazol in Controlling Tree Growth

Abstract

Paclobutrazol (PAC), is a synthetic compound which inhibits the gibberellin biosynthesis pathway. A range of studies were conducted investigating PAC's reactivity, detection, movement, effects and, introduction into trees. PAC was subjected to a range of experiments to detect its presence either 'in vitro' or 'in vivo'. PAC is a relatively inert compound and neither chemical reactivity nor optical recognition could be used to identify it 'in vitro'. In vivo, PAC could be detected using a linear displacement technique, ¹⁴C-PAC tracer studies and bioassays. The relative success of each detection method was related to its complexity, with radioactive tracer studies proving most effective in identifying PAC in vivo. Field trials using PAC were established in Armidale, New South Wales, Australia. Three street tree species which posed a potential threat to overhead powerlines were chosen for study. 'Liquidambar styraciflua' responded favourably to PAC showing sustained shoot growth inhibition for up to 18 months. 'Pistachia chinensis' responded to PAC, but its inhibitory effects were not as well sustained over the trial period. 'Eucalyptus nicholii' responded poorly to PAC in comparison to the other test species. The level of response of a tree species to the inhibitory effects of PAC is discussed, and is dependent on anatomical, physiological and environmental factors. PAC was believed to be exclusively xylem mobile in plants. This thesis presents evidence using HPLC detection and radiolabelled ¹⁴C-PAC that the compound is also phloem mobile in 'Ricinus communis'.