Paclobutrazol: A Promising Growth Inhibitor for Injection into Woody Plants

Paclobutrazol [(2RS,3RS)-1-(4-chlorophenyI)-4,4-dimethyl-2-(1,2,4-triazol-1-yl-)pentan-3-ol)] was injected into bean plants ( Phaseolus vulgaris L. ‘Black Valentine’), California privet seedlings ( Ligustrium ovalifolium Hassk.), saplings of red maple ( Acer rubrum L.), yellow poplar ( Liriodendron tulipifera L.), white ash ( Fraxinus americana L.), American sycamore ( Platanus occidentalis L.), and one-year-old trees of ‘Golden-Delicious’ apple ( Malus domestica Borkh), to evaluate growth inhibition response. Also, the extent of translocation and metabolism of injected 14 C — paclobutrazol was determined in apple. The height growth, weight, and leaf size of bean plants decreased as the dose of paclobutrazol increased from 0.01 to 1,000 μg/plant. Sprout growth from stumps of privet was reduced as the dose of paclobutrazol increased from 1 μg to 1000 μg/tree. The shoot growth of sapling trees in the field was controlled for at least 2 months with doses of paclobutrazol ranging from 5 mg to 40 mg per tree. When obvious inhibition occurred in apple trees (27 days after injection), 23% of the 14 C-activity had been translocated acropetally to the apical shoots. A high percentage of 14 C-activity detected was paclobutrazol; 90% of the 14 C-activity found in the xylem and phloem and over 85% in the shoot tissue chromatographed with paclobutrazol.