What is Paclobutrazol?

1. What is Paclobutrazol?

Paclobutrazol (PBZ) is a triazole plant growth regulator that inhibits gibberellin synthesis in plants, limiting plant cell division and elongation, without changing growth patterns or causing toxicity to plants.

2. Uses of Paclobutrazol

– It is an inhibitor of cell elongation and internode elongation that slows plant growth. As a result, shoots with the same number of leaves and internodes are shorter in length.

– Increases the production of abscisic acid hormone (ABA) and phytol chlorophyll components to help plants grow well.

– Changes in leaf morphology, such as smaller stomata, thicker leaves, increased number and size of surface appendages.

– Increases root density, thereby improving tolerance to environmental changes.

– Has fungicidal activity due to its ability as a triazole to inhibit sterol synthesis which helps control fungal diseases on plants.

3. Mechanism of action

– Several scientific studies have shown that optical isomers with S configuration at the asymmetric carbon bearing the hydroxyl group are inhibitors of gibberellin synthesis. Paclobutrazol inhibits the oxidation of ent-kaurene to ent-kauronoic acid by inactivating cytochrome P450 oxidase.

– Paclobutrazol affects the synthesis of abscisic acid and phytol hormones. Abscisic acid is also synthesized via the terpenoid pathway (Figure 2). When gibberellin synthesis is blocked, more precursors in the terpenoid pathway accumulate and are diverted to promote abscisic acid formation.

– Paclobutrazol has also been reported to inhibit the normal catabolism of ABA. The effects of PBZ on both synthesis and catabolism result in increased leaf ABA concentrations. Increased ABA levels result in reduced stomatal openings, reduced stem growth, and reduced surface area for transpiration, increased root growth for water uptake, and anatomical changes in leaves that create a water loss barrier.

*** References:

1. Chaney WR. Growth retardants: A promising tool for managing urban trees. Purdue Extension document FNR-252-W; 2005.

2. Marshall JG, Rutledge RG, Blumwald E, Dumbroff EB. Reduction in turgid water volume in Jack pine, white spruce and black spruce in response to drought and paclobutrazol. Tree Physiol. 2000;20:701–7.

3. Rademacher E. Growth retardants: effects on gibberellin biosynthesis and other metabolic pathways. Annu Rev Plant Physiol Mol Biol. 2000;51:501–31.

4. Rademacher W. Bioregulation of crop plants with inhibitors of gibberellins biosynthesis. Proc Plant Growth Regul Soc Am. 1997;24:27–31.