Paclobutrazol (PBZ) is the ISO common name for an organic compound that is used as a plant growth retardant and triazole fungicide.

[4] 4-Chlorobenzaldehyde and pinacolone are combined in an aldol condensation to form a chalcone which is hydrogenated using Raney nickel as catalyst to give a substituted ketone.

The final reduction reaction uses sodium borohydride, which in cold methanol gives almost exclusively the diastereomer pair having the absolute configuration (2R,3R) and its enantiomer (2S,3S), with only about 2% of the alternative (2R,3S) and (2S,3R) isomers.

[5][6] However, the commercial product (developed under the code number PP333) was the racemic material, since separation of the isomers was unnecessary when both components had utility in agriculture.

The (2S,3S) isomer inhibits the enzyme ent-kaurene oxidase[5][8] which is on the main biosynthetic pathway to gibberellins, which are important plant hormones.

[9][10] A secondary effect arising from the inhibition of ent-kaurene oxidase is that its precursor, geranylgeranyl pyrophosphate accumulates in the plant and some of this is diverted into additional production of the phytol group of chlorophyll and the hormone abscisic acid.

[5] Many other azole derivatives including propiconazole and tebuconazole show this type of activity, so the main commercial opportunity for paclobutrazol was as a plant growth retardant and it was first marketed by ICI in 1985 under the trade names Bonzi, Clipper, Cultar and Parlay.

[16][17][18] PBZ has been extensively used as a means to improve the quality of turf on golf courses, where it reduces the need for mowing and by increasing chlorophyll content has the effect of greening the grass.

For example, the Arabidopsis allele gai-t6 (of the giberellic acid interacting gene) confers resistance to paclobutrazol's damage to vegetative growth.