Plant growth retardants are applied in agronomic and horticultural crops to reduce unwanted longitudinal shoot growth without lowering plant productivity. Most growth retardants act by inhibiting gibberellin (GA) biosynthesis. To date, four different types of such inhibitors are known: (a) Onium compounds, such as chlormequat chloride, mepiquat chloride, chlorphonium, and AMO-1618, which block the cyclases copalyl-diphosphate synthase and ent-kaurene synthase involved in the early steps of GA metabolism. (b) Compounds with an N-containing heterocycle, e.g. ancymidol, flurprimidol, tetcyclacis, paclobutrazol, uniconazole-P, and inabenfide. These retardants block cytochrome P450-dependent monooxygenases, thereby inhibiting oxidation of ent-kaurene into ent-kaurenoic acid. (c) Structural mimics of 2-oxoglutaric acid, which is the co-substrate of dioxygenases that catalyze late steps of GA formation. Acylcyclohexanediones, e.g. prohexadione-Ca and trinexapac-ethyl and daminozide, block particularly 3ss-hydroxylation, thereby inhibiting the formation of highly active GAs from inactive precursors, and (d) 16,17-Dihydro-GA5 and related structures act most likely by mimicking the GA precursor substrate of the same dioxygenases. Enzymes, similar to the ones involved in GA biosynthesis, are also of importance in the formation of abscisic acid, ethylene, sterols, flavonoids, and other plant constituents. Changes in the levels of these compounds found after treatment with growth retardants can mostly be explained by side activities on such enzymes.

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Reduction of shoot growth caused by the norbornanodiazetine derivative tetcyclacis and by the triazoles BAS 110¨ W, BAS 111¨ W and LAB 150 978 is probably due primarily to an inhibition of endogenous gibberellin (GA) formation. Treated plants are lower in biologically active GAs; reduction of shoot growth can be overcome by applying an appropriate dose of GA3; in the fungi Gibberella fujikuroi and Sphaceloma manihoticola the formation of GAs is inhibited by these compounds; in a cell‐free system of pumpkin endosperm the reactions leading from entkaurene to ent‐kaurenoic acid are blocked by relatively low concentrations of the growth retardants. The oxidative steps from ent‐kaurene to ent‐kaurenoic acid are known to be dependent on cytochrome P‐450. A structural feature common to tetcyclacis and the triazole‐type compounds is an sp2‐hybridised nitrogen atom located at the periphery of the molecule in a heterocycle. It appears likely that the lone electron‐pair on this atom interacts with the central iron atom of cytochrome P‐450 in the enzymes, resulting in inhibition. Previous work has shown that the activity of other enzymes containing cytochrome P‐450 is much less affected by these compounds than that of the ent‐kaurene oxidase. Therefore, it is concluded that distinct cytochrome P‐450 species occur in plants and that tetcyclacis, BAS 110 ¨ W, BAS 111 ¨ W and LAB 150 978 are efficient and specific inhibitors of GA biosynthesis at low concentrations. At higher concentrations and in special cases they may, however, also interfere with other metabolic reactions involving cytochrome P‐450.