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.

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Abstract. During the incubation of undifferentiated cell suspensions of sunflower (Helianthus annuus L. cv. Spanners Allzweck) ethylene production was effectively inhibited by the novel oxime ether derivative LAB 181 508, [[(Isopropyliden)-amino]-oxy]-acetic acid-2-(methoxy)-2-oxoethylester (PACME). The compound was most active during the first 6 days of incubation exhibiting a value of 50% inhibition at 9.5 • 10 -6 mol x L -~. The pattern of changes in the internal l-aminocyclopropanecarboxylic acid (ACC) and N-malonyl-ACC (MACC) levels paralleled the influence on ethylene formation. While the addition of ACC fully restored ethylene production, applied S-adenosyl-Lmethionine (SAM) was not effective. Experiments with [~4C]indole-3-acetic acid (IAA) revealed that LAB 181 508 did not affect auxin uptake into suspension cells of sunflower. The results suggest that LAB 181 508 reduces ethylene formation by inhibiting the conversion of SAM to ACC in the biosynthetic pathway. In comparison to the structurally related inhibitor of ACC synthase, aminoethoxyvinylglycine (AVG), LAB 181 508 reduced growth and viability of the suspension cells only slightly. Low phytotoxicity of LAB 181 508 combined with a less complicated chemical synthesis might offer interesting aspects for physiological research and horticultural and agricultural practice.Ethylene influences many processes in the development and yield formation of plants (e.g., fruit set and ripening, senescence, and stress reactions)