BIOREGULATORY EFFECTS OF THE FUNGICIDAL STROBILURIN KRESOXIM-METHYL IN WHEAT (TRITICUM-AESTIVUM)

Apart from its fungicidal effect, the strobilurin kresoxim-methyl (BAS 490 F) was found to induce physiological and developmental alteration s in wheat (Triticum aestivum L.) which are seen in connection with im proved yield. In a series of biotests including heterotrophic maize an d photoautotrophic algal cell suspensions, duckweed, isolated mustard shoots and germinating cress seeds, kresoxim-methyl showed a similar r esponse pattern to standard auxins (e.g. indol-3-ylacetic acid, IAA; 2 -(1-naphthyl)acetic acid, alpha-NAA). Auxin-like activity of kresoxim- methyl was also found when stem explants of tobacco were cultured on a hormone-free medium. Kresoxim-methyl stimulated shoot formation, part icularly at 10(-7) M. The same effect was induced by 10(-8) M IAA. The determination of phytohormone-like substances in shoots of wheat plan ts foliar-treated with 7 x 10(-4) M kresoxim-methyl revealed only slig htly changed levels of endogenous IAA, gibberellins and abscisic acid. In contrast, the contents of dihydrozeatin riboside-type cytokinins i ncreased to 160% of the control, while trans-zeatin riboside- and isop entenyladenosine-type cytokinins remained nearly unchanged. The most r emarkable alterations were the reductions in 1-aminocyclopropane-1-car boxylic acid (ACC) levels and ethylene formation which were demonstrat ed in intact plants, leaf discs and the shoots of wheat subjected to d rought stress. Kresoxim-methyl affected the induction of ACC synthase activity which converts S-adenosyl-methionine to ACC in ethylene biosy nthesis. In shoots from foliar-treated wheat plants, 10(-4) M kresoxim -methyl inhibited stress-induced increases in endogenous ACC synthase activity, ACC levels and ethylene formation by approximately 50%. Redu ctions in ACC synthase activity and ACC levels of 30% were also obtain ed at low concentrations of alpha-NAA (10(-6) M). In contrast, ACC syn thase activity in vitro was not influenced by adding the compounds. In wheat leaf discs, the inhibiting effect of kresoxim-methyl, alpha-NAA and IAA on ethylene formation was accompanied by delayed leaf senesce nce, characterized by reduced chlorophyll loss. However, in contrast t o kresoxim-methyl which showed only inhibitory activity on ethylene sy nthesis over a wide range of concentrations applied, the auxins stimul ated ethylene production at high concentrations of about 10(-4) M. The inhibition of ethylene biosynthesis by kresoxim-methyl, together with an increase in endogenous cytokinins could explain the retardation of senescence and the intensified green leaf pigmentation in wheat expos ed to this strobilurin.