Carl Fedtke

Effect of different photosystem II inhibitors on chloroplasts isolated from species either susceptible or resistant toward s-triazine herbicides

Abstract In chloroplasts isolated from susceptible and atrazine-resistant Amaranthus retroflexus, the inhibition of photosynthetic electron transport by various classes of herbicides has been investigated. Resistance of mutant Amaranthus is not restricted to s-triazines but also extends to uracils, 1,2,4-triazine-5-ones, and ureas. For 1,2,4-triazin-5-ones and chloroplasts of both biotypes, a correlation between inhibition of photosynthetic electron transport and the partition coefficient could be established. In the case of phenolic herbicides only modestly decreased or even higher sensitivity of chloroplasts from the resistant biotype as compared to the susceptible one could be observed. These results are confirmed by binding of radioactively labeled herbicides to chloroplasts of both plants. Specific binding of atrazine or metribuzin to resistant chloroplasts is completely abolished, and that of diuron or phenisopham diminished as compared to susceptible chloroplasts. In contrast, binding of phenolic herbicides generally is enhanced in resistant chloroplasts. Photoaffinity labeling of thylakoids from both biotypes by 2-azido-4-nitro-6-[2′,3′-3H]isobutylphenol yields almost identical labeling patterns. These results are consistent with a recently proposed model (W. Oettmeier, K. Masson, and U. Johanningmeier, Biochim. Biophys. Acta 679, 376 (1982) of two different herbicide binding proteins at the reducing side of photosystem II: a 32- to 34-kdalton protein responsible for binding of triazines, triazinones, ureas, and related herbicides and a photosystem II reaction center protein for binding of phenolic herbicides.

Influence of photosynthesis-inhibiting herbicides on the regulation of crop plant metabolism

Abstract Potato plants (Solanum tuberosum L.) treated with metribuzin (4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one) and spring wheat (Triticum aestivum L.) treated with methabenzthiazuron (N-(benzothiazol-2-yl)-NN′-dimethylurea) were used in the present work. The changes observed in these plants in response to the herbicidal treatment were largely the consequence of a temporary lowered concentration of reducing sugars caused by a partial inhibition of photosynthesis. The herbicidal treatment led to a strongly enhanced nitrate reductase activity (5.5 times), which was accompanied by higher levels of nitrate (up to 10,000%), soluble amino acids (400%) and soluble protein (163%). The same results could be obtained by growing the plants under low light intensity. Rates of photosynthesis and respiration were found to be lowered significantly in the treated plants.

Influence of methabenzthiazuron on ATP-level and protein synthesis in wheat

Abstract Spring wheat (Triticum aestivum L. “Kolibri”) was grown in vermiculite treated with the photosynthesis inhibiting herbicide methabenzthiazuron (1-(benzothiazol-2-yl)-1,3-dimethylurea) either before germination or when the plants were 13 days old. The plants were analyzed from 2 days before until 11 days after treatment or up to a plant age of 23 days. The results are interpreted to fit the concept of a “shade adaptation reaction” caused by a herbicidal photosynthesis inhibition. Soluble reducing sugars have been found to decrease 4 hr after treatment and to stay at a low level throught the experiment. The ATP level was decreased 1 day after treatment but increased above control values 4 days later. This increase was accompanied by a decrease of the chlorophyll a b ratio. The explanation is given that the ATP level was first decreased because of a lowered carbohydrate supply for substrate and oxidative phosphorylation, and was afterwords increased as a result of an increased cyclic photophosphorylation activity. The soluble protein content and the incorporation of [14C]leucine into protein were increased 4 days after herbicidal treatment. The total protein was slightly decreased beginning 1 day after treatment. The nitrate concentration and the in vitro nitrate reductase activity were both increased 1 day after treatment. The increase of the nitrate concentration occurred in two phases: a first increase by 50% 1 and 2 days after treatment and a second, much stronger increase beginning on the third day. The first increase is interpreted as the result of a decreased in vivo nitrate reductase activity. The second increase possibly was the result of an increased rate of nitrate uptake.

Herbicide Induced Changes in Wheat Chloroplast Ultrastructure and Chlorophyll a/b Ratio

Summary Wheat plants (Triticum aestivum L. cv. Kolibri) treated with the photosynthesis inhibiting herbicide methabenzthiazuron (N-(benzothiazol-2-yl)-N,N′-dimethylurea) responded by a decreased chlorophyll a/b ratio and an increased proportion of grana to stroma lamellae. The concentration of soluble reducing sugars was simultaneously reduced, but the concentrations of soluble amino acids and soluble proteins were increased. These results are integrated into the results from earlier experiments with methabenzthiazuron treated wheat plants, and the correlations with the available evidence on chloroplast ultrastructure in photosynthetically inhibited and in shaded plants are discussed.

BEHAVIOUR OF METRIBUZIN IN TOLERANT AND SUSCEPTIBLE SOYBEAN VARIETIES

Abstract Two tolerant (Bragg, Tracy M) and two susceptible (Tracy, Coker 156) soybean varieties were compared. In the tolerant plants metribuzin (4-amino-6- tertbutyl 3-(methylthic)- as -triazin-5 (4 H )-one) was more rapidly deaminated in vivo and in vitro. The stem exhibited high rates of metabolic conversion, also leading to less metribuzin arriving in the leaves of the tolerant plants.

Characterization of the Metamitron Deaminating Enzyme Activity from Sugar Beet ( Beta vulgaris L.) Leaves

Abstract The enzymatic activity from sugar beet leaves which is responsible for the detoxification of the herbicide metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-1, 2, 4-triazin-5-one, trade name Goltix®) has been characterized in vitro. The detoxification occurs by rapid deamination in vivo as well as in vitro. However, the deamination in vitro is only maximal under reducing conditions, i. e. with an electron donor and in a nitrogen atmosphere. The electron donor may be cystein, glutathione, dithionite or ascorbate. The enzymatic deamination further requires the addition of cytochrome c and a “supernatant factor”, which may be replaced by FMN, FAD or DCPIP. However, in the presence of FMN or DCPIP cytochrome c is not essential but only stimulatory. The particulate as well as the soluble metamitron deaminating enzyme preparations obtained take up oxygen when supplied with cysteine and FMN. The particulate enzyme appears in the peroxysome-fraction. It is therefore suggested, that the enzymatic deamination of metamitron in sugar beet leaves is mediated by a proxisomal membrane bound electron transport system which alternatively may reduce oxygen or metamitron (deaminating).

Intramolekulare Wasserstoffverschiebung in der Hexosephosphatisomerase-Reaktion bei der photosynthetischen Stärkebildung in Chlorella

Chlorella pyrenoidosa was illuminated in the presence of glucose and fructose echa having carbon-14 and tritium markers. Starch and sucrose were degraded to obtain the intramulocular distribution of 14C and tritium. Glucose-1-14C-2-T was incorporated into the glucose of starch and into the glucose moiety of sucrose without change of the T/14C ratio. Fructose-1-14C-1-T (labelled stereospecifically in the isomerase position) was incorporated into starch, yielding predominately glucose-1-14C-2-T, whereas fructose-1-14C-1-T (labelled in the nonisomerase position) gave mainly glucose-1-14C-1-T. The results, in particular the transfer of tritium from C-1 of fructose to C-2 of glucose, are interpreted as being a strong indication for an intramolecular hydrogen transfer in the hexosephosphat-isomerase reaction in a multienzyme system of the photosynthetic starch biosynthesis in vivo, shielding the hexoses of T-exchange reactions with water.

Chloroacetanilides, oxyacetamides, tetrazolinones: mode of action. 1. Cross resistance and oleic acid incorporation in algal model systems

Results presented suggest that chloroacetamides, oxyacetamides, tetrazolinones, and possibly cafenstrole, act at the same site, although the precise molecular mode of action is still unknown, despite much research effort.

Antagonistic interactions of mefenacet with inhibitors of monooxygenases

Abstract The monooxygenase suicide inhibitor piperonyl butoxide (PBO) antagonizes the inhibitory activity of the herbicide mefenacet in a root regeneration test system with etiolated cut oat stems. When studying the behaviour of [14C]mefenacet [2-(2-benzothiazolyloxy)-N-methyl-N-phenylacetamide] in cut oat stems that had been pretreated with PBO for 24 h after cutting, an increased rate of uptake and increased concentrations of mefenacet in the tissue were found after PBO as compared to water pretreatment. Increased uptake rates and increased rates of metabolism after PBO pretreatment, supposedly by increased monooxygenation, were also observed with the substrates diuron, 2,4-D and cinnamic acid. Mefenacet induced the same responses as PBO in all systems. The similar actions of mefenacet and PBO are interpreted to suggest that mefenacet might interfere with monooxygenase enzymes. Similar responses, i.e. stimulation of monooxygenation of suitable substrates, have been reported for herbicide safeners. An environmentally controlled regulatory response system is suggested to be triggered by the different classes of compounds, mefenacet and similar herbicides, monooxygenase inhibitors, and safeners, and to respond with increased enzyme activities in the monooxygenase and glutathione detoxification pathways.

Synergistic activity of the herbicide safener dichlormid with herbicides affecting photosynthesis.

Abstract Dichlormid, a safener for thiolcarbamate herbicides, was tank-mixed with several herbicidal inhibitors of photosystem II, or with the herbicide acifluorfen, and applied postemergence to Ipomoea hederacea plants. Dichlormid had no visible effects on the plants when applied alone, but interacted synergistically with the herbicides in the combination treatments. Dichlormid strongly decreased the ascorbic acid levels in the Ipomoea hederacea cotyledons. Ascorbate is known to protect plant tissue from photooxidative damage. The herbicides which interacted synergistically with dichlormid are believed to generate their phytotoxic action via the production of excess singlet oxygen. It is suggested that the decreased ascorbate levels in the Ipomoea hederacea cotyledons after dichlormid treatment result in an impaired singlet oxygen scavenging system and consequently lead to increased plant damage in the presence of singlet oxygen generating herbicides.