Eiki Nagano

Diphenylether-like physiological and biochemical actions of S-23142, a novel N-phenyl imide herbicide

Abstract Several physiological and biochemical actions of a new experimental herbicide, S-23142 [N-(4-chloro-2-fluoro-5-propargyloxyphenyl)-3,4,5,6-tetrahydrophthalimide], have been investigated. S-23142 was active under the presence of light and oxygen. Photosynthetic CO2 fixation in soybean began to decrease 4–5 hr after the foliar treatment of S-23142, being accompanied by the appearance of visible bleaching and wilting of the plants. A large amount of ethane, the products of peroxidation of unsaturated fatty acids, was produced from the cotyledon discs of cucumber (Cucumis sativus L.) treated with S-23142. Leakage of ATP was also observed. S-23142 did not inhibit photosynthetic oxygen evolution of the discs just after the application; however, the oxygen evolution rate decreased as the treated discs produced ethane. The results suggest that cell membrane and chloroplast membrane were deteriorated by the membrane lipid peroxidation. S-23142 also induced ethylene production and a high level of phenylalanine-ammonia lyase activity in cucumber cotyledon, which was regarded as the phenomena of stress response. Only the ethylene production was inhibited by aminoethoxyvinylglycine and cycloheximide, while the ethane production was not affected. All of these actions of S-23142 were essentially the same as those of acifluorfen ethyl except that the activity of S-23142 was more than 10 times higher than that of acifluorfen ethyl. These data strongly suggest that S-23142 belongs to the same group as diphenylethers in its mechanisms of action despite the difference in chemical structure.

Activities of the N-phenyl imide S-23142 in carotenoid-deficient seedlings of rice and cucumber☆

Abstract The activity of S-23142 [N-(4-chloro-2-fluoro-5-propargyloxyphenyl)-3,4,5,6-tetrahydrophthalimide], an experimental herbicide having diphenyl ether-type action, was examined in carotenoid-deficient materials. White seedlings of rice mutant which show tolerance to diphenyl ethers were completely killed by 0.1 μM S-23142 and the tolerance to a diphenyl ether acifluorfen-ethyl (25 μM) was lost under high light intensity (92 μE/m2/sec). S-23142 was also active in cucumber seedlings with carotenoids depleted by fluridone grown under low intensity (0.14 μE/m2/sec PAR) light that did not cause photobleaching of chlorophyll. The specific wavelength of light required for the action of S-23142 in yellow cucumber cotyledons treated with tentoxin, an inhibitor of chlorophyll formation, was 400 nm. As the effectiveness of the 400-nm light for inducing S-23142 action was not altered by the depletion of carotenoids by fluridone, the influence of carotenoids can be ruled out. The effectiveness of red light of 650 nm was greatly reduced in tissue grown under low light intensity or treated with tentoxin. These results indicate that S-23142 is active without the involvement of carotenoids and multiple photoreactions are involved in the herbicidal action of S-23142 in normal green tissue. The results also strongly suggest that chlorophyll or its related pigment(s) serves as one of the photoreceptors, although S-23142 is also active in achlorophyllous tissue.

Herbicidal Efficacy of Protoporphyrinogen Oxidase Inhibitors

Compounds which inhibit protoporphyrinogen oxidase (Protox) were known as “photobleaching herbicides” before their site of action was discovered. Photobleaching herbicides cause very strong bleaching of the treated part of higher plants. It was known that a photobleaching herbicide requires oxygen and light to express its herbicidal activity. After intensive investigations on the mode of action of photobleaching herbicide, protoporphyrinogen oxidase has been identified as the molecular target. It is generally accepted that the inhibition of Protox and oxidation of protoporphyrinogen leads to the accumulation of a strong photosensitizer, namely protoporphyrin IX (PP IX), followed by activation of oxygen and lipid peroxidation (Duke et al. 1993). This mechanism explains the necessity of light and oxygen. After discovery of the molecular target of photobleaching herbicides, several chemical structures were known to inhibit Protox. It is possible to classify Protox inhibitors into several groups according to their chemical structure.