The mode of action of paraquat has been studied with particular reference to the roles of superoxide and hydrogen peroxide. A variety of redox compounds was investigated for an ability to mediate chloroplastic oxygen reduction via photosynthetic electron transport. In spiteof the relatively wide range of chemical types studied, most compounds were able to catalyse superoxide and hydrogen peroxide formation, but this was not in direct proportion to their one-electron redox potentials. The most active compounds were also shown to stimulate oxygen uptake in isolated asparagus mesophyll cells and in a whole plant screen, produced phytotoxic symptoms reminiscent of paraquat treatment. The role of superoxide in the herbicidal action of paraquat in flax cotyledons was investigated using penicillamine-copper complex (PA-Cu) which possessed superoxide dismutase (SOD) activity. Paraquat-induced chlorophyll and carotenoid pigment breakdown were inhibited by PA-Cu. The evolution of ethane, as an indicator of herbicide induced membrane lipid peroxidation was inhibited by the complex and correlated with chloroplast fatty acid levels notably linolenic acid, isolated from treated cotyledons. Similarly, ultrastructural examination revealed that cellular damage was markedly reduced by PA-Cu. These results indicated that the paraquat stimulation of superoxide production was of major importance in the herbicidal action of the compound. The simple hydrocarbons ethylene and ethane are produced during wounding and lipid peroxidative reactions respectively and active oxygen species are believed to be involved in both the biosynthetic pathways. A series of model reactions showed that ethylene production from methionine and pyridoxal phosphate was promoted by SOD, PA-Cu and ferredoxin, but was inhibited by catalase and paraquat. In contrast, ethane formation was stimulated by a-linolenic acid, paraquat and DCMU, but was inhibited by SOD, PA-Cu and the carotenoid, crocin. The results indicated that ethylene and ethane were formed via different biosynthetic pathways. The mechanism of production of ethylene appeared to involve hydrogen peroxide, whereas ethane formation required superoxide and/or singlet oxygen. A study of the mechanism of resistance of a biotype of Conyza linefolia to paraquat showed that tolerance was not based on reduced uptake of the herbicide. A comparison of the SOD enzymes showed that the paraquat resistant biotype possessed almost three times the amount of SOD found in susceptible plants. This was shown to be due at least in part, to the additional presence of two cyanide-insensitive isozymes in the resistant plant. CO fixation 2 in the resistant biotype was stimulated on prolonged incubation with paraquat. A mechanism of herbicide resistance is proposed based on a decreased reduction of paraquat by the chloroplast which could account for the observed differences in CO2 fixation and SOD levels between the biotypes. In summary, superoxide would appear to occupy a central role in the herbicidal action of paraquat, but this does not preclude the possible involvement of other active oxygen species, notably singlet oxygen.
|Date of Award||1980|