Mode of interference of chlorosis‐inducing herbicides with peroxisomal enzyme activities

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In rye leaves (Secale cereale L. cv. Petkus “Kustro”) bleached in the presence of the chlorosis‐inducing herbicides aminotriazole, haloxidine, San 6706 or difunone in white light of 54.2 W m‐2 (5000 lx), catalase activity was very low. In addition, the activities of glycolate oxidase and hydroxypyruvate reductase were strongly diminished in treatments with San 6706 and difunone. The lowering of the peroxisomal enzyme activities was observed in red, but not in blue light and did not occur after treatment with the non‐bleaching pyridazinone derivative San 9785. The deficiencies of the peroxisomal enzymes did not appear to be involved in the initiation of the chlorosis. Instead they are probably produced as secondary consequences of the bleaching. Low peroxisomal enzyme activities were also obtained without herbicide treatment by growing the leaves in an atmosphere of 2% O2 and 3% CO2, but in this case were not accompanied by an increased sensitivity of the Chl to photooxidative bleaching. The peroxisomal enzymes reached as high activities as in untreated controls when the herbicide‐treated leaves were grown at a low light intensity of 0.106 W m‐2 (10 lx). After transfer of herbicide‐treated leaves grown under 0.106 W m‐2 to 306 W m‐2 (30 000 lx), catalase was strongly inactivated, even at 0°C. In treatments with San 6706 and difunone the increase of the activities of glycolate oxidase and hydroxypyruvate reductase was either stopped, remaining unchanged, or the enzymes were slightly inactivated after exposure to 306 W m‐2 (30 000 lx). The observations suggest that the inactivation of peroxisomal enzymes results from photooxidative events in the chloroplasts.