Comparison of the Action of Bleaching Herbicides

Among chlorosis-inducing herbicides that interfere with carotenoid synthesis two groups o f different potency can be discriminated (group 1; aminotriazole amd haloxidine; group 2 with more extensive photodestructions: pyridazinone herbicides and difunon). After application of herbicides o f group 2 colored carotenoids were completely absent and preexisting chlorophyll was degraded by photochemical reactions requiring high light intensity and 0 2, that occurred also at 0°C . In treatments with group 1 herbicides direct photodegradation of chlorophyll was not sufficient to generate the chlorosis. Light-induced interference with constituents o f the chloroplast protein synthesis apparatus being more sensitive to photooxidative dam age than chlorophyll, appeared to indirectly mediate the chlorosis. In the absence o f chloroplast protein synthesis further chlorophyll accumulation is prevented. Photodegradation o f chlorophyll in the presence o f group 2 herbicides involved the participation o f 0 2~ radicals and was accom panied by lipid peroxidation. In all herbicide treatments the catalase activity o f the leaves was very low. Only in the presence o f group 2 herbicides chloroplast enzymes o f cytoplasmic origin (e.g. NADP-glyceraldehyde-3-phosphate dehydrogenase) were also inactivated. Rapid inactivation o f catalase as well as of NADP-glyceraldehyde-3-phosphate dehydrogenase was induced by exposure o f dim-light-grown herbicide-treated leaves to bright light, also at 0 °C . In treatments with herbicides o f group 2 also other peroxisomal enzymes (e.g. glycolate oxidate, hydroxypyruvate reductase) were affected. The elimination o f these peroxisomal enzymes also appeared to depend on photooxidative processes o f the chloroplast.