Protoporphyrin IX Content Correlates with Activity of Photobleaching Herbicides

Abstract

Several laboratories have demonstrated recently that photobleaching herbicides such as acifluorfen and oxadiazon cause accumulation of protoporphyrin IX (PPIX), a photodynamic pigment capable of herbicidal activity. We investigated, in acifluorfen-treated tissues, the in vivo stability of PPIX, the kinefics of accumulation, and the correlation between concentration of PPIX and herbicidal damage. During a 20 hour dark period, PPIX levels rose from barely detectable concentrations to 1 to 2 nanomoles per 50 cucumber (Cucumis sativus L.) cotyledon discs treated with 10 micromolar acifluorfen. When placed in 500 micromoles per square meter per second PAR, PPIX levels decayed logarithmically, with an initial half-life of about 2.5 hours. PPIX levels at each time after exposure to light correlated positively with the cellular damage that occurred during the following 1 hour in both green and yellow (tentoxin-treated) cucumber cotyledon tissues. PPIX levels in discs incubated for 20 hours in darkness correlated positively with the acifluorfen concentration in which they were incubated. In cucumber, the level of herbicidal damage caused by several p-nitrodiphenyl ether herbicides, a p-chlorodiphenylether herbicide, and oxadiazon correlated positively with the amount of PPIX induced to accumulate by each of the herbicide treatments. Similar results were obtained with acifluorfen-treated pigweed and velvetleaf primary leaf tissues. In cucumber, PPIX levels increased within 15 and 30 minutes after exposure of discs to 10 micromolar acifluorfen in the dark and light, respectively. These data strengthen the view that PPIX is responsible for all or a major part of the photobleaching activity of acifluorfen and related herbicides.The cause of the photobleaching activity of NDPE2 herbicides has been a continuing mystery (1,20). Recent reports (8, 19, 21, 23-26, 30, 36) phyrin synthesis strongly inhibit the activity of the herbicide. PPIX presumably acts as a photodynamic pigment, generating singlet oxygen in the presence of light and molecular oxygen. The absorption spectrum of PPIX, with peaks in the blue, green, and red spectral regions, fits the published action spectra for these herbicides (11,12,31,32). Duke and Kenyon (7) argued that the toxic oxygen species in NDPE action is singlet oxygen and Haworth and Hess (15) recently demonstrated that singlet oxygen is the active toxic oxygen species in NDPE action. After dark incubation with the herbicide, the temperature independence of the light-induced photodestruction indicates that a photodynamic dye is responsible for the activity (17). If exposed to metabolic inhibitors, such as respiratory inhibitors, during the dark incubation period, the activity of NDPE herbicides is strongly reduced upon exposure to light (9, 22). Photobleaching NDPE, cyclic imide, and oxadiazole herbicides inhibit Chl synthesis (13,34,35). Furthermore, there is good evidence that photosynthetic light reactions are not required for activity of these herbicides (6, 9, 12, 15) or accumulation ...

Effect of Diphenyl Ether Herbicides on Oxidation of Protoporphyrinogen to Protoporphyrin in Organellar and Plasma Membrane Enriched Fractions of Barley

“…'The study was Several laboratories have found the photobleaching DPE2 herbicides to increase greatly the intracellular levels of the photodynamic tetrapyrrole, protoporphyrin, causing light dependent phytotoxic effects (2-4, 6, 14-16, 18, 22-24, 26, 29). DPE herbicides also inhibit Protox, which is the last enzyme of the common branch of the heme and Chl biosynthetic pathways in plants and oxidizes protoporphyrinogen to protoporphyrin (1,5,12,20,21,27,30 (2,19). In addition, these findings are compatible with previous in vivo localization studies in intact plant cells showing that DPE treated plant cells exhibit a rapid accumulation of porphyrins at nonplastidic sites, such as the plasma membrane or in the cytoplasm (16).…”

Associations Between Herbicides and Glyphosate in Agronomic Performance of RR2 Intact Soybean

“…The lactofen acts in the formation of free radicals in the cytoplasm that are highly reactive in the presence of light and oxygen . the first moments after the application of the herbicide, these radicals cause lipid peroxidation of membranes (Becerril and Duke, 1989), compromising the structure of chlorophyll and optimizing the process of the appearance of chlorotic symptoms. For crops such as soybeans which has selectivity to this herbicide, chlorosis symptoms are common but have a recovery being that the new leaves do not exhibit these symptoms.…”

5-Aminolevulinic Acid Induced Photodynamic Damage of the Photosynthetic Electron Transport Chain of Cucumber (Cucumis sativus L.) Cotyledons

“…Recently, it has been reported that the treatment of plants with diphenyl ether and oxadiazon herbicides causes accumulation of excess protoporphyrin IX that acts as a sensitizer for the production of singlet oxygen in the plants. This leads to photodynamic damage and consequent death (3,4,7,27). ALA-induced photodynamic death of the plant could be due to the impairment of various vital functions.…”