Ultrastructural effects of the diphenyl ether herbicide acifluorfen and the experimental herbicide M&B 39279

Abstract

The sequence of the herbicidal effects of acifluorfen and the novel herbicide M&B 39279 (5-amino-4-cyano-1-(2,6-dichloro-4-trifluoromethylphenyl)pyrazole) on ultrastructure in excised Galium aparine L. leaves has been examined in detail and supported by physiological measurements. Plant responses to acifluorfen and M&B 39279 were indistinguishable, the latter compound therefore being deemed to share a common mode of action with nitrodiphenyl ethers. After 3 hr in 50 μmol m −2 sec −1 photosynthetic photon flux density light, chloroplast swelling and invagination/evagination of chloroplast envelopes was observed in the presence of 100 μ M acifluorfen and M&B 39279. Tonoplast lysis and/or development of cytoplasmic vesicles occurred by 15 hr whilst plasmalemma disruption, membraneous vesicle formation, and abumdant electron opaque material were apparent between 20 and 30 hr. Chloroplasts, often fused together with highly modified envelope membranes, were the last identifiable cell structures remaining. Thylakoids were notably persistant, retaining integrity for about 20 hr. Under identical conditions, leaf photosynthetic capability diminished immediately following herbicide treatment. Leaf chlorophyll content remained constant for over 15 hr, after which a decline was accompanied by leakage of electrolytes and malondialdehyde from treated leaves. These results indicate that the diphenyl ether-type mode of action is initiated at the chloroplast envelope, but is independent of photosynthesis and involves lipid peroxidation.