Plant Physiol. Journal of Pharmacology and Experimental Therapeutics
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Plant Physiology 71:235-240 (1983)
© 1983 American Society of Plant Biologists

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Articles

Mode of Action of a Herbicide 1

Johnsongrass and Methanearsonic Acid

Francis C. Knowles and Andrew A. Benson

Marine Biology Research Division, A-002, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093

Johnsongrass (Sorghum halepense (L.) Pers.) is sensitive to methanearsonate, foliar application resulting in a topkill. Investigation of the pattern of photosynthesis by radioautography revealed an accumulation of malate in methanearsonate-treated leaves. Accumulation of malate was attributed to an inhibition of NADP+-malic enzyme which was found to be sensitive to sulfhydryl group reagents including arsenosomethane, CH3AsO. Methanearsonate was found to act as an oxidant in the Hill reaction using spinach chloroplasts, the photoproduct being a sulfhydryl group reagent.

These results suggest that methanearsonate inhibits CO2 release from malate in bundle sheath cells, depriving the plant of its source of carbon for sucrose production. The mechanism of inhibition of enzymes sensitive to sulfhydryl group reagents by arsenosomethane is addressed.

1 Supported by National Science Foundation Grant OCE 78-25707. A preliminary and partial account of these studies has been published: Benson AA, FC Knowles 1981 Arsenic metabolism and photosynthetic productivity. In G Akoyunoglou, ed, Photosynthesis, Vol 6. Balaban International Science Services, Philadelphia, pp 33-37.






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