Comparative Phytotoxicity Among Four Arsenical Herbicides’

Cacodylic acid (hydroxydimethylarsine oxide) was more phytotoxic than monsodium methanearsonate (MSMA), sodium arsenate, or sodium arsenite when foliarly-applied. MSMA was much more effective on dicotyledonous than on monocotyledonous species. Sodium arsenite and arsenate had little effect on grasses. A comparative study of absorption, transport, and metabolism in beans (Phaseolus vulgaris L. ‘Black Valentine’) revealed that cacodylic acid and MSMA were transported about equally from the leaves to the terminal bud and expanding leaves whereas negligible amounts of sodium arsenite and arsenate were translocated. The latter two compounds caused more rapid contact injury to the treated leaves than either organic arsenical. There was no indication that cacodylic acid or MSMA was demethylated to form inorganic arsenicals or reduced to trivalent arsenic compounds. Studies with “C-MSMA indicated that about 40% of the “C and arsenic recovered was bound rapidly to another molecule to form a ninhydrin-positive complex. In small amounts, arsenate combined with some component of plant tissues. Also, arsenite probably was oxidized to arsenate. In ‘Received for publication November 25, 1970. *Senior author on leave from the Dep. of Environ. Hort., Univ. of Calif., Davis, Calif. 95616, from September 1969 to September 1970. Y’lant Physiologist, Plant Sciences Laboratories, Fort Detrick, Frederick, Maryland. Current address: Wood Science Laboratories, Colo. State Univ., Fort Collins 80521. 558 beans, root-applied sodium arsenite was more phytotoxic than sodium arsenate and both were much more phytotoxic than cacodylic acid and MSMA. Most differences in phytotoxicity could not be explained by differences in rates of absorption by bean roots. Arsenite caused considerable contact injury to the root system, probably accounting for its relatively great phytotoxicity. Both cacodylic acid and MSMA were more phytotoxic per mole of tissue arsenic when foliarly-applied than when root-applied. I N T R O D U C T I O N ARSENICALS, inorganic and organic, have been used to control certain forms of cancer and syphilis and as trypanocides, amebicides, insecticides, and herbicides (9, 21). Although they have been studied for many decades, the modes of action of arsenicals in different organisms are not well understood and subject to many interpretations (21). Species selectivity, whether in plants or animals, is a particularly difficult problem, since many arsenicals inhibit respiration or oxidative phosphorylation or inactivate sulfhydryl enzymes, which are fundamental to all organisms. Selectivity aside, several hypotheses concerning the effect of different kinds of arsenicals have been proposed that involve differences in rates of absorpt ion, transport, metabolic conversion of the applied arVolume 19, Issue 5 (September), 1971 SACHS AND MICHAEL : PHYTOTOXICITY AMONG ARSENICALS senical to other more (or less) toxic compounds, or 3 shunt mechanism in resistant organisms (2, 7, 8, 12, 14, 17, 18, 19, 21). The earliest arsenical herbicide used was sodium nrsenite (or arsenic trioxide dissolved in sodium hydroxide); it proved to be an excellent soil sterilizer, although it could be used for selective weed control (4). Sodium, calcium, and lead arsenates have since been used for preemergence weed control in grasses (11). Most recently three organic arsenicals, MSMA, disodium methanearsonate (DSMA), and cacodylic acid, have been used extensively for selective and general weed control. MSMA and DSMA are particularly valuable as selective, preemergence and postemergence herbicides in cotton (Gossy#lum hirsatum L.) and turf. Cacodylic acid is used for general weed control and is an excellent herbicide for monocotyledonous weeds. Skogley and Ahlgren (16) concluded that cacodylic acid was a more potent soil sterilizer than MSi\fA, DSMA, or sodium arsenite. Since arsenicals are valuable herbicides, considerable attention has been given to dosage, time of application, and residual effects in the soil (4, 6, 11, 15); a few studies have correlated herbicidal activity with factors affecting absorption and transport (1, 3, 12). The literature contains relatively. few references to comparative herbicidal studies exammmg both inorganic and organic arsenicals (11, 12, 15, 16) and none comparing activity with application to the leaves and roots. This paper reports the results of investigations on comparative activity with root and foliar applications and also on relative absorption, transport, and metabolism of sodium arsenite, sodium arsenate, MSMA, and cacodylic acid. MATERIALS AND METHODS Seven-day-old ‘Black Valentine’ bean plants with the primary leaves fully expanded and the first trifoliolate leaf half-expanded were used for root and leaf absorption studies. The plants were grown in artificially-lighted, temperature-controlled chambers at 25 -+2 C and 60 t 10% relative humidity with a 16-hr photoperiod. Light intensity was about 14,000 lumens/m2 at plant tops provided by a mixture of fluorescent and incandescent lamps. The plants were immersed in aerated half-strength Hoagland’s solution in plastic containers of approximately 1-L capacity. Four to six plants were used for each treatment. Foliar applications were made by micropipet to the upper surface of each primary leaf; five lo-uL droplets were applied to each leaf. Applications to the root system were made by adding measured amounts of arsenicals from stock solutions to give the required concentration when the container was brought to full volume. Initial and final height measurements were made to compute stem elongation during the experimental period. For root applications, the stems were severed above the container level, the roots rinsed thoroughly in large volumes of tap water, and both the roots and shoots dried overnight in a 70 C oven. In separate tests with control plants dipped in arsenical solutions of concentrations equivalent to those used in the experiments, the root rinsing procedure removed all but 0.5 to 5% of the arsenic that was absorbed in a ‘I-day period. For foliar applications, the treated primary leaves were severed at the first node just Volume 19, Issue 5 (September), 1971 below the axillary buds. Thus the plant was divided into three portions: treated leaves, the shoot system above the treated leaves, and the stem and root system below the treated leaves. Each portion was dried as above. Dry weights were recorded and the samples analyzed for arsenic content by methods described elsewhere (13). All values in the tables are corrected for apparent arsenic of the untreated plants: the background values were 0.6 ppm for the stem and leaf tissues and 0.3 ppm for the root system. Comparative phytotoxicity of spray applications was measured on soil-grown plants raised and treated in a greenhouse in which the minimum temperature was approximately 20 C. Applications were made approximately 7 days after emergence to two 15-cm pots of each of the following species thinned to two plants per pot. ‘Black Valentine’ bean, soybean (Glycine max. (L.) Merr. ‘Lincoln’), ivyleaf morningglory (Ipomea hederqea (L.) Jacq. ‘Heavenly Blue’), radish (Rhaphanus s&us (L.) ‘Scarlet Globe’). Oats (Arena sntiua (L.) ‘Clinton’) and rice (Oryza sntiua (L.) ‘Colusa’), planted thickly, about 30 seed per 15-cm pot, also were treated. Equimolar, aqueous solutions containing 0.5% surfactant (Atplus 401) were applied with a glass atomizer at an operating air pressure of 140 to 350 g/sq cm. Visual observations were made on all treatments 1, 2, 4, and 7 days after treatment. On the seventh day the plants were harvested, dried, and weighed. The maximum inhibition attainable was about 90% since the initial dry weight of the plants was not substracteci from the final value. There was an approximate lo-fold increase in dry weight in the control plants during the 7 days following treatment. For chromatographic analyses of the treated plants, the dried material was extracted with hot water and the extract filtered and reduced in volume as described elsewhere (13). All chromatography was with Whatman 3MM paper and a descending technique in sealed glass tanks. Solvents used were reagent grade chemicals. Paper strips were eluted with 50% methanol. Each experiment was repeated, but only the data from one experiment is recorded in the tables and figures. Sodium arsenite, sodium arsenate, and cacodylic acid were reagent grade crystalline chemicals. MSMA was supplied as a 58% solution. Chromatographic analysis of sodium arsenite and arsenate using four solvent systems revealed no other arsenic-containing compounds. There was some arsenate (up to 1% of the stock solution) in the cacodylic acid and MSMA samples. Radioactivity was measured with a Nuclear-Chicago Mark I liquid scintillation counter and a Tracer Lab GM, gas-flow, 4rr strip-scanner. The scintillation medium used for counting aqueous solutions contained 10 g of 2,5diphenyloxazole (PPO) and 80 g of naphthalene per liter of dioxane. Paper strips were counted in a toluene-based scintillation medium containing 0.4% PPO and 0.0050/, POPOP (P-bis[2-(5-phenyloxazolyl)] benzene).