In the greenhouse, we examined approximately 2,700 strains of soybeans [Glycine max (L.) Merr.] for response to 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), a herbicide which inhibits photosynthesis. Duration of survival when grown in soil containing 0.84 kg/ha was the initial selection criterion. Strains thus selected as most tolerant and most susceptible were grown then for 3 to 3 ½ weeks in soil containing 0.45 kg/ha of atrazine. The dry weight of shoots expressed as a percentage of each strain's own untreated check was used to measure the strains' tolerance of atrazine. Tolerance, thus measured, generally increased as seed size increased. Regression analysis indicated that 80% of the variation in response was attributable to variation in seed size. We suggest the possibility of minimizing soybean injury from atrazine (and perhaps other herbicides) by planting large seed.

We applied (2,4-dichlorophenoxy)acetic acid (2,4-D), (2, 4,5-trichlorophenoxy)acetic acid (2,4,5-T), 4-amino-3,5,6-trichloropicolinic acid (picloram), and 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat) alone and in various combinations by aircraft on arborescent species in Texas and Puerto Rico. Paraquat defoliated trees rapidly but did not give long-term control. Picloram defoliated a greater number of species than the other herbicides and defoliation extended over a longer period. No treatment killed all trees in the mixed forest or prevented regrowth and secondary succession for a period of more than 1 year. Higher herbicide rates were necessary to defoliate woody plants in tropical Puerto Rico than in subtropical Texas.

Absorption and translocation of (2,4-dichlorophenoxy)-acetic acid (2,4-D) and 4-amino-3,5,6-trichloropicolinic acid (picloram) were measured in defoliated and intact field bindweed (Convolvulus arvensis L.) plants. Radioactivity was applied as 5 μl of triethylamine salts of 2,4-D and picloram with an activity of 0.15 μc to a middle leaf in intact and partially-defoliated plants. After treatment for 96 hr, 14C-activity in untreated leaves, stems, growing tips, and roots was measured. Removal of all leaves significantly reduced absorption but not translocation of both 2,4-D and picloram. A greater percentage of the applied picloram than 2,4-D was absorbed in all treatments. Translocation of 14C of both 2,4-D and picloram was greater when (a) upper and lower leaves were removed, (b) upper leaves only were removed, and (c) lower leaves only were removed than (d) when plants were intact. Removal of lower (older) leaves reduced translocation more than removal of upper (younger) leaves.

Twelve field trials were conducted in western Oregon during 1963 to 1966 to evaluate the effect of 4-amino-3,5,6-trichloropicolinic acid (picloram) on the seed production and seed quality of eight grass species. Seed production of creeping bentgrass (Agrostics palustris Huds., vars. Penncross and Seaside), Colonial bentgrass (Agrostis tenuis Sibth., var. Highland), perennial ryegrass (Lolium perenne L., var. Linn), Italian ryegrass (Lolium multiflorum Lam., var. Gulf), and tall fescue (Festuca arundinacea Schreb., var. Alta) treated with picloram was not statistically different from the untreated check. Seed production of Colonial bentgrass (Agrostis tenuis Sibth., var. Astoria) was reduced by picloram at rates of 1.0 and 1.5 lb/A. Seed production of Kentucky bluegrass (Poa pratensis L., var. Newport) was not statistically different from the untreated check when picloram was applied in October but was significantly lower than the check following some March treatments. Seed production of orchardgrass (Dactylis glomerata L., var. Potomac) and creeping red fescue (Festuca rubra L., var. Pennlawn) was reduced by most picloram treatments made either in October or March. Germination of the creeping red fescue seed was significantly reduced when picloram was applied at 2 lb/A in March. None of the treatments affected seedling development.

Chemical analyses for carbohydrates, fats, and protein and C14O2 tracer techniques were employed to study the influence of repeated applications of amine methylarsonate (hereinafter referred to as AMA) on food reserves in purple nutsedge (Cyperus rotundus L.). Purple nutsedge plants were grown under greenhouse conditions and treated at 3-week intervals with an aqueous solution of AMA. Chemical analyses suggested that repeated applications of AMA accelerated starch hydrolysis in tubers but had little effect on the fat and protein content. The disappearance of starch was correlated with an increase in the arsenic (hereinafter referred to as As) content of tubers following repeated applications. In leaf tissue, applications of AMA significantly reduced the sugar content and increased the protein content. Tracer studies on the rate of C14O2 fixation and evolution suggested that, in general, AMA-treated plants had a higher rate of utilization of the products of photosynthesis than untreated plants. Apparently, the carbohydrate fraction of food reserves was utilized in preference to fats and protein.

Although similar in mode of action, 1,2-dihydroxybenzene (catechol) and 1,1′-dimethyl-4,4′bipyridinium ion (paraquat) at different levels were required to kill plants. Both chemicals were fast-acting, they both required light to be active, and herbicides that inhibit photosynthesis protected the plants temporarily against the action of both compounds. Paraquat and catechol had additive effects on black nightshade (Solanum nigrum L.) but not on redroot pigweed (Amaranthus retroflexus L.). Catechol protected redroot pigweed against the action of paraquat.

Benzadox [(benzamidooxy)acetic acid] applied at 1, 2, 3, and 4 lb/A as a postemergence treatment controlled kochia [Kochia scoparia (L.) Schrad.] selectively in sugarbeets (Beta vulgaris L.). Control increased as the rate of benzadox increased, but some kochia survived at the 4-lb/A rate. Competition from these plants reduced the yields of sugarbeet roots and sucrose. Where surviving kochia plants were removed by hand 7 weeks after emergence, all sugarbeets treated with benzadox yielded as well as the hand-weeded checks. In growth chamber studies, the activity of benzadox was increased by temperature and decreased by simulated rain which occurred within 4 hr following application. Under a temperature regime of 70 F day and 40 F night, if simulated rain was delayed for 8 hr, the control of kochia was identical to that obtained where no simulated rain followed treatment. The growth chamber studies confirmed our results with the performance of benzadox under different environmental conditions in the field.