Glyphosate [N-(phosphonomethyl)glycine] significantly decreased the chlorophyll content of field-grown soybeans (Glycine maxL. Merr. ‘Williams’) within 48 h after a 2.24-kg/ha treatment. In laboratory studies, 0.0001, 0.001, 0.01, 0.1, and 1 mM glyphosate reduced chlorophyll content of 7-day-old etiolated barley (Hordeum vulgareL. ‘Barsoy’) shoots 8, 12, 25, 49, and 77%, respectively, following 8 h dark incubation and 24 h illumination. Reduction of chlorophyll accumulation ranged from 6% with 0.001 mM glyphosate to 82% with 1.0 mM glyphosate. In studies with 8-day-old etiolated corn (Zea maysL. ‘Pioneer Brand 3535’) shoots, 0.1, 1.0, and 10.0 mM glyphosate decreased chlorophyll content of corn shoots 24, 42, and 50%, respectively, after 12 h of illumination. The rate of chlorophyll accumulation in corn shoots was significantly reduced 64% by 1.0 mM glyphosate over a 15-h illumination period. These rapid and substantial effects on chlorophyll accumulation suggest that interference with greening may be important in the mechanism of action of glyphosate.

In field and greenhouse studies, tank mixing 0.3 and 0.4 kg ai/ha of the butyl ester of fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid} with 0.4 kg ai/ha of acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} reduced control of large crabgrass [Digitaria sanguinalis(L.) Scop. # DIGSA] and itchgrass (Rottboellia exaltataL.f. # ROOEX) compared to fluazifop applied alone in soybeans [Glycine max(L.) Merr.]. Less antagonism between the two herbicides was observed in a year when conditions were optimum for large crabgrass control with fluazifop. Application of acifluorfen 1 to 3 days before application of fluazifop decreased large crabgrass control. Antagonism between fluazifop and acifluorfen was avoided when fluazifop was applied 3 to 5 days before acifluorfen. No antagonism was observed when fluazifop at 0.3 or 0.4 kg/ha was tank mixed with acifluorfen at 0.4 kg/ha for control of itchgrass. Itchgrass was more susceptible to fluazifop than large crabgrass.

The effect of glyphosate [N-(phosphonomethyl) glycine] on barley (Hordeum vulgareL.) and corn (Zea maysL.) shoot δ-aminolevulinic acid (ALA) production was examined by monitoring ALA content in the tissue and measuring incorporation of14C precursors into ALA and chlorophylla. Barley shoot ALA content was significantly decreased by 1 mM glyphosate after 9, 11, and 15 h of illumination. ALA production by treated barley shoots was 30 nmoles•g fresh weight-1•h-1at each interval tested, compared with 75 to 120 nmoles•g fresh weight-1•h-1for the control. In corn shoots, ALA content was reduced 32, 45, and 58% by 0.1, 1.0, and 10.0 mM glyphosate, respectively, after 12 h illumination. Incorporation studies with14C-glutamate,14C-α-ketoglutarate, and14C-glycine into ALA showed a 77, 92, and 91% inhibition, respectively, in barley shoots treated with 1 mM glyphosate. Incorporation of14C-ALA into chlorophyllawas not affected by 1 mM glyphosate. Thus, the site of action of glyphosate may involve two enzyme pathways:one controlling the conversion of α-ketoglutarate to ALA, and the other controlling the condensation of glycine with succinyl CoA to form ALA and carbon dioxide. Inhibition of ALA synthesis blocks synthesis of chlorophyll, as well as all other porphyrin ring compounds found in higher plants. Thus, inhibition of ALA synthesis may be an integral component of the herbicidal mode of action of glyphosate.

Glasshouse studies were undertaken to determine the effect of imposed moisture stress on the phytotoxicity of haloxyfop; the absorption, translocation, and metabolism of14C-haloxyfop; and14C-photoassimilate partitioning in johnsongrass and large crabgrass. Following foliar applications of haloxyfop at 30 and 25 g ai ha–1to large crabgrass and johnsongrass, respectively, control 15 days after treatment was 92% for nonstressed plants and less than 12% for water-stressed plants. Foliar absorption of14C-haloxyfop was reduced by moisture stress 1, 3, 5, and 24 h after treatment (HAT) in large crabgrass and 1, 3, 5, 48, and 72 HAT in johnsongrass. Regardless of stress treatment, absorption in both species reached a maximum by 24 HAT. Translocation of the radiolabel from the treated leaf to plant parts above and below the node of the treated leaf was inhibited by moisture stress in large crabgrass and johnsongrass at all harvest intervals beginning 5 and 24 HAT, respectively. Metabolism of14C-haloxyfop was not altered by moisture stress. Fixation of14CO2and subsequent distribution of the14C-photoassimilates were reduced by moisture stress. Decreases in photoassimilate translocation were similar to reductions in14C-haloxyfop translocation. Moisture stress reduced the phytotoxicity of haloxyfop in the two grasses, and the reduced activity of haloxyfop appeared to be partially related to changes in herbicide absorption and translocation.

A field investigation was conducted to determine the influence of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] and trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine] herbicides on soybean [Glycine max(L.) Merr. ‘Centennial’] —Bradyrhizobium japonicumsymbiosis. Pendimethalin and trifluralin applied at rates of 1.1, 1.7, and 2.2 kg ai/ha delayed emergence and injured soybean seedlings grown on an Olivier silt loam. Nodule number, dry weight, and nitrogen (N2) fixation measured by acetylene (C2H2) reduction were decreased by all herbicide rates during the vegetative growth stages in 1984, with occasional decreases in nodulation noted during the late reproductive growth stage. Seedling injury was less severe in 1985 than in 1984, with most injury occurring at 1.7 and 2.2 kg/ha for both herbicides. Nodule number and dry weight in 1985 were decreased from vegetative through the early reproductive growth stages with little influence on N2(C2H2) fixation. Inhibitory effects of the herbicides on nodulation and N2(C2H2) fixation did not influence seed formation since seed yield was not affected either year.