Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Laboratory, greenhouse, and field investigations were conducted to determine the influence of pendimethalin (N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine), trifluralin (2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine), and other selected herbicides on the Bradyrhizobium japonicum (3I1B110)-soybean (Glycine max, (L.) merr.) symbiosis. Bradyrhizobium japonicum, strain 110 growth on yeast extract mannitol (YEM) agar was not inhibited by most herbicides in direct contact with cells when applied at 10 to 2000 mg L('-1). Pendimethalin and trifluralin inhibited B. japonicum growth at levels over 2000 mg L('-1). Bradyrhizobium japonicum growth in YEM broth was not inhibited by pendimethalin or trifluralin at 100 mg L('-1). Metolachlor (2-chloro-N-(2-ethyl-6-methylphehyl)-N-(2-methoxy-1-methylethyl)acetamide at 10 mg L('-1) increased growth in YEM broth during the first four days of a 13-day incubation period. Respiration by B. japonicum was decreased by pendimethalin at 100 mg L('-1), even when arabinose or glucose was added to the inoculated media. Evolution of CO(,2) was higher when arabinose was included as the carbon source. Nodule number, dry weight of nodules, roots, and plants, and N(,2)(C(,2)H(,4)) fixation per plant of Forrest cv. soybeans were significantly decreased by trifluralin at 0.90 and 1.80 kg ha('-1) equivalent in the greenhouse. Linuron (N'-(3,4-dichlorophenyl)-N-methoxy-N-methylurea) inhibited nodule number, and nodule and plant dry weight. Pendimethalin decreased nodule dry weight and C(,2)H(,4) production per plant of Centennial cv. soybeans at 0.75, 1.00, and 1.25 kg ha('-1) in the greenhouse. Plant dry weight was decreased at 1.25 kg ha('-1). Trifluralin decreased nodule and plant dry weight at 1.00 and 1.25 kg ha('-1), and C(,2)H(,4) production per plant at 0.75, 1.00, and 1.25 kg ha('-1). Pendimethalin and trifluralin injured Centennial cultivar soybeans at 1.12, 1.68, and 2.24 kg ha('-1) when grown on an Olivier silt loam. Nodule number and dry weight, and C(,2)H(,4) production were decreased during vegetative growth at all rates in 1984. Seedling injury caused by pendimethalin and trifluralin was not as severe in 1985, with most injury occurring at 1.68 and 2.24 kg ha('-1). Nodule number and dry weight were decreased up to the R3 growth stage. Occasional inhibition of C(,2)H(,4) producton was found. Inhibitory effects were not present at seed formation, and seed yield was not affected in either year.