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Imazethapyr is a selective herbicide used to control a wide spectrum of weeds in several legume crops, including pea. The effect of imazethapyr on pea-Rhizobiumsymbiosis was studied through its effect on microorganism growth, microorganism nodulation ability and plant growth. Symbiotic plants were damaged by imazethapyr concentrations higher than 1.73 μM applied preemergence. The number of nodules per plant was affected more than nodule size, suggesting a direct imazethapyr effect on the nodule initiation rather than on nodule development. However, imazethapyr did not directly affectRhizobiumbecause doses higher than 0.34 mM were required to cause slight effects onRhizobiumgrowth in a defined medium. Also, nodulation ability of bacteria treated with imazethapyr was not affected. These results suggest that imazethapyr inhibits the growth of the symbiotic plant rather than having a direct effect on the bacteria. Accordingly, symbiotic plants were less affected than the corresponding nitrate-reducing pea plants.

Chlorsulfuron was preemergence applied in field trials at La Corufia, Spain, at rates from 5 to 30 g ai ha-1 (1987-88) and from 5 to 100 g ai ha71 (1988-89) for weed control in winter and spring wheat. Excellent control of broadleaf weeds was obtained in both; however, narrowleaf control at the last evaluation (heading) was poor. Chlorsulfuron at 30 g ha-1 did not cause a residual problem in 1987-88, with only 0.24 jig kg-1 of dry soil recovered after both a winter and spring wheat cropping season. However, chlorsulfuron persistence at the same rate was very high in the dry year 1988-89, with 0.43 jg kg71 and 0.53 jg kr1 recovered in winter and spring wheat, respectively. 'Cardeno' spring wheat showed no phytotoxic symptom at any rate of chlorsulfuron from 5 to 100 g ha71. Its yield and yield components (spikes m-2, grain number spike-1, grain weight) were greater with all chlorsulfuron rates, but not in 'Fiel' winter wheat for rates of 30 g ha71 or higher. Nomenclature: Chlorsulfuron, 2-chloro-N-(((4-methoxy- 6 - methyl - 1,3,5-triazin - 2 - yl)-amino)carbon- yl)benzenesulfonamide; wheat, Triticum aestivum L. Cardeno and Fiel. Additional index words. Bioassay, efficacy, tolerance.