Exogenous ethylene inhibited nodulation on the primary and lateral roots of pea, Pisum sativum L. cv Sparkle. Ethylene was more inhibitory to nodule formation than to root growth; nodule number was reduced by half with only 0.07 ML/L ethylene applied continually to the roots for 3 weeks. The inhibition was overcome by treating roots with 1 gM Ag', an inhibitor of ethylene action.Exogenous ethylene also inhibited nodulation on sweet clover (Melilotus alba) and on pea mutants that are hypernodulating or have ineffective nodules. Exogenous nodulation of intact plants seemed warranted, testing it on young plants at the stage when nodules are initiated.We have been using the pea cv Sparkle and a family of its near-isogenic symbiosis mutants to study the formation and function of nodules (3,8). In mutants at sym-5, there are few cortical cell divisions or nodule primordia in advance of the infection thread. However, nodules form (3, 7) if the roots are treated with inhibitors of ethylene formation (AVG)2 (16) or action (Ag+) (2). Another pea mutant E107 (brz) takes up excess ions and forms few nodules. Its nodule number is increased, though not to normal, by treatment of roots with Ag+ (8). Thus, ethylene may be involved in decreasing nodulation of these mutants. Similarly, the inhibition of nodulation by light may be mediated by ethylene (13).In this report, we establish that nodulation of intact plants is extremely sensitive to low concentrations of exogenous ethylene applied during the time of nodule initiation. We also determine the developmental stage at which exogenous ethylene inhibits nodule development on pea. MATERIALS AND METHODSSeeds were planted individually in coarse vermiculite in conical pots ('conetainers') that had 16 small holes in the upper 10 cm. Pea (Pisum sativum) cv Sparkle, its mutant line E135F (11), cv Rondo, and its mutant line nod-3 (9) were inoculated with Rhizobium leguminosarum bv. viciae strain 128C53. Sweet clover (Melilotus alba, U389) and soybean (Glycine max L. cv Ransom) were inoculated with R. meliloti strain 1021 and Bradyrhizobium japonicum strain 61A92, respectively. Plants were inoculated with rhizobia on planting or 2 DAP, and were subirrigated with nutrient solution containing 5 mM nitrate (10). Plants were grown under highpressure sodium vapor and metal halide lamps (550 ,umol m-2 s-' PPFD) in a 16/8 h, 20/150C light/dark regime.For treatment with exogenous ethylene, conetainers were placed in 10-L chambers so that the roots were continually exposed to air containing ethylene (12). Air was scrubbed of ethylene by passage through Purafil (Purafil Inc., Atlanta, GA) and pumped to a four-outlet manifold. To three of the outlets, ethylene was added via micro valves (Nupro Co. Plant Physiol. Vol. 100, 1992 At harvest, nodules on the upper 10-cm region of pea roots and on whole roots of sweet clover and soybean were counted. The length of the primary and the longest lateral roots, and the dry weight of roots and shoots were also recorded. In some trials, the upperm...

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Nitrogen fixation was estimated by acetylene reduction assays for 21 soybean [Glycine max(L)Merr.] cultivars from five maturity groups. Some cultivars were studied for 2 or 3 years. Fixation by four cultivars was also estimated by the isotope dilution method and by comparing N accumulation to that of a nonnodulating cultivar. Nitrogen fixation was related to maturity group. Late maturing cultivars fixed more N2 than early maturing cultivars. Within maturity groups there was little variation in nitrogen fixing activity. During a season when N2 fixation was low, there was little agreement among three methods of measuring fixation. When available soil N was artifically reduced by amendment with carbohydrate, N2 fixation was higher and there was good correlation among the techniques used. The acetylene reduction assay gave lower values than the other two methods.