Fast-growing rhizobia have been isolated from soybean root nodules collected in China. These new isolates are physiologically distinct from slow-growing soybean rhizobia. They formed effective nitrogen-fixing associations with wild soybean and an unbred soybean cultivar from China, but were largely ineffective as nitrogen-fixing symbionts with common commercial cultivars of soybeans.

In a split-root system of soybeans (Glycine max L. Merr), inoculation of one half-side suppressed subsequent development of nodules on the opposite side. At zero time, the first side of the split-root system of soybeans received Rhizobium japonicum strain USDA 138 as the primary inoculum. At selected time intervals, the second side was inoculated with the secondary inoculum, a mixture of R. japonicum strain USDA 138 and strain USDA 110. In a short-day season, nodulation by the secondary inoculum was inhibited 100% when inoculation was delayed 10 days. Nodulation on the second side was significantly suppressed when the secondary inoculum was delayed for only 96 hours. In a long-day season, nodule suppression on the second side was highly significant, but not always 100%. Nodule suppression on the second side was not related to the appeaance of nodules or nitrogenase activity on the side of splitroots which were inoculated at zero time. When the experiments were done under different light intensities, nodule suppression was significantly more pronounced in the shaded treatments.The nitrogen-fixing symbiotic association formed between leguminous plants and soil bacteria of the genus Rhizobium has recently become an area of intense scientific research because of the economic and agricultural benefits derived from cropping systems using nodulated legumes. One of the specific goals of recent investigations has been to understand the mechanisms involved in the formation of the legume-Rhizobium association. Although both the host plant and the bacteria contribute to the specificity of the association (6), the mechanisms by which each partner exerts its influence remain poorly understood (1,4,13

Soybean lectin labeled with fluorescein isothiocyanate combined specifically with all but 3 of 25 strains of the soybean-nodulating bacterium Rhizobium japonicum. The lectin did not bind to any of 23 other strains representative of rhizobia that do not nodulate soybeans. The evidence suggests that an interaction between legume lectins and Rhizobium cells may account for the specificity expressed between rhizobia and host plant in the initiation of the nitrogen-fixing symbiosis.

Indigenous rhizobia in soil present a competition barrier to the establishment of inoculant strains, possibly leading to inoculation failure. In this study, we used the natural diversity of rhizobial species and numbers in our fields to define, in quantitative terms, the relationship between indigenous rhizobial populations and inoculation response. Eight standardized inoculation trials were conducted at five well-characterized field sites on the island of Maui, Hawaii. Soil rhizobial populations ranged from 0 to over 3.5 x 104 g of soil-1 for the

A split-root growth system was employed to evaluate the effect of NaCl on nodule formation by soybean (Glycine max L. Meff. cv Davis). By applying the salinity stress and rhizobial inoculum to only one-half the root system, the effects of salinity on shoot growth were eliminted in the nodulation process. Rhizobium colonization of inoculated root surfaces was not affected by the salt treatments (0.0, 26.6, 53.2, and 79.9 millimolar NaCI). While shoot dry weight remained unaffected by the treatments, total shoot N declined from 1.26 grams N per pot at 0.0 millimolar NaCI to 0.44 grams N per pot at 79.9 millimolar NaCl. The concentration of N in the shoot decreased from 3.75% N (0.0 millimolar NaCI) to 1.26% N at 79.9 millimolar NaC. The decrease in shoot N was attributed to a sharp reduction in nodule number and dry weight. Nodule number and weight were reduced by approximately 50% at 26.6 millimolar NaCl, and by more than 90% at 53.2 and 79.9 millimolar NaO. Nodule development, as evidenced by the average weight of a nodule, was not as greatly affected by salt as was nodule number. Total nitrogenase activity (C2H2 reduction) decreased proportionally in relation to nodule number and dry weight. Specific nitrogenase activity, however, was less affected by salinity and was not depressed signficantly ntil 79.9 milimolar NaCI. In a second experiment, isolates of Rhizobium japonicum from nodules formed at 79.9 millimolar NaC did not increase nodulation of roots under salt stress compared to nodule isolates from normal media (0.0 millimolar NaCl). Salt was applied (53.2 millimolar NaC) to half root systems at 0, 4, 12, and 96 hours from inoculation in a third experiment. By delaying the application of salt for 12 hours, an increase in nodule number, nodule weight, and shoot N was observed.Nodule formation in the 12-and 96-hour treatments was, however, lower than the control. The early steps in nodule initiation are, therefore, extremely sensitive to even low concentrations of NaCl. The sensitivity is not related to rhizobial survival and is probably due to the salt sensitivity of root infection sites.Rhizobium growth and survival are generally more tolerant in vitro to high osmotic pressures than are their respective host legumes (8,15, 17). Tu (22), however, observed reduced colonization of soybean root surfaces by Rhizobium japonicum when plants were grown in salinized culture medium.Legumes grown in saline environments exhibit reduced yield potential and reduced numbers and weight of root nodules (1,14,15, 22, 24). There were, however, serious limitations in the above studies for evaluating the effects of salinity on the early stages of nodule formation. With the exception of the work of Lakshmi et al. (14), inoculation of seedlings with Rhizobium preceded the salinization of the rooting medium. It is likely that in these studies some critical steps of rhizobial attachment and infection thread formation could have occurred before the introduction of the salt stress. In the study of Lakshmi et aL, (14) plant...

Fast-growing, acid-producing soybean rhizobia were examined to determine their biochemical relatedness to each other, to typical slow-growing Rhizobium japonicum strains, and to other fast-growing species of Rhizobium. Although both the fast-and slow-growing soybean rhizobia were positive for catalase, urease, oxidase, nitrate reductase, and penicillinase, the fast-growing strains grouped with other fast-growing species of Rhizobium in that they tolerated 2% NaCl, were capable of growth at pH 9.5, utilized a large variety of carbohydrates (notably disaccharides), and produced serum zones in litmus milk. In addition, these fastgrowing strains were similar to other fast-growing species of Rhizobium in that they produced appreciable levels of P-galactosidase and nicotinamide adenine dinucleotide phosphate-linked 6-phosphogluconate dehydrogenase but had no detectable hydrogenase activity. The fast-growing soybean rhizobia share symbiotic host specificity with Brudyvhizobium japonicum, but appear to be related biochemically to the other fast-growing species of Rhizobium.

Application of fluorescent-antibody (FA) techniques to the study of rhizobia as free-living soil bacteria was explored. Antiserum to a particular strain of Rhizobium japonicum proved specific in both agglutination and FA tests. Within the R. japonicum group, 2 of 12 strains were stained by the conjugate and these fluoresced brightly; all others were entirely negative. FA tests were negative for 7 strains of R. meliloti, 9 strains of R. leguminosarum, 9 strains of R. trifolii, 6 strains of R. phaseoli, and 65 unidentified bacteria isolated from 12 soils. R. japonicum grew in autoclaved soil and was readily detectable by FA examination of contact slides. The FA technique also detected antibody-reacting bacteria in a field soil whose rhizobial content was unknown. Fluorescent cells were probably R. japonicum, since nodules developed on soybean plants grown in the same soil sample and FA preparations of the crushed nodules proved positive. Autofluorescence was not a problem, but nonspecific adsorption of conjugate restricted observations to microscopic fields free from soil particles. Nonspecific adsorption was substantial, irrespective

Wild-type soybean (Glycine max [LU Merr. cv Bragg) and a nitrate-tolerant supemodulating mutant (nts382) were grown in split root systems to investigate the involvement of the autoregulation response and the effect of timing of inoculation on nodule suppression. In Bragg, nodulation of the root portion receiving the delayed inoculation was suppressed nearly 100% by a 7-day prior inoculation of the other root portion with Bradyrhizobium Japonkum strain USDAI 10. Significant suppression was also observed after a 24-hour delay in inoculation. Mutant nts382 in the presence of a low nitrate level (0.5 millimolar) showed little, if any, systemic suppression. Root fresh weights of individual root portions were similar for both wild type and nts382 mutant. When nts382 was grown in the absence of nitrate, a 7-day delay in inoculation resulted in only 30% suppression of nodulation and a significant difference in root fresh weight between the two sides, with the delayed inoculated side always being smaller. Nodulation tests on split roots of nts382, nts1116, and wild-type cultivars Bragg, Williams 82, and Clark demonstrated a difference in their systemic suppression ability. These observations indicate that (a) autoregulation deficiencies in mutant nts382 result in a reduction of systemic suppression of nodulation, (b) some suppression is detectable after 24 hours with a delayed inoculation, (c) the presence of low nitrate affects the degree of suppression and the root growth, and (d) soybean genotypes differ in their ability to express this systemic suppression.