Acetylene-reducing bacteria isolated from the setts (stem cuttings used as seed pieces) and roots of two sugar cane varieties propagated aseptically from stem cuttings were identified using a computer-assisted scheme based on 75 biochemical tests. Because 106 to 108 acetylene-reducing bacteria per gram (fresh weight basis) were found in the roots, while 10 to 100 times fewer were present in the sett, we suggest that the root is the site of bacterial multiplication. Sterilization of the sett surface before planting or root sterilization at harvest reduced or completely removed acetylene-reducing bacteria and associated whole plant acetylene-reducing activity. This indicates that most of the active bacteria were on the sett and root exteriors. Setts did not exhibit acetylene-reducing activity until after emergence of the roots. Since shoot emergence was not necessary for acetylene-reducing activity, the extensive carbohydrate supply of the sett itself must have provided the carbon substrate for bacterial N2 fixation. The acetylene-reducing bacteria isolated were facultative anaerobes of the families Enterobacteriaceae and Bacillaceae. Klebsiella pneumoniae, Enterobacter cloacae, Erwinia herbicola, and Bacillus polymyxa were present inside the sett and the roots but E. herbicola was the dominant bacterium on the root exterior. No Beijerinckia spp. or Azotobacter spp. were found associated with the sett or the roots.

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Summary Differences in Nz-fixation by Phaseolus vulgaris bean cultivars were successfully evaluated in the field using ~ SN isotope dilution technique with a non-fixing test crop of a different species (wheat). The Phaseolus cultivars could have been similarly ranked for N2-fixation capacity from either seed yield or total nitrogen yield, but the isotope method provided a direct measure of N 2-fixation and made it possible to estimate the proportion of fixed to total nitrogen in the crop and in plant parts.Amounts of nitrogen fixed varied between 24.59 kg N/ha for the 60-day cultivar Goiano precoce to 64.91 kg N/ha for the 90-day cultivar Carioca. The per cent of plant nitrogen due to fixation was 57-68% for the 90-day cultivars and 37% for Goiano precoce (60-day cultivar).Fertilizer utilization was 17-30% of a 20kgN/ha fertilizer application. 100 kg N/ha fertilizer application decreased N2-fixation without suppressing it totally.Differences in yield between the highest yielding (Carioca) and the lowest (Moruna) 90-day cultivars were also due apparently to varietal differences in efficiency of conversion of nitrogen to economic matter i.e. seed, as well as to differences in capacity of genotypes for Nz-fixation.

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