Isolation and identification of N2-fixing bacteria associated with sugar cane (Saccharum sp.)

Abstract

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.