Colonization of wheat (Triticum vulgare L.) by N -fixing cyanobacteria: IV. Dark nitrogenase activity and effects of cyanobacteria on natural N abundance in the plants.

Two cyanobacterial soil isolates, Nostoc 2S9B and Anabaena C5, that had previously been shown to form different types of association with the roots of wheat plants grown in liquid culture, were tested for heterotrophic nitrogenase activity and the ability to colonize the roots of plants grown in sand. Nostoc 2S9B showed substantial nitrogenase activity when associated with the roots of plants grown in liquid culture in medium free of combined N, even with the roots maintained and with assays performed in the dark (29 % of the rate shown by root-associated Nostoc 2S9B grown and assayed in the light). When grown heterotrophically in the dark, at the expense of fructose, free-living Nostoc 2S9B showed a similar nitrogenase activity to root-associated Nostoc 2S9B in the dark. In contrast, Anabaena C5 showed no nitrogenase activity in the dark, under these conditions. When three different wheat cultivars were grown in sand that had previously been surface-inoculated with Nostoc 2S9B or with the cultured symbiotic cyanobacterium Nostoc LBG1, isolated from the bryophyte Anthoceros, there was colonization of the plant roots; there was no colonization of roots by Anabaena C5 under these conditions. Some increases in plant biomass and nitrogen content were observed, but these were dependent on the wheat cultivar and cyanobacterial inoculum used. Wheat plants grown in sand that had been pre-inoculated with Nostoc 2S9B, Nostoc LBG1 or Anabaena C5 in medium free of combined N had lower δ N values in both roots and shoots than plants grown under identical conditions without a cyanobacterial inoculum. The observed N/ N fractionation indicates that N fixed by the cyanobacteria contributed to the nitrogen economy of the wheat plants, irrespective of whether they were closely associated with the plant roots.