A simplified model for assessing critical parameters during associative 15N2 fixation between Azospirillum and wheat

Detailed studies in field experiments have shown repeatedly that the transfer of 15 N 2 fixed by diazotrophic bacteria to wheat tissue is minimal. Here, a simple and convenient laboratory co-culture model was designed to assess important features of the association between Azospirillum brasilense and wheat, such as the rate of nitrogen fixation (acetylene reduction), ammonia excretion from the bacterium and the transfer of newly fixed 15 N 2 from the associative diazotroph to the shoot tissue of wheat plants. After 70 h, in this model, insignificant amounts of newly fixed N 2 were transferred from an ammonia-excreting strain of A. brasilense to the shoot tissue of wheat. However, when malate was added to the co-culture the 15 N enrichment of the shoot tissue increased 48-fold, indicating that 20% of shoot N had been derived from N 2 fixation. Thus, the inability of the host plant to release carbon in the rhizosphere is a significant constraint in the development of associative N 2 -fixing systems. These specific results suggest that wheat plants with an increased release ofphotosynthate to the rhizosphere should be a priority for the future development of broad-acre agricultural systems that are more self-sufficient for nitrogen nutrition. The simplicity of the model for assessing the critical parameters of associative 15 N 2 fixation may allow large-scale surveys of plant-bacterial interactions to be conducted and a selection of improved associations for further study.