A simplified model for assessing critical parameters during associative N-15(2) fixation between Azospirillum and wheat

Detailed studies in field experiments have shown repeatedly that the transf er of N-15(2) fixed by diazotrophic bacteria to wheat tissue is minimal. He re, a simple and convenient laboratory co-culture model was designed to ass ess important features of the association between Azospirillum brasilense a nd wheat, such as the rate of nitrogen fixation (acetylene reduction), ammo nia excretion from the bacterium and the transfer of newly fixed N-15(2) fr om the associative diazotroph to the shoot tissue of wheat plants. After 70 h, in this model, insignificant amounts of newly fixed N-2 were transferre d from an ammonia-excreting strain of A. brasilense to the shoot tissue of wheat. However, when malate was added to the co-culture the N-15 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 re lease carbon in the rhizosphere is a significant constraint in the developm ent of associative N-2-fixing systems. These specific results suggest that wheat plants with an increased release of photosynthate 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 simplicit y of the model for assessing the critical parameters of associative N-15(2) fixation may allow large-scale surveys of plant-bacterial interactions to be conducted and a selection of improved associations for further study.