Since its rediscovery in 1975 by Döbereiner and collaborators (18), Azospirillum has been the subject of much controversy in the field of biological nitrogen fixation in relation to plant nutrition. This controversy stems from non-reproducible results of Azospirillum inoculations in pot and field trials. Based on the natural abundance of diazotrophic Azospirilla in the rhizosphere of tropical vegetations on low N-content soils, it was postulated that these bacteria actively contribute to the nitrogen demand of plants. Moreover since these tropical vegetations are often composed of non-leguminous plants, a new niche for research on biological nitrogen fixation was created, the so called associative nitrogen fixation. With the introduction of N balance and 15N techniques, the contribution of biological nitrogen fixation in the plant growth promotion effect of Azospirillum inoculations had to be reconsidered. The uptake of biologically fixed N2 by the plant turned out to be less than expected, and insufficient to account for the increased N content of inoculated plants versus non-inoculated plants (2,7). However, it is important to stress that an associative interaction versus a symbiotic interaction is more difficult to reconstruct. There are no such things like time of emergence, numbers and size of nodules that can be used to monitor a successful interaction. Therefore it is crucial that for Azospirillum the molecular genetics of both nitrogen fixation and plant interaction are developed.
KeywordsNitrogen Fixation Tropical Vegetation Plant Growth Promotion Effect Azospirillum Inoculation Indigenous Plasmid
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