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Fig. 1 | BMC Biology

Fig. 1

From: Are we there yet? The long walk towards the development of efficient symbiotic associations between nitrogen-fixing bacteria and non-leguminous crops

Fig. 1

Different types of nitrogen-fixing associations with plants. The three challenges of biological nitrogen fixation are solved with different efficiency by these types of interactions—energy source, oxygen protection, and transfer of fixed nitrogen to the plant. The efficiency of each bacterial partner is indicated by + (low), ++ (moderate), or +++ (high). The nitrogen fixation rates depend on the efficiency of the interaction. a Root nodule symbiosis, 50–465 kg N ha−1 y−1 [17, 18]; b associative nitrogen fixation, 2–170 kg N ha−1 y−1 [19,20,21,22,23]; and c, d free-living nitrogen fixation, 1–80 kg N ha−1 y−1 [24,25,26]

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