The energy costs for utilizing N2 as a nitrogen source are much higher than for the utilization of NO3-
As shown in Figure 11.8, at least six molecules of NADH are consumed in the production of one molecule of NH4+ during molecular nitrogen fixation. Assimilation of nitrate, in contrast, requires only four NAD(P)H equivalents for the formation of NH4+ (Fig. 10.1). In addition, nodule formation costs the plant much metabolic energy. Therefore it is cheaper for plants to satisfy their nitrogen demand by nitrate assimilation instead of N2 fixation with the help of the symbionts. As a consequence the formation of nodules has to be balanced according to the cellular demands and environmental conditions. Nodules are formed only when the soil is nitrate-deficient. The advantage of this symbiosis is that legumes and actinorhizal plants can grow in soils with very low nitrogen content, where other plants do not survive.
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