Annals of Botany

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Emergency air supply piped to nodules

Although the nitrogenase enzyme involved in N-fixation is inhibited by oxygen, the overall process has a high demand for respiratory energy and is thus very sensitive to anoxia. This is nicely shown by the work of Thomas et al., Campinas, Brazil (pp. 1191–1198) on N-fixing root nodules in soybean. Flooding the root system led to oxygen availability being reduced by approx. 90 % and an almost complete disappearance of the metabolic products of N-fixation (ureides and glutamine). However, there was also a rapid morphological response: adventitious roots, derived from divisions of the pericycle, appeared in the area of the stem–root junction; aerenchyma formed in the same region as well as in the taproot, lateral roots, the newly formed adventitious roots and at a later stage in the less deeply submerged nodules. Over a period of 21 days, three different mechanisms for aerenchyma formation were observed. In the taproot and lateral roots, the initial formation of aerenchyma was by lysigeny of cortical cells, but the cortex was later displaced by a spongy parenchyma (secondary aerenchyma) formed by cell division in the pericycle. In adventitious roots, aerenchyma was initially formed by separation of cell layers (schizogeny) followed by lysigeny of cortical cells and eventually, after about 21 days, secondary aerenchyma was also formed. By contrast, in both the stem–root junction and in nodules, aerenchyma arose almost exclusively via cell division (from the pericycle in the former; from the phellogen in the latter). Aerenchymatous nodules had a very different appearance from non-flooded nodules but nevertheless possessed a normal complement of leghaemoglobin, suggesting a capability for N-fixation. Evidence that they actually did fix N was that between 7 and 10 days after the start of the flooding treatment, ureides and glutamine were once again detectable in significant amounts in the xylem. The formation of aerenchyma had thus led to the restoration of normal nodule function.