Recovery of nitrogen fixation after short-term flooding of the nodulated root system of soybean

Abstract

Nitrogen fixation of terrestrial legumes is strongly and rapidly diminished under flooding. Although recovery is possible with the formation of aerenchyma, information is scarce regarding recovery after draining following short-term flooding, before the appearance of morphological adaptations. This study used soybean (Glycine max) plants nodulated with Bradyrhizobium elkanii to determine xylem sap glutamine as an indication of nitrogen fixation activity during recovery from different periods of flooding. Xylem glutamine levels showed rapid recovery (within 90min) following periods of flooding up to 4h. Recovery was progressively slower after longer periods of flooding. After 48h flooding very little recovery could be observed within the first 120min after draining but recovery was possible within 48h. Consistent with the changes in xylem glutamine, direct measurements of apparent nitrogenase activity carried out immediately on draining revealed rapid recovery after flooding for 1h and slow recovery following 48h of flooding. In the latter case, nitrogenase activity largely recovered 24h after draining. Experiments with 15N2 incorporation into amino acids exported in the xylem sap revealed that glutamine was by far the most highly labelled amino acid in sap collected over the first 30min of exposure to the isotope. This is conclusive evidence that xylem sap glutamine is an immediate product of N2 fixation and export. The changes in xylem sap glutamine seen on flooding (decline) and after draining (recovery) can therefore be attributed to changes in nitrogenase activity. The data show that xylem sap glutamine is a useful means for assessing changes in nitrogenase activity, especially when the root system is submersed in water and activity cannot be measured directly.