The activities of nitrate reductase and glutamine synthetase were evaluated in young plants of Faidherbia albida, a tropical woody legume, fed with different N sources under hydroponic conditions. Results showed that assimilation of both NO3 - and NH4 + preferentially took place in shoots. A basal amount of nitrate reductase activity was detected in shoots of plants grown with an NO3 - -free solution or placed under N2 -fixing conditions, and also in nodules of N2 -fixing plants. This strongly suggests that constitutive nitrate reductase activity is present in these organs. Analyses of the soluble nitrogenous content showed that the major form of N in the different organs was α-amino acids (particularly amides), irrespective of the N status of the culture conditions. The same result was obtained for nodulated plants grown in local sandy soil. In this case, amide-N generally accounted for more than 40% of the total soluble N. This was especially true in nodules. Ureide-N never exceeded 9% of the total soluble N and did not appear to increase with increasing nodule nitrogenase activity. Amides were also predominant in three N2 -fixing Sahelian acacias (Acacia seyal, A. nilotica and A. tortilis), showing that F. albida does not differ from Sahelian Acacia in terms of the metabolism of fixed N. However, like another Sahelian acacia growing preferentially near water (A. nilotica), F. albida can be distinguished from acacias growing strictly in arid zones (A. seyal and A. tortilis) in terms of initial growth, water and nitrate management.
Keywords: Acacia; Faidherbia albida; N fixation; Sahel; amides; glutamine synthetase; nitrate reductase; ureides.