Nitrogen Fixation, Assimilation and Transport During the Initial Growth Stage of Phaseolus vulgaris L

In nodulated common bean (Phaseolus vulgaris L.), there is typicallya period of N stress between 15 and 20 d after emergence (DAE),due to a lack of synchronization between the depletion of Nin the cotyledons and the beginning of N² fixation and transport.Screening trials identified some Rhizobium leguminosarum bv.phaseoli strains with which symptoms of N deficiency were notvisible (‘precocious’ strains). Cultivar Negro Argelwas then inoculated with two ‘traditional’ strains(C-05 and CIAT 727) and two ‘precocious’ strains(CNPAF 146 and CNPAF 512), and plants were harvested from 8to 30 DAE. There were no differences between the two groupsof strains in nodule dry weight or in the acetylene reductionrates between 8 and 16 DAE. However, nodules induced by the‘precocious’ strains showed earlier onset of glutaminesynthetase (GS) (EC 6.3.1.2[EC]) and glutamate synthase (GOGAT)(EC 1.4.1.14[EC]) activities, and ureide synthesis. The N concentrationin the nodules formed by ‘precocious’ strains variedfrom 4.2 to 4.5%, whereas with the ‘traditional’strains, it increased from 3.2% at 8 DAE to 65% at 18 DAE, atwhich time plants exhibited N-deficiency symptoms. By 21 DAE,GS and GOGAT activities in ‘traditional’ noduleswere increased, as well as the ureide-N-concentration in thexylem sap, nodule N content declined to 4.5% and the leavesbecame green. These results suggest that the N stress with ‘traditional’strains is not a limitation in early N² fixation activity butrather in the rates of expression of the processes of N assimilationand transport. Key words: Glutamate synthase, glutamine synthetase, nitrogen fixation, Phaseolus vulgaris, Rhizobium