NITROGEN-FIXATION, ASSIMILATION AND TRANSPORT DURING THE INITIAL GROWTH STAGE OF PHASEOLUS-VULGARIS L

In nodulated common bean (Phaseolus vulgaris L.), there is typically a period of N stress between 15 and 20 d after emergence (DAE), due to a lack of synchronization between the depletion of N in 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 not visible ('precocious" strains). Cultivar Negro Argel was 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 8 to 30 DAE. There were no differences between the two groups of strains in nodule dry weight or in the acetylene reduction rates between 8 and 16 DAE. However, nodules induced by the 'precocious' strains showed earlier onset of glutamine synthetase (GS) (EC 6.3.1.2) and glutamate synthase (GOGAT) (EC 1.4.1.14) activities, and ureide synthesis. The N concentration in the nodules formed by 'precocious' strains varied from 4.2 to 4.5%, whereas with the 'traditional' strains, it increased from 3.2% at 8 DAE to 6.5% at 18 DAE, at which time plants exhibited N-deficiency symptoms. By 21 DAE, GS and GOGAT activities in 'traditional' nodules were increased, as well as the ureide-N-concentration in the xylem sap, nodule N content declined to 4.5% and the leaves became green. These results suggest that the N stress with 'traditional' strains is not a limitation in early N2 fixation activity but rather in the rates of expression of the processes of N assimilation and transport.