Soybean plants, inoculated with Rhizobium japonicum, were grown in vermiculite with a nitrogen free nutrient solution in a greenhouse, in the Experimental Station of Campinas, Instituto Agronômico, State of São Paulo, Brazil. The effects of NO3- and NH4+ were studied in terms of ureide and amino acid levels in the xylem sap. Nodulated plants were treated with nutrient solution either containing NO3- (15 mM), or NH4+ (10 mM), or without nitrogen (control) for 7 days, near the flowering period. Higher proportion of alantoic acid was found relative to allantoin, and this proportion increased in plants grown on mineral N. The NH4+ ion had an intermediary effect compared to the other N-assimilatory processes, with respect to the quantity of N transported to the aerial parts, suggesting that the mecanism of transport depended on the interaction between the soil mineral nitrogen and the energy available. Asparagin was the amino acid in the greatest quantity in the xylem, independent of the treatment. Ammonium did not alter the levels of N-NH4+ in the sap, but increased slightly the level of NO3-. Nitrate caused slight increase in aspartic acid and large in NO3- content in the sap. These results suggested that the differences in the amino acid synthesis proceeded of a specific pathway of nitrogen uptake.
soybean; amino acids; ureides; symbiotic fixation; nitrogen nutrition