Metabolic assimilation of nitrogen in rubber tree seedlings grown with nitrate or ammonium

The objective of this work was to evaluate metabolic assimilation of nitrogen in rubber tree seedlings grown with nitrate or ammonium, by quantifying the activity of the enzymes nitrogen reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH). The following treatments were evaluated: nitrogen sources - control, without N application; 8 mmol L-1 ammonium; and 8 mmol L-1 nitrate -, and exposure times - 0 and 27 days -, in a 3x2 factorial arrangement, in a completely randomized block design with four replicates. The ammoniacal form favored higher N metabolic assimilation, which lead to a higher aminoacid accumulation. Both nitrate and ammonium assimilations occurred mainly by GS/GOGAT route; part of the nitric nitrogen is assimilated in the roots, with little GDH participation, and the other part is translocated to other organs. Ammonium nitrogen is totally assimilated in roots, with a significant participation of GDH. In the leaves, GS participates in the primary assimilation, as well as in the photorespiratory ammonium re-assimilation, irrespective of the N source, while GDH has a secondary role, independently of the source or of the exposure time. In vitro RN activity is detected only in the roots and stems.