Source Year: 2000

The effects of different NO3-/NH4+ proportions (0/0, 8/0, 6/2, 4/4, 2/6 or 0/8 mM) on the concentration of total N and soluble protein, and on the activities of nitrate reductase (RN, E.C.1.6.6.1), glutamine synthetase (GS, E.C. 6.3.1.2) and glutamate synthase (Fd-GOGAT, E.C. 1.4.1.7 and NADH-GOGAT, E.C. 1.4.1.14) in roots and leaves of young Hevea brasiliensis trees were investigated. The experiment was carried out under controlled environmental conditions for 120 days, in plastic pots containing 4 litres of washed sand. Bolle-Jones nutrient solution, at pH 6, was prepared to give the desired NO3-/NH4+ ratio, significantly increased total N concentration in leaves, stems and roots. The activities of GOGAT in leaves and roots and of GS in leaves did not differ between plants treated exclusively with either NO3-/NH4+ , but were significantly higher than in plants not treated with N. In roots, the activities of NR and GS were higher in plants treated exclusively with NO3- and decreased significantly when NH4+ was added to the nutrient solution. No NR activity was detected in leaves in this experiment, suggesting that in young rubber trees, NO3- reduction and primary NH4+ assimilation take place only in roots. Because NO3- reduction was absent in the leaves, it is hypothesized that the main role of the high GS and GOGAT activities found in the leaves is to reassimilate NH4+ released during photorespiration.