With the objective of evaluating the effect of exclusive supply of ammonium or nitrate, or both, on the growth of eucalypt seedlings, two experiments were carried out under greenhouse conditions. Eucalyptus urophylla and E. camaldulensis were cultivated in the first experiment and Eucalyptus pellita, E. grandis and E. cloeziana in the second. Initial growth (32 days) was carried out on washed sand bed. Then the seedlings were transferred to nutrient solution with three ammonium/nitrate ratios (100-0, 50-50 e 0-100%) where they grew for 70 days (experiment 1) or for 105 days (experiment 2). Plant heights were measured and the seedlings transferred to a no-nitrogen solution (experiment 1) or to 0.36 mmol L-1 N (experiment 2) where they were left for 48 hours. After this period, part of the plants were taken to determine the dry matter of stem, leaf and root and the other part was submitted to N exhaustion for 10 hours in the presence and absence of P in the nutrient solution with 0.36 mmol L-1 N. The exclusive supply of NO3- reduced the biomass for all species with exception of E. cloeziana which presented higher growth with increase in nitrate concentration. Approximately 80% of the plants submitted to exclusive ammonium medium died. With the exception of E. cloeziana, the shoot/root biomass ratio was reduced with increase in NO3- concentration. Under NO3- medium, the absence of P sharply reduced the efficiency of NO3- absorption by E. grandis and E. pellita and slightly modified it for NH4+. Passing from a pure nitric or ammoniacal medium to a medium with both forms, Km-NH4+ was reduced and Km-NO3- increased. Eucalyptus urophylla and E. pellita grew better under NH4+, whereas E. camaldulensis and E. grandis grew equally well under both forms of N in solution; E. cloeziana showed better growth under NO3- medium.
mineral nutrition; eucalypt; nitrogen