Nitrogen metabolism of Neotropical tree seedlings with contrasting ecological characteristics

Despite the importance of nitrogen for plant performance, few studies have addressed the nitrogen assimilation and amino acid metabolism of Neotropical trees. This study analyzed the nitrogen use strategies of tree species native to the Brazilian Atlantic Forest with different degrees of shade tolerance: Heliocarpus popayanensis (a shade-intolerant species), Cariniana estrellensis (a shade-tolerant canopy species) and Eugenia brasiliensis (a shade-tolerant understory species). We analyzed the growth and nitrogen uptake, assimilation and translocation of seedlings cultivated hydroponically with nitrate (NO3−) or ammonium (NH4+). 15N incorporation into amino acids was monitored after incubation with 15NO3− or 15NH4+. H. popayanensis showed a preference for NO3−, characterized by high NO3− uptake rates and intense NO3− assimilation occurring primarily in the leaves. E. brasiliensis assimilated NO3− primarily in the roots and showed a preference for NH4+ uptake. The canopy species C. estrellensis assimilated NO3− predominantly in the leaves and absorbed NO3− and NH4+ at similar rates. Both shade-tolerant species were highly tolerant to NH4+ and showed glutamine and compounds related to nitrogen remobilization as important nitrogen carriers in their xylem sap. In contrast, H. popayanensis seedlings had their growth reduced by NH4+ and showed asparagine as the major amino acid translocated in the xylem sap. Variations in root and leaf amino acid metabolism among the species were also observed. The canopy species has nitrogen use strategies intermediate to shade-intolerant and understory species. The different responses might be related to adaptations to the different light conditions of their habitats, since high light intensities could favor the use of NO3−.