Abstract

The characteristics of symbiotic nitrogen fixation in tropical forest tree species have received little attention but it is well established that mineral N negatively affects symbioses in crop legumes such as soybean and common bean. Nevertheless, ecophysiological mineral nitrogen dynamics in terms of nitrogen xylem transport of native Amazonian legume has been not documented so far.The objective of this study was to evaluate nodulation and amino acid xylem transport of Inga edulis, a tropical Amazonian perennial legume, in response to mineral N. Plant growth, nodule number, nodule dry weight and xylem sap amino acid contents were determined in nodulated I. edulis in response to nutrition with ammonium, nitrate or no mineral N (total dependence on N₂fixation).  In addition, free individual amino acids were quantified in the different plant tissues and in the xylem. Plant growth (total dry weight) responded significantly to both nitrate and ammonium, especially the latter. Nitrate negatively affected nodulation and total amino acid transport to the shoot while ammonium increased nodule dry weight, but not total amino acid contents of the xylem which diminished. On the other hand, mineral-N showed only discrete changes in the amino acid composition of the xylem where asparagine was the predominant form. Our results suggest that I. edulis responds to ammonium nutrition through greater plant and nodule growth compared to the other N sources, consistent with ammonium being the principal source of mineral N in the acid soils of the natural habitat of this species.

Key words: Tropical tree species, nitrogen fixation, glutamine, symbiosis.