Nitrogen use plasticity in response to light intensity in neotropical tree species of distinct functional groups

Environmental conditions influence the use of different nitrogen (N) sources by plants. We hypothesized that an increase in light intensity favors the use of nitrate (NO₃ ^- ) relative to ammonium (NH₄ ⁺ ) by seedlings of neotropical tree species from different functional groups, that is, Cecropia pachystachya (a shade-intolerant species), Cariniana estrellensis (a shade-tolerant canopy species), and Guarea kunthiana (a shade-tolerant understory species). We analyzed the growth and N metabolism in seedlings simultaneously provided with NH₄ ⁺ and NO₃ ^- , under lower (LL) and higher (HL) light intensity. ¹⁵ N incorporation into amino acids was monitored after incubation with ¹⁵ N-labeled NH₄ ⁺ or NO₃ ^- . Under HL, all species showed decreased leaf area ratio, and increased growth, nitrate reductase activity and assimilated N content. Cecropia pachystachya increased the use of both N sources under HL, with substantial increases in ¹⁵ N-amino acids derived from ¹⁵ NO₃ ^- (12.5- and 4.0-fold in roots and leaves, respectively) and ¹⁵ NH₄ ⁺ (4.5- and 3.0-fold in roots and leaves, respectively). Guarea kunthiana showed the greatest plasticity in N use, as the assimilation of ¹⁵ NO₃ ^- in roots and leaves increased substantially under HL (11.2- and 17.0-fold, respectively). Cariniana estrellensis increased the assimilation of ¹⁵ NH₄ ⁺ in roots and ¹⁵ NO₃ ^- in leaves under HL. Therefore, the responses of N use strategies to light intensity varied with the species according to their ecological characteristics.