Ammonium and urea as nitrogen sources for bromeliads

Epiphytic bromeliads have no contact with the pedosphere, so they need to draw their nutrients from the atmosphere as well as from the host tree and animal debris. Terrestrial bromeliads, like Ananas comosus, generally depend on the soil as their main nutrient source. The aim of this study was to investigate and compare some aspects of the nitrogen metabolism of two bromeliads with different growth habits: Ananas comosus, a terrestrial bromeliad, and Vriesea gigantea, an epiphytic tank bromeliad. Nitrogen-starved plants were grown in vitro for 3, 7, 15, 30, and 60 days, either with 5 mmol L⁻¹ ammonium (NH₄)₂SO₄] or urea as the sole nitrogen source. When NH₄⁺ was supplied to the plants, it stimulated a faster increase of chlorophyll content in A. comosus than in V. gigantea. In the presence of urea, after 15 days of the plants in culture, there was a significant increase in tissue free-NH₄⁺ and total amino acids for V. gigantea only. V. gigantea presented a higher level of total free amino acids than A. comosus when nitrogen was supplied to the plants. Asparagine was the main amino acid accumulated in both bromeliads when plants were transferred to the medium with nitrogen. When the ratio of the main individual free amino acids between the bromeliads grown in NH₄⁺ and urea was compared, values such as 7.2 for asparagine, 5.3 for glutamate, and 1.8 for aspartate in A. comosus, and values such as 2.3 for asparagine, 1.1 for glutamate and 0.7 for aspartate in V. gigantea were observed, demonstrating that the last is more efficient in assimilating urea. The results prompted us to support the idea that V. gigantea, an epiphytic tank bromeliad, is better adapted to absorb and assimilate organic nitrogen, such as urea, while A. comosus, a terrestrial plant, is better adapted to inorganic nitrogen forms, such as ammonium. The natural exposure of tank bromeliads to urea is discussed in the paper.