title: Ammonium and nitrate uptake in gap, generalist and understory species of the genus Piper.
abstract: We studied root net uptake of ammonium (NH4+) and nitrate (NO3-) in species of the genus Piper (Piperaceae) under high, intermediate and low photosynthetically active photon flux densities (PFD). Plants were grown hydroponically, and then transferred to temperature controlled (25° C) root cuvettes for nutrient uptake determinations. Uptake solutions provided NH4+and NO3-simultaneously (both) or separately (single). In the first experiment, seven species of Piper, from a broad range of rainforest light habitats ranging from gap to understory, were screened for mineral nitrogen preference (100 μM NH4+and/or 100 μM NO3-) at intermediate PFD (100 μmol m-2s-1). Preference for NH4+relative to NO3-, defined as the ratio of NH4+(both):NO3-(both) net uptake, was higher in understory species than in gap species. Ammonium repression of NO3-uptake, defined as the ratio of NO3-(single): NO3-(both) net uptake, was also higher in understory species as compared to gap species. In a second set of experiments, we examined the effect of nitrogen concentration (equimolar, 10 to 1000 μM) on NH4+preference and NH4+repression of NO3-net uptake at high (500 μmol m-2s-1) and low (50 μmol m-2s-1) PFD in a gap (P. auritum), generalist (P. hispidum) and understory species (P. aequale). All species exhibited negligible NH4+repression of NO3-net uptake at high PFD. At low PFD, NH4+preference and repression of NO3-net uptake occurred in all species (understory > generalist > gap), but only at intermediate nitrogen concentrations, i.e. between 10 and 200 μM. Ammonium repression of net NO3-uptake decreased or increased rapidly (in < 48 h) after transitions from low to high or from high to low PFD respectively. No significant diurnal patterns in NO3-or NH4+net uptake were observed.
authors.affiliation: Department of Plant Biology, Carnegie Institution of Washington, 290 Panama Street, 94305, Stanford, CA, USA.
authors.forename: Arthur L
authors.initials: AL
authors.lastname: Fredeen
authors.affiliation: Department of Plant Biology, Carnegie Institution of Washington, 290 Panama Street, 94305, Stanford, CA, USA.
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Field
keywords: Ammonium uptake; Neotropics; Nitrate reductase; Nitrate uptake; Piper