Ammonium and nitrate uptake in gap, generalist and understory species of the genus Piper.

We studied root net uptake of ammonium (NH 4+ ) and nitrate (NO 3- ) 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 NH 4+ and NO 3- 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 NH 4+ and/or 100 μM NO 3- ) at intermediate PFD (100 μmol m-2 s-1). Preference for NH 4+ relative to NO 3- , defined as the ratio of NH 4+ (both):NO 3- (both) net uptake, was higher in understory species than in gap species. Ammonium repression of NO 3- uptake, defined as the ratio of NO 3- (single): NO 3- (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 NH 4+ preference and NH 4+ repression of NO 3- net uptake at high (500 μmol m-2 s-1) and low (50 μmol m-2 s-1) PFD in a gap (P. auritum), generalist (P. hispidum) and understory species (P. aequale). All species exhibited negligible NH 4+ repression of NO 3- net uptake at high PFD. At low PFD, NH 4+ preference and repression of NO 3- 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 NO 3- 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 NO 3- or NH 4+ net uptake were observed.