Nitrogen metabolism of two contrasting poplar species during acclimation to limiting nitrogen availability

To investigate N metabolism of two contrasting Populus species in acclimation to low N availability, saplings of slow-growing species (Populus popularis, Pp) and a fast-growing species (Populus alba Populus glandulosa, Pg) were exposed to 10, 100, or 1000 M NH4NO3. Despite greater root biomass and fine root surface area in Pp, lower net influxes of NH4 and NO3 at the root surface were detected in Pp compared to those in Pg, corresponding well to lower NH4 and NO3 content and total N concentration in Pp roots. Meanwhile, higher stable N isotope composition (N-15) in roots and stronger responsiveness of transcriptional regulation of 18 genes involved in N metabolism were found in roots and leaves of Pp compared to those of Pg. These results indicate that the N metabolism of Pp is more sensitive to decreasing N availability than that of Pg. In both species, low N treatments decreased net influxes of NH4 and NO3, root NH4 and foliar NO3 content, root NR activities, total N concentration in roots and leaves, and transcript levels of most ammonium (AMTs) and nitrate (NRTs) transporter genes in leaves and genes involved in N assimilation in roots and leaves. Low N availability increased fine root surface area, foliar starch concentration, N-15 in roots and leaves, and transcript abundance of several AMTs (e.g. AMT1;2) and NRTs (e.g. NRT1;2 and NRT2;4B) in roots of both species. These data indicate that poplar species slow down processes of N acquisition and assimilation in acclimation to limiting N supply.