Differential contributions of NO 3 − /NH 4 + to nitrogen use in response to a variable inorganic nitrogen supply in plantlets of two Brassicaceae species in vitro

Background The primary sources of nitrogen for plants have been suggested to be nitrate (NO3−) and ammonium (NH4+). However, when both nitrate and ammonium are simultaneously available to plants, it is very difficult to differentially quantify NO3−/NH4+ utilization in culture media or soil. Consequently, the contribution of NO3−/NH4+ to total inorganic nitrogen assimilation cannot be determined.