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

The primary sources of nitrogen for plants have been suggested to be nitrate (NO ) and ammonium (NH ). However, when both nitrate and ammonium are simultaneously available to plants, it is very difficult to differentially quantify NO /NH utilization in culture media or soil. Consequently, the contribution of NO /NH to total inorganic nitrogen assimilation cannot be determined. We developed a method called the bidirectional stable nitrogen isotope tracer to differentially quantify the nitrate and ammonium utilization by ( ) and ( ) plantlets in vitro. The utilization efficiency of nitrate was markedly lower than the utilization efficiency of ammonium for plantlets of both and . In both and , the proportion of NO /NH utilization did not show a linear relationship with inorganic nitrogen supply. The plantlets assimilated more nitrate than the plantlets at the lowest inorganic nitrogen concentration. Quantifying the utilization of nitrate and ammonium can reveal the differences in nitrate and ammonium assimilation among plants at different inorganic nitrogen supply levels and provide an alternate way to conveniently optimize the supply of inorganic nitrogen in culture media.