Effects of Nitrate and Ammonium on Growth and Nitrogen Utilization of Wasabi (Eutrema japonicum) Using 15N Tracers

Nitrogen (N) form is a key factor influencing plant growth, N utilization efficiency (NUE), and sustainable crop production. Information on N-source effects on N uptake, allocation, and growth of ‘Daruma’ wasabi remains limited. This study evaluated the effects of nitrate (NO3−), ammonium (NH4+), and their combination on plant growth, N uptake and allocation, NUE, and selected free amino acid concentrations using a 15N tracer approach in a sterilized sand culture system. Five N treatments were applied: N-free solution; 2.5 mM 15NO3− + 2.5 mM NH4+; 2.5 mM NO3− + 2.5 mM 15NH4+; 5.0 mM 15NO3−; and 5.0 mM 15NH4+. Measurements were conducted at 90 and 180 days after treatment initiation. Plants supplied with NO3− alone or combined with NH4+ showed greater growth and biomass than those under sole NH4+ or N-free conditions. Total plant N increased with NO3− supply. Isotopic analysis showed preferential allocation of NO3−-derived N to leaves and petioles, whereas NH4+-derived N was largely retained in the rhizome. At 90 DAIT, sole NO3− supply showed the highest 15NUE, whereas at 180 DAIT, the NO3−-labelled fractions under sole and mixed-N supply showed the highest 15NUE. NO3− supply was also associated with higher concentrations of selected free amino acids. The 50:50 NO3−/NH4+ supply may therefore support NUE and stable wasabi growth under the present conditions.