Elucidation of inorganic nitrogen utilization characteristics in Coix lacryma-jobi L. seedlings under variable ammonium/nitrate ratios via the 15N tracer technique

The coordination between nitrogen assimilation and carbon fixation is a key determinant of crop productivity. However, for Coix lacryma-jobi L.—an underutilized cereal with high nutritional and medicinal value—the interplay between nitrogen source and photosynthetic performance remains unknown, limiting the development of precision cultivation practices. In this study, ¹⁵N-labeled ammonium chloride (¹⁵NH₄Cl, 10 atom% 15N) was employed as the tracer source for ammonium, and unlabeled sodium nitrate (NaNO3) with natural ¹⁵N abundance was used as the nitrate. Four ammonium/nitrate ratios (10/90, 30/70, 50/50, 70/30) were established under hydroponic conditions. On the basis of the 15N labeling technique, the nitrogen accumulation amount derived from ammonium and nitrate utilization could be quantified for Coix seedlings grown under different ammonium/nitrate ratios. Under the 50/50 ratio of ammonium/nitrate treatment, the Coix seedlings achieved the maximum carbon accumulation, which may be attributed to the synergistic effect of minimized energy expenditure for nitrogen assimilation (approximately 60% ammonium utilization) and a concomitant boost in photosynthetic capacity. However, excessive ammonium assimilation (> 70%) under the highest ammonium/nitrate ratio significantly reduced both photosynthetic capacity and the utilization efficiency of ammonium and nitrate in Coix seedlings. The high reductant demand from intense nitrate assimilation (> 85%) under the lowest ammonium/nitrate ratio limited photosynthetic carbon fixation, thereby reducing total carbon accumulation in Coix seedlings. The integration of the photosynthetic performance with the characteristics of inorganic nitrogen utilization contributes to revealing nitrogen metabolism efficiency and its coordination with carbon assimilation. Furthermore, quantifying the nitrogen accumulation amount derived from ammonium utilization and nitrate utilization offers insights into precision nitrogen management for Coix seedlings, contributing to sustainable agricultural practices.