Physiological and molecular insights into nitrogen rate and planting density interactive regulation of black sesame nitrogen use efficiency, growth, yield, and seed quality

Abstract Background Optimizing nitrogen (N) application and planting density is a green and efficient agronomic strategy to increase crop yield and nitrogen use efficiency (NUE). However, the sesame responses to the interactive effects of N dose and planting density have not been fully elucidated. Here, we investigated the effects of different planting densities and N rates on the growth performances, physiological traits, N and carbon metabolism, yield components, and seed quality of two black sesame varieties (JHM and PYH). Two-year field experiments were conducted combining three planting densities (110,000, 160,000, and 330,000 plants.ha− 1) and three nitrogen rates (45, 90, and 135 kg.ha− 1). Results Analyses revealed that sesame’s response to the combined effects of planting density and N rate is developmentally regulated and varietal-specific. Notably, we found that a moderate nitrogen dose of ≤ 90 kg.ha⁻¹ coupled with dense planting of ⁓330,000 plants per hectare improved sesame growth, NUE, yield, and seed quality. This optimal planting condition significantly improved N allocation to seeds, maximized the yield of JHM and PYH to 3.47 t·ha⁻1 and 3.84 t·ha⁻1, respectively, and considerably enhanced seed oil and unsaturated fatty acid contents. AMT1 and NRT2.13 A were identified as promising candidate regulatory genes of sesame NUE and modulators of N metabolism under dense planting density and moderate N rate. Other candidate N regulatory and sucrose metabolism-related genes were also identified. Conclusions This study exposes the complexity of mechanisms underlying sesame response to dual effects of planting density and N rate, and identifies a potential optimal planting condition to improving yield per unit area and seed quality. This agronomic optimization is likely to reduce environmental risk compared with high-N regimes. However, the proposed regime is optimal for the two tested varieties and the specific soil–climate conditions studied, and it should be validated in additional germplasm and environments before broader extrapolation. .