Nitrogen use efficiency of flue−cured tobacco genotypes: physiological basis and relative contributions of nitrogen uptake and utilization

Understanding the physiological processes that regulate nitrogen uptake efficiency (NUpE), nitrogen utilization efficiency (NUtE), and nitrogen use efficiency (NUE) in crops is essential for developing nitrogen-efficient varieties. We conducted a two−year field study (2021–2022, Shixing, southern China) comparing three flue−cured tobacco genotypes (Yunyan 87, Yueyan 97, K326) under two nitrogen application rates: traditional (150 kg ha -1 ) and reduced (105 kg ha -1 , −30%). Across both rates, the high−NUpE genotypes (Yueyan 97, K326) showed substantially greater root biomass, length, surface area, volume, vigor, bleeding sap and nitrate flow rates, and higher activities of key N−metabolism enzymes than low−NUpE Yunyan 87. These root and physiological traits were positively correlated with nitrogen uptake efficiency, indicating that they are major correlates of NUpE and may contribute to its variation. Compared with Yunyan 87 and K326 (both low−NUtE genotypes), Yueyan 97 (a high−NUtE genotype) exhibited a significantly lower respiratory rate despite a lower net photosynthetic rate. This pattern is consistent with higher NUtE being associated with a balance between net photosynthesis and respiration that favors reduced respiratory consumption. Path analysis indicated strong conditional associations of both NUpE and NUtE with NUE across genotypes. While path coefficients do not imply causality, the results suggest that jointly improving NUpE and NUtE may be a promising avenue for achieving high yield and improved NUE in flue-cured tobacco. In conclusion, this study identifies physiological traits that are strongly associated with NUpE and NUtE in flue-cured tobacco and provides insights to guide future efforts aimed at enhancing NUE in this crop.