The impact of reduced nitrogen application on carbon and nitrogen metabolism in two low-nitrogen-tolerant tobacco varieties

Nitrogen is an essential nutrient element for the growth and development of flue-cured tobacco. An insufficient nitrogen supply can inhibit tobacco metabolism, delay growth progression, and reduce yield. However, in actual production, excessive nitrogen fertilizer application is common, leading to potential environmental risks. Therefore, it is imperative to explore the feasibility of nitrogen reduction and its regulatory mechanisms while ensuring the yield and normal growth of flue-cured tobacco. In this study, the flue-cured tobacco varieties Longyan 101 and Longjiang 911 were used as experimental materials. Through nitrogen reduction treatment at the seedling stage combined with physiological and transcriptomic analyses, we investigated the physiological and molecular response mechanisms of flue-cured tobacco to nitrogen reduction stress. The results showed that nitrogen reduction significantly inhibited the chlorophyll content, photosystem activity, and carbon assimilation capacity in both varieties. However, under the 1/2N treatment, Longyan 101 exhibited significantly better physiological performance than Longjiang 911, with Chl a, Chl b, and Chl (a+b) showing no significant difference from those in the normal nitrogen application treatment (1N). Transcriptomic analysis revealed that under nitrogen reduction conditions, the differentially expressed genes (DEGs) between the two varieties were enriched primarily in pathways related to photosynthesis, photosynthetic antenna proteins, carbon assimilation, porphyrin and chlorophyll metabolism, and nitrogen metabolism. Further comparative analysis indicated that the extent of downregulation in the expression of genes associated with chlorophyll synthesis, photosystem activity, and carbon-nitrogen metabolism was lower in Longyan 101 than in Longjiang 911, which may underlie its stronger tolerance to low nitrogen. This study provides a theoretical basis for variety selection and regulation strategies in nitrogen-reduced cultivation of flue-cured tobacco.