Objectives To explore the application potential of nano-molybdenum fertilizer in tobacco production, this study systematically compared the regulatory effects of nano-molybdenum trioxide (MoO₃ NPs) on photosynthesis and nitrogen metabolism in flue-cured tobacco seedlings, using traditional molybdenum fertilizer (ammonium molybdate) as a control. The aim was to identify an efficient molybdenum source and determine its suitable application concentration.

Methods Using 'Yunyan 87' as the test material, a hydroponic experiment was conducted with seven treatments established: a control (CK), ammonium molybdate (40, 80, 160 mg Mo/L), and nano molybdenum trioxide (40, 80, 160 mg Mo/L). The tobacco seedling biomass, leaf chlorophyll content, photosynthetic gas exchange parameters, activities of key nitrogen metabolism enzymes, and the accumulation of molybdenum and nitrogen in the plants were analyzed.

Results The results indicated that foliar application of MoO₃ NPs significantly increased seedling biomass, chlorophyll content, net photosynthetic rate (Pn), and the activities of key nitrogen metabolism enzymes. Among all treatments, 40 mg/L MoO₃ NPs (NMo40) showed the best effect. Under this treatment, the fresh weight and dry weight of the shoot of the seedlings increased by 63.53% and 60.94%, respectively, compared to the control (CK). The contents of chlorophyll a, chlorophyll b, and total chlorophyll in the leaves were significantly increased, establishing a material foundation for improved photosynthesis. The net photosynthetic rate rose by 61.48%, with concurrent improvements in stomatal conductance and intercellular CO₂ concentration. This suggests that the enhancement in photosynthesis is associated with both stomatal and non-stomatal factors. Furthermore, the activity of key nitrogen metabolism enzymes—nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and ribulose-1,5-bisphosphate carboxylase (Rubisco)—was significantly activated, and nitrogen accumulation in the plants markedly increased. This indicates that nitrogen assimilation and conversion processes were effectively promoted. Compared to traditional ammonium molybdate, treatments with equal concentrations of MoO₃ NPs consistently demonstrated superior performance across all measured indicators, highlighting the unique enhancement effect characteristic of nanomaterials. However, high-concentration (160 mg/L) MoO₃ NPs treatment inhibited tobacco seedling growth, leading to reduced biomass accumulation and weakened photosynthetic performance. This reflects a typical dose-dependent response of "low-concentration promotion and high-concentration inhibition."

Conclusions Nano-MoO₃ has a small particle size, which facilitates absorption and translocation, enabling more efficient allocation of molybdenum to underground plant parts. Consequently, at the same equivalent concentration, it can more significantly increase leaf chlorophyll content, enhance photosynthetic efficiency, elevate the activities of key enzymes such as nitrate reductase (NR), and synergistically promote dry matter accumulation and nitrogen assimilation compared with conventional molybdenum fertilizers. However, excessively high foliar application concentrations (e.g., 160 mg/L) may cause stomatal blockage, trigger reactive oxygen species accumulation, and inhibit photosynthesis as well as root growth. In tobacco production, when replacing ammonium molybdate with nano-molybdenum fertilizer, particular attention should be paid to controlling the foliar spray concentration.