Foliar spraying nano MoO₃ effectively increases photosynthesis and nitrogen metabolism of tobacco seedlings

Objectives Using conventional molybdenum fertilizer (ammonium molybdate) as the control, this study systematically compared the regulatory effects of foliar application of molybdenum trioxide nanoparticles MoO₃ NPs) on photosynthesis and nitrogen metabolism in flue-cured tobacco seedlings, aiming to screen 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 (Mo 40, 80, 160 mg/L), and nano molybdenum trioxide (Mo 40, 80, 160 mg/L). The tobacco seedling biomass, leaf chlorophyll content, photosynthetic gas exchange parameters, the activities of key nitrogen metabolism enzymes, and the accumulation of molybdenum and nitrogen in the plants were analyzed.

Results 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) was the most effective. Under this treatment, shoot fresh and dry weights increased by 63.53% and 60.94%, respectively, compared with the control (CK). The contents of chlorophyll a, chlorophyll b, and total chlorophyll in leaves were significantly increased, providing a basis for improved photosynthesis. The net photosynthetic rate increased by 61.48%, with concurrent improvements in stomatal conductance and intercellular CO₂ concentration. This suggests that the enhancement of photosynthesis is associated with both stomatal and non-stomatal factors. Furthermore, the activities of key nitrogen metabolism enzymes—nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and ribulose-1,5-bisphosphate carboxylase (Rubisco)—were significantly enhanced, and nitrogen accumulation in plants markedly increased. This indicated that nitrogen assimilation and conversion were effectively promoted. Compared with conventional ammonium molybdate, treatments with equivalent concentrations of MoO₃ NPs consistently demonstrated superior performance across all measured indicators, highlighting the unique enhancing effects of nanomaterials. However, treatment with high-concentration(160 mg/L) MoO₃ NPs inhibited tobacco seedling growth, reducing biomass accumulation and weakening photosynthetic performance. This reflects a typical dose-dependent effect: promotion at low concentrations and inhibition at high concentrations.

Conclusions Nano-MoO₃ has a small particle size, which facilitates absorption and translocation, and enables 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.