Lanthanum nitrate enhances aroma precursors of tobacco leaves via influencing chemical components and photosynthetic characteristics

Aroma precursors are key biochemical factors determining the aroma quality of flue-cured tobacco (Nicotiana tabacum L.). While rare earth elements (REEs) are known to enhance tobacco growth, their role in the biosynthesis of aroma precursors and overall quality remains insufficiently understood. In this study, a controlled pot experiment was conducted using flue-cured tobacco treated with a gradient of lanthanum nitrate [La(NO₃)₃] solutions (0, 25, 50, 90, 130, and 170 mg L⁻¹) applied via foliar spraying to systematically evaluate dose-dependent effects. The impacts of these treatments on aroma precursors, photosynthetic characteristics, chemical components, and agronomic traits were assessed. A hormetic concentration-dependent response showed: 25-90 mg L⁻¹ lanthanum nitrate significantly increased the content of polyphenols, plastid pigments, and chemical components, while improving photosynthetic efficiency and promoting shoot and root growth. However, supraoptimal concentrations above 130 mg L^{- 1} exhibited inhibitory effects, significantly reducing growth parameters, polyphenol content, plastid pigment accumulation, and photosynthetic efficiency, particularly at 170 mg L^{- 1}. Partial least squares path modeling (PLSPM) revealed that the promotion of aroma precursor biosynthesis was indirectly mediated through improvements in chemical components and photosynthetic characteristics. Based on comprehensive optimization, foliar application of 90 mg L^{- 1} lanthanum nitrate is proposed as the optimal concentration, achieving 33.16% and 22.20% enhancements in total phenol and total chlorophyll content, respectively, while maintaining an optimal growth-physiology balance. These findings provide novel insights into REE-mediated quality improvement mechanisms and establish a scientific basis for the precision application of lanthanum in premium tobacco production. Future studies need to employ molecular techniques such as transcriptomics and metabolomics to further elucidate how rare earth elements influence biosynthetic pathways associated with phenolic and carotenoids metabolism.