Sustainable application of rare earth elements (lanthanum and cerium) to enhance growth and nutrient uptake in Paramichelia baillonii seedlings via hormonal regulation

Rare earth elements (REEs) enhance plant growth and quality in agricultural systems; however, their excessive and unregulated use raises environmental concerns due to REE accumulation in soil and water. Despite growing interest, limited research has assessed their impact on medicinal and endangered tree species such as Paramichelia baillonii, which faces population decline and low reproductive capacity. This study investigated the effects of foliar application of lanthanum (La) and cerium (Ce) at 10, 20, 40, and 80 mg L^－1 on seedling growth and physiolgical response in P. baillonii. Results showed a biphasic response, with plant growth, chlorophyll content, and nutrient acquisition improving at lower concentrations and declining at higher levels. The optimal concentration of 20 mg L^－1 La increased seedling height, stem diameter, and total dry weight by 35 %, 48 %, and 26 %, respectively, compared to the control, while Ce achieved increases of 27 %, 4 %, and 2 %. At this level, La and Ce significantly enhanced uptake of potassium, phosphorus, and nitrogen, improved root morphology, and stimulated antioxidant enzyme activity. Levels of gibberellic acid (GA₃) and indole-3-acetic acid (IAA) also peaked at 20 mg L^－1, increasing by 30.22 % and 36.00 %, respectively. In contrast, abscisic acid (ABA) rose significantly at higher concentrations, indicating stress. These findings suggest that 20 mg L^－1 of La and Ce is an environmentally sustainable threshold for promoting growth and physiological function in P. baillonii, while excessive application may pose ecological risks due to potential phytotoxicity and environmental accumulation of REEs.