Application of Salicylic Acid Improves the Production of Medicinal Components in Mucuna macrocarpa Wall by Regulating Endogenous Hormone and Nutrient Balance

The <i>Mucuna macrocarpa</i> Wall, a traditional Chinese medicinal plant, exhibits significant cultivation-dependent variations in the accumulation and yield of its medicinal components. Salicylic acid (SA) has demonstrated the potential to regulate plant growth, which can be strategically used to enhance medicinal yield, offering a promising approach for high-yield cultivation in medicinal plants. This study aimed to investigate the changes in the medicinal components of <i>Mucuna macrocarpa</i> seedlings (4 months old) at different concentrations of SA (0, 0.1, 0.5, 0.9, and 1.3 mM) in a pot experiment. The results indicate that SA significantly increased the basal diameter (BD) by 2.9% to 20.61% and the total biomass (TB) by 14.28% to 48.57%. Notably, SA treatments resulted in alterations in the endogenous hormone content, including indole-3-acetic acid (IAA), abscisic acid (ABA), gibberellin A₃ (GA₃), and trans-zeatin-riboside (ZR), and the balance in leaves. SA regulated the content and balance of nitrogen (N), phosphorus (P), and potassium (K) in all organs, and K content and K: P in roots, stems, and leaves was significantly higher than that of the control under 0.9 mM SA treatment. Crucially, SA significantly enhanced the content of bioactive compounds. The total phenolic content (TPC) and total flavonoid content (TFC) in stems peaked at 0.9 mM (14.89 mg·g^-1 and 3.73 mg·g^-1, respectively), which were 11.87% and 11.68% higher than those in the control. Moreover, compared to the control, SA treatments increased total phenolic production by 20.00% to 61.45% and total flavonoid production by 3.89% to 90.56%. In addition, 0.9 mM SA was found to be more effective than other treatments for increasing total phenolic and d total flavonoid content. In summary, this study investigated the effect of SA as an eco-friendly elicitor to improve the total phenolic and total flavonoid production of <i>Mucuna macrocarpa</i>.