Responses of Coix lacryma-jobi L. to Exogenous Phenolic Acid Treatments: Effects on Growth, Antioxidant Responses, and Leaf Metabolome

Phenolic acids are known allelopathic compounds that may serve as the primary cause of continuous cropping obstacles in Coix lacryma-jobi L. (Coix). However, the concentrations, types, and specific stress responses of Coix to these phenolic acids still require further investigation. In this study, the cultivar ‘Wanyi 2′ was used to examine the effects of different phenolic acids and their concentrations on the growth of Coix. Four concentrations (0 mg/L, 10 mg/L, 100 mg/L, and 1000 mg/L) and four phenolic acid types (p-hydroxybenzoic acid, salicylic acid, cinnamic acid, and ferulic acid) were used to assess their influences on plant growth, leaf physiological parameters, and metabolic pathways under greenhouse conditions. In this greenhouse pot experiment, the effects of the four phenolic acids showed a similar tendency: a low concentration (10 mg/L) tended to promote the growth and root development of Coix seedlings, whereas high concentrations (100 and 1000 mg/L) generally showed inhibitory effects. Among these phenolic acids, ferulic acid exhibited the strongest inhibitory effect at the highest concentration (1000 mg/L), while salicylic acid showed the most pronounced growth-promoting effect at low concentrations (10 mg/L). In addition, high levels of phenolic acids markedly increased antioxidant enzyme activities and oxidative stress-related substances in Coix leaves, while reducing soluble sugar (SS) and soluble protein (SP) contents. Our data suggest that under phenolic acid stress, Coix leaves exhibited changes in the metabolism of phenolic acids (e.g., 4-methoxysalicylic acid, gallic acid) and amino acids (e.g., glutathione, proline), which may be associated with the adaptive response to allelochemical-induced stress. Overall, this study provides insights that may support strategies to optimize plant growth regulators and mitigate continuous cropping barriers in Coix.