Phytotoxicity of L-Menthol-Based Hydrophobic Deep Eutectic Solvents

Hydrophobic deep eutectic solvents (HDES) have emerged as promising green alternatives to conventional organic solvents, yet their environmental safety remains largely unexplored. This study investigates the phytotoxic effect of five L-menthol-based HDES-prepared with caprylic acid, oleic acid, lauric acid, (R)-(+)-limonene, and (-)-α-pinene-on wheat seeds (Triticum aestivum L.). The individual components and common organic solvents (dimethyl sulfoxide, pyridine, methylene chloride) were also evaluated for comparison. Germination assays revealed that several HDES, particularly those containing oleic acid, limonene, and pinene, exhibited lower phytotoxicity than L-menthol alone, with germination percentages comparable to control at 0.1 and 0.05 mg/mL. Most treatments significantly increased lipid peroxidation levels, indicating oxidative stress. Additionally, elevated phenylalanine ammonia-lyase and guaiacol peroxidase activities, along with total phenolic content, suggested activation of antioxidant defense mechanisms. Notably, some HDES induced metabolic shifts without affecting total phenolic content, implying potential redirection of phenylpropanoid metabolism. These findings provide novel insight into the phytotoxic profile of HDES and highlight the importance of assessing their environmental safety prior to agricultural or industrial use.