LED Light Treatments Induce Activation of the Antioxidant Defense System in Thymus mastichina L.

This study investigated how different spectral ranges of LED light affect the synthesis of photosynthetic pigments and antioxidant systems in Thymus mastichina L., focusing on two ecotypes with distinct chemotypes: linalool and eucalyptol. The ecotypes were exposed to white, red, blue, red-blue (70:30), white-blue, or white-red light for 30 days under a 16/8 h light/dark cycle (115 μmol/m²s). Photosynthetic pigment content, lipid oxidative damage, antioxidant capacities, and both enzymatic (SOD, CAT) and non-enzymatic (tocopherols and polyphenols) antioxidant systems were assessed. For the linalool chemotype, red-blue light significantly increased carotenoid content, antioxidant capacity, and catalase activity, while elevating levels of plastochromanol-8 and phenolic compounds such as salvianolic acid B, rosmarinic acid, and 6-OH-apigenin-7-hexoside, thereby reducing oxidative stress. In contrast, for the eucalyptol chemotype, pure red light produced the most significant enhancements in carotenoid synthesis and antioxidant defenses, substantial increases in key compounds such as salvianic, neochlorogenic, rosmarinic, and lithospermic acids, and salvianolic acids E and B, and higher levels of plastochromanol-8. Additionally, both SOD and CAT activities increased, providing greater protection against lipid oxidation. These findings highlight the importance of customizing light treatments not only based on plant species but also according to chemotype to obtain optimal biochemical and physiological outcomes.