Enzymatic and non-enzymatic antioxidant components of Passiflora incarnata seedlings are modulated by exogenously applied thymol

Thymol is a natural antioxidant compound that has the potential to stimulate plant development. Therefore, the present work aimed to evaluate the influence of thymol—via seed soaking, foliar spraying or their combination—on the germinability, growth, and antioxidant metabolism of the medicinal plant Passiflora incarnata L . To this end, germination and vigor of seeds, biometry of seedlings, activity of antioxidant enzymes, and content of flavonoids and total phenolic compounds were evaluated. Seed soaking in thymol-containing solution did not improve germination percentage and speed index. However, it increased the concentration of flavonoids and phenolic compounds in the seedling leaves (34.65 and 29.93%, respectively, over control plants). Foliar spraying increased the shoot dry mass and decreased the activity of superoxide dismutase—SOD, catalase—CAT, and ascorbate peroxide—APX (decreases of 37.30, 57.44, and 46.79%, respectively), suggesting that thymol can modulate both (i) the biomass allocation pattern and (ii) the antioxidant machinery in seedling leaves. The combined application of thymol (seed soaking plus foliar spray) further reduced the activity of antioxidant enzymes (total decreases of 57.84, 84.43, and 64.51% for SOD, CAT and APX, respectively), reinforcing evidence of thymol interference in the antioxidant machinery. Taking together, data indicates that thymol can influence physiology and antioxidant metabolism at early stages of the development of P. incarnata seedlings, and this influence depends on the thymol application methods.