Seed treatments with essential oils protect radish seedlings against drought

Establishment of seedlings of economic crops is often reduced if there is not a steady supply of water. Essential oils (EO) from plants are increasingly used instead of synthetic chemicals to protect plant and animal products against biotic and abiotic stresses. We investigated priming radish seeds by soaking or by matriconditioning with synthetic or natural compounds as a means of inducing resistance to drought stress, thus maintaining crop yield. Priming radish seeds for two hours in solutions of essential oils (EO) thymol and carvacrol derived from Origanum syriacum, with “oregano natural product” (ONP; a solution of the residue remaining after EO extraction), or with the gibberellin synthesis inhibitor trinexapac ethyl (TE), was much more effective in inducing drought resistance than was matriconditioning with the same compounds in sawdust for two days. The latter treatment induced considerable fungal and bacterial infection in treated seeds if the substrate-matrix was not heat-treated beforehand. The increase in specific leaf area in plants from treated seeds was mostly consistent with an increase in leaf water content. Seed treatments with EO, ONP, and especially TE led to a three-fold increase in radish seedling survival compared with water-treated controls, when 21 day-old seedlings were irrigated after 6 days of drought. Under drought conditions, seedlings from treated seeds had a 2–3-fold increase in relative water content increased 2–3-fold, while membrane permeability decreased 20–50-fold as a result of the treatments. However, the physical benefits of the treatments often did not correlate with treatment-induced increases in physiological parameters such as pigments (chlorophyll, carotenoid, anthocyanin), pigment ratios (chlorophyll a/b, carotenoid/chlorophyll), or antioxidant activity. Seed treatments with biostimulants can be as effective as treatments with synthetic compounds in inducing drought resistance in seedlings.