Seed Priming with Cold Plasma and Vacuum Increases the Amounts of Phenolic Compounds and Antioxidant Activity in Lavender Herb

Seed processing effects induced by two types of cold plasma, CP (low-pressure plasma, LCP; dielectric barrier discharge plasma, DBD), and vacuum (V) treatments were compared by estimating changes in the emergence and growth of lavender seedlings, the density of leaf trichomes, and the biochemical composition of leaf extracts, including the content of photosynthetic pigments, TPC, antioxidant activity, and the amounts of two hydroxycinnamic acids associated with antioxidant activity—chlorogenic and rosmarinic acid. DBD treatment for 3 min stimulated the emergence and growth of seedlings but induced negative or neutral effects on biochemical parameters. All treatments, except DBD3, increased the density of glandular trichomes in leaves. Short-term treatments with LCP (0.5 min), DBD (2 min), and V (2 min) increased the total phenolic compound (TPC) content by 15–25%, and the first two treatments enhanced antioxidant activity (21–32%). HPLC analysis revealed that V (2 min) treatment was the most effective, increasing the content of chlorogenic (49%) and rosmarinic (14%) acid. LCP (1 min) and DBD (2 min) treatments increased chlorogenic acid content by 9% and 26%, respectively. The obtained results support the potential of pre-sowing seed treatments with CP and vacuum to produce raw lavender material enriched by biologically active compounds for pharmaceutical and nutraceutical applications.