Intraplant and Interspecific Antioxidant Interactions in Origanum vulgare and Mentha aquatica

The antioxidant activity of Origanum vulgare L. and Mentha aquatica L. has been widely reported; however, interaction effects within and between different plant parts remain insufficiently characterized. This study aimed to evaluate the antioxidant behavior of methanolic extracts from leaves, flowers, and rhizomes of both species and to assess the nature of intraplant and interspecific interactions using combination analysis. Antioxidant activity was determined for individual extracts and their binary mixtures using DPPH and ABTS radical scavenging assays. Phytochemical analysis was performed by LC-MS/MS. In O. vulgare, all intraplant mixtures exhibited synergistic effects, suggesting complementary contributions of phenolic acids and flavonoids across plant organs. In contrast, M. aquatica showed more variable responses, with additive to antagonistic interactions, particularly in combinations involving rhizomes with lower phenolic content. Interspecific mixtures further demonstrated that interaction outcomes depended on the qualitative and quantitative composition of phytochemicals: leaf mixtures showed synergism, whereas flower and rhizomes mixtures tended toward antagonism. Comparable interaction trends were observed in both radical scavenging assays. These results indicate that antioxidant activity in plant mixtures is not simply additive but is strongly influenced by phytochemical composition and plant part, highlighting the importance of empirical testing when designing multicomponent plant-based antioxidant formulations.