Moderate drought and signal transducer induced biosynthesis of relevant secondary metabolites in thyme (Thymus vulgaris), greater celandine (Chelidonium majus) and parsley (Petroselinum crispum)

•Drought stress increases the concentration of secondary metabolites.•Short-term drought stress increases the overall contents of natural compounds.•Stress signal transducers elicit biosynthesis of plant secondary metabolites.•Drought and signal transducer applications improve the quality of medicinal plants. This study was aimed to analyze the impact of moderate drought stress and stress signal transducers (methyl jasmonate and salicylic acid) on the concentration and overall content of secondary metabolites in Thymus vulgaris (monoterpenes), Chelidonium majus (alkaloids) and Petroselinum crispum (flavones and essential oils). Short-term exposure to moderate drought stress increased concentrations as well as overall contents of the relevant secondary metabolites in T. vulgaris (+40% and +15%, respectively) and C. majus (+33% and +8%, respectively). Biosynthetic activity was clearly enhanced by the stress related over-reduced states. However, longer drought phases led to marked decreases in the overall contents of relevant natural products due to greater reductions in growth. P. crispum was very sensitive to drought, and even short-term stress caused considerable reductions in growth. SA treatments did not cause any positive effects, but the impact of MeJa applications varied depending on plant species. The application of this stress signal transducer strongly enhanced the concentrations of benzyl isoquinoline alkaloids in C. majus (+46%) and flavones in P. crispum (+70%). The main outcome was that concentrations as well as overall contents of relevant natural compounds in spice and medicinal plants could be enhanced by stress treatments, such as drought stress and signal transducer applications (MeJa and SA). Based on the results of this greenhouse study, corresponding cultivation recommendations for agricultural production of quality-enhanced spice and medicinal plants should be elaborated.