Gallic acid as a phytostimulant enhancing yield and quality of Mentha spicata L. under deficit- and well-watered conditions

Here, we propose gallic acid (GA) as a phytostimulant for Mentha spicata subsp. spicata L. (spearmint), an aromatic herb of great global economic importance. Our evidence would suggest the potential of GA in promoting the acclimation of spearmint to drought stress (DS), also improving yield and quality of its essential oil (EO). We observed that GA and DS did not induce significant morphometric (e.g., stem length) and physiological (e.g., redox state, water content) modifications, indicating an innate tolerance of M. spicata to both these factors. However, spectrophotometric, HPLC-DAD and GC-MS analyses revealed that GA tended to induce the accumulation of some phytochemicals (e.g., terpenoids), mainly responsible for fragrance, quality and bioactivity of EO. Additionally, plants exposed to GA increased the number of peltate glandular trichomes and their content in mono- and sesqui-terpenes. Finally, we demonstrated that DS determined a reduction of spearmint EO yield, while GA significantly favoured it, especially at low doses, both in deficit- and well-watered conditions. Since an increase of GA concentration in roots and/or leaves of the treated plants was not observed, we hypothesised that the proposed phytostimulant exerted its activity indirectly, that is by modulating rhizobiota and/or soil structure and features or triggering signalling transduction pathways starting from the rhizodermis.