The complexity of plant responses to hormonal treatments in vivo – A case study with basil (Ocimum basilicum L.) and marjoram (Origanum majorana L.)

Methyl jasmonate (MeJa) and salicylic acid (SA) are plant hormones with numerous functions in plant development and metabolism. The aim of this three-year field study was to investigate how they affect the accumulation of volatiles in the essential oils (EO) of basil ( Ocimum basilicum L.) and marjoram ( Origanum majorana L.). Prior to harvesting, plants were repeatedly sprayed with 0.1 mM and 2.0 mM doses of the elicitors. Growth and biomass hardly changed, except in 2020, when 2 mM MeJa induced a 10 % increase in fresh mass of basil and 2 mM SA elevated the height of marjoram by 20 %. The treatments with 2 mM MeJa significantly improved the EO content of marjoram in all three years, as well as that of basil in 2021 and 2022. SA produced only insignificant changes. In terms of EO volatile composition, the treatments produced no notable changes. In basil oil, the ratio of the main compound linalool was adversely affected by 2 mM SA during the years of the experiment. Minor compounds (eugenol, α-bulnesene, cis-γ-cadinene, iso -borneol) changed in some cases but not uniformly throughout the years. In marjoram EO, cis -sabinene hydrate was decreased due to the SA treatments in two of the experimental years, with a parallel increase of terpinene-4-ol ratios. The overall ratios of sabinene and terpinene type compounds, however, were not significantly affected by any of the treatments. The study demonstrated that besides the type and dosage of the elicitors, the actual stress status and growth behaviour of the plants might heavily influence the results. Comprehensive studies under standardized cultivation practices could deliver reliable results. In separate file • Methyl jasmonate and salicylic acid had no significant effect on herbal yield. • 2 mM MeJa elevated the essential oil content in most cases in both species. • Quantitative changes in composition did not influence the quality of the oils. • In vivo elicitation may provide advanced herb quality for the food industry.