Improvement of the quality in hydroponically grown fresh aromatic herbs by inducing mild salinity stress is species-specific

Profitable hydroponic production requires high quality fresh water, which is often not available for agricultural use, while desalinisation of salty water is an expensive and unsustainable technology. In the present study, we assessed the effect of mild salinity stress during the soilless cultivation of fresh peppermint and spearmint in the floating system on biomass yield, produce quality and plant secondary metabolite content. Peppermint and spearmint plants were grown for 25 days on a nutrient solution (NS) supplemented with three different NaCl concentrations (0 mM, 10 mM or 20 mM NaCl). The plant height, root length, fresh and dry weight were recorded and composition was determined on fresh tissue. The composition of essential oil was determined upon hydrodistillation and that of polyphenolic compounds by targeted ultra-performance liquid chromatography coupled with mass spectrometer (UPLC-MS/MS). Plant growth was not suspended by the addition of NaCl in the NS, except for the plant height at the highest salinity level. In peppermint, the nutritional composition was not affected by the salinity, whereas it was significantly improved in spearmint as confirmed by the nitrate content decrease and the total antioxidant capacity, total soluble phenol, total carotenoid and essential oil content increases. Simultaneously, no effect of the salinity on essential oil or polyphenolic composition in both plants was induced. In conclusion, peppermint and spearmint production is feasible in the floating system even under mild salinity conditions, without negatively affecting either the crop yield or the plant's essential oil or phenolic composition. Indeed, low salinity levels improved the nutritional composition of spearmint plants.