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

Soil salinisation is one of the detrimental effects of climate change, and it is often combined with low quality water during irrigation and poor soil drainage, especially in semi-arid regions all over the world (Arslan et al., 2018; Rezaie et al., 2019). Moreover, fresh groundwater is slowly converted into saline water and thus rendered unsuitable for agricultural use, due to excessive pumping for irrigation use followed by seawater fusion, waterlogging and nutrient leaching (Alaghmand et al., 2013; Pulido-Bosch et al., 2018). These factors have triggered growers’ willingness to shift towards hydroponic vegetable production (Niu et al., 2018), which however requires the use of optimum quality water that is unfortunately not always available for agricultural activities (Niu et al., 2018; Yousefi et al., 2020). Deterioration of groundwater quality often imposes the need for establishment of desalinisation water treatment technologies, such as the reverse osmosis technique, which not only are substantially expensive but also simultaneously convert only part of the source water into water suitable for irrigation and reject the remaining, which typically amounts to around 60%, due to extreme salinity loading (Arslan et al., 2018; Niu et al., 2018; Pulido-Bosch et al., 2018; Atzori et al., 2019).

Using low quality water with high electrical conductivity (EC) carrying a large amount of salts such as sodium chloride (NaCl) causes the absorption and over-accumulation of sodium and chloride ions in tissues, which subsequently induce severe salt toxicity, tissue dehydration, nutrient imbalance and oxidative stress (Munns, 1993; Lazof et al., 1998). On the other hand, it has been demonstrated that modification of the EC in the nutrient solution (NS) has efficiently improved the quality of some vegetables, and such a practice has already been recommended for use in agricultural production (Rouphael et al., 2012). Furthermore, salinity stress may even induce secondary metabolism in plants and, consequently, the accumulation of human health promoting secondary metabolites, such as terpenes and polyphenolic compounds (Tarchoune et al., 2013; Tounekti et al., 2011; Verma and Shukla, 2015; Thakur et al., 2019).

The floating tray system exhibits many advantages in comparison to other hydroponic systems, such as low installation cost, simplicity and functionality of operation, high water and nutrient use efficiency, accelerated cultivation cycles and eventually total absence of soil residues from the harvested produce (Rodríguez-Hidalgo et al., 2010). Several aromatic plants of the Lamiaceae family are suitable to be grown in this hydroponic system, given that there is a significant demand for a high volume of water in order to increase crop yield (Peter, 2001). Peppermint (Mentha × piperita L.) and spearmint (Mentha spicata L.) are two perennial herbs of high economic value, from which a combination of uses can be obtained, including in cooking, phytomedicine and food industry (Charles, 2013). They are consumed fresh or dry and are rich in phytonutrients, such as vitamins, minerals and natural antioxidants (Peter, 2001; Charles, 2013).

Spearmint cultivation using the deep flow technique has already been successfully tested (Vimolmangkang et al., 2010; Chrysargyris et al., 2017); however, there is no available literature relevant to peppermint production. Moreover, the response of these two species to salinity has only been studied under extreme treatments with high salt concentration.

Therefore, the aim of this study was to investigate the feasibility of a commercial soilless production of peppermint and spearmint on the floating trays system, as well as to assess the possibility of adapting mild salinity conditions in the NS of a non-recirculating closed hydroponic system as a tool to improve the quality and nutritional value of fresh peppermint and spearmint plants.

In the floating system, as water and nutrient uptake is a continuous function during plant growth and development, an NS depletion as well as a nutrient imbalance may be exhibited due to the absorption of specific ions at a higher rate than others, thus causing a modification of both EC and pH of the solution. A rapid increase in EC may be observed when low quality saline water is used, especially in arid and semi-arid areas, as non-essential ions as Na⁺ and Cl⁻ accumulate in the NS (Urrestarazu and García, 2000). Changes in the pH of the NS are also induced, as roots release H⁺ and HCO₃⁻ ions during nutrient uptake (Hinsinger et al., 2003). In the present study, the rapid increase of EC and the changes in the pH of the solutions cannot be attributed solely to the presence of NaCl, as the control solution followed a similar trend, and therefore may also be attributed to the NS depletion due to evapotranspiration and the change of ionic ratios (Urrestarazu and García, 2000).

Extreme levels of salinity impose a harmful effect on plant growth; the most common morphological, physiological and biochemical parameters that indicate salinity stress are stunted growth, water stress, disruption of metabolic processes and ion uptake, high respiration, synthesis of osmoregulators, disruption of photosynthesis, ion imbalance and inhibition of enzyme activity (Estaji et al., 2018). Although a decrease in plant height was observed in both peppermint and spearmint plants and in root length of spearmint, the fresh and dry biomass and the total soluble solids content were not affected by the presence of NaCl in the NS, possibly because of the low levels of salinity and the short period of cultivation. The hydroponic production of herbs in NS of medium to high salinity (20–130 mM NaCl) has shown various responses in plant growth, depending on the species or cultivar, the level of salinity, the type of salt and the period of abiotic stress. Indeed, a negative effect on plant growth was observed in herbs, such as basil, marjoram, mint timija, sage, lavender and mint, grown either hydroponically under salinity or in pots and irrigated with NS with increased EC due to NaCl presence (Bernstein et al., 2010; Jelali et al., 2011; Taârit et al., 2011; Tarchoune et al., 2013; Kasrati et al., 2014; Yu et al., 2015; García-Caparrós et al., 2017). The decrease in biomass and yield in plants treated with high amounts of NaCl may be attributed to stomatal limitations, which in turn restrict carbon dioxide (CO₂) diffusion into the leaf and induce further osmotic and salt-specific effects, as reported by Noecleous et al. (2017) in melon (Cucumis melo L.) plants grown hydroponically in recirculating NS.

Although the chlorophyll content may be used as an indicator of salt stress, it is affected differently depending on the plant sensitivity and may either decrease or remain unaffected in sensitive species and even increase in tolerant ones (Jungklang, 2003; Lee et al., 2004; Qiu and Lu, 2003). Similar to our results, the chlorophyll content in basil plants was not affected by salinity up to 130 mmol · L⁻¹ NaCl, implying that it is not a reliable indicator of salinity stress for these species.

One quality trait of high importance in green leafy herbaceous plants is their nitrate content. The accumulation of nitrates has triggered the attention of the medical society, because it has been associated with severe human health risks and many countries have adopted regulations setting the maximum accepted nitrate levels in many species produced for human consumption (Santamaria, 2006). The decrease in nitrate content that was observed in spearmint plants may be attributed to the competition between nitrate and chloride ions for the same transporter in the root system (Blom-Zandstra and Lampe, 1983). A similar decrease in nitrate content has been observed in celery grown hydroponically under high saline conditions (50–300 mmol · L⁻¹ NaCl) (Pardossi et al., 1999), as well as in lettuce grown in a floating hydroponic system after addition of 5,000–10,000 mmol · L⁻¹ CaCl₂ in the NS (Borghesi et al., 2013).

Salinity stress may also have a positive or negative impact in the production of secondary metabolites in plants (Verma and Shukla, 2015; Thakur et al, 2019) and may improve their antioxidant capacity by increasing the synthesis of compounds that act as scavengers, protecting the cells against reactive oxygen species (Dat et al., 2000). Several secondary metabolites, such as carotenoids, polyphenols and essential oil components, are important antioxidants in herbs (Wojdyło et al., 2007; Riachi and De Maria, 2015). Irrespective of the NaCl concentration in the NSs, the main constituents in the essential oil of peppermint were menthone (55%), menthol (15%), menthofuran (6.5%) and isomenthone (6%) (Table 3) and in the spearmint were carvone (40%), limonene (17.5%), 1–8 cineole (16.5%) and myrcene (4.5%). Higher contents of carvone (50–57%) and lower of limonene (7–10%) were found in the aerial parts of spearmint plants that were collected from naturally growing populations (Chauhan et al., 2010; Zekri et al., 2019; Farahbakhsh et al., 2021), which may be explained by different genotypes, location or agronomic conditions, such as harvesting time, plant age, crop density, as well as growing substrate (soil against soilless NS). Rosmarinic acid and hesperidine were the main phenolic components in both peppermint and spearmint plants, which is in line with the findings of a more recent study in Iran (Farahbakhsh et al., 2021), which, too, observed significant antioxidant activity.

In our study, an increase in the above antioxidant components was observed only in spearmint grown in NS containing 20 mmol · L⁻¹ NaCl. Similar results have been observed in the carotenoid content of basil plants grown under different levels of saline conditions (Bernstein et al., 2010), as well as in the total phenolic content of marjoram and sage and the total antioxidant capacity of the latter (Jelali et al., 2011; Taârit et al., 2012), where the content of these components increased relatively while increasing the levels of salinity. Phenolics content and antioxidant capacity in lettuce plants grown in a floating hydroponic system and harvested 5 weeks after sowing were favoured by the moderate salt conditions induced after CaCl₂ supplement in the NS (Borghesi et al., 2013).

High salinity may also influence individual polyphenolics’ composition, as observed in rosemary plants grown under NSs where various salts had been added, but in the NaCl treatment no differences were found in phenolic diterpene concentrations (Tounekti et al., 2011), which is similar to the findings of the present study. Many researchers have reported contradictory results regarding variations in the essential oil content and its constituents in herbs grown under saline conditions. In less resistant species, like the Canadian mint and mint timija, a decrease in the essential oil content was observed when grown in high salinity of >50 mmol · L⁻¹ NaCl (Kasrati et al., 2014; Yu et al., 2015). However, in more resistant species, such as marjoram, sage and basil, mild salinity stress may induce the production of essential oil until a maximum limit of salinity level and beyond that a decrease was observed (Bernstein et al., 2010; Jelali et al., 2011; Taârit et al., 2011). The type of salt also constitutes an important factor, as the essential oil content of basil plants decreased while grown in NS containing 25 mmol · L⁻¹ Na₂SO₄ and increased while grown in 50 mmol · L⁻¹ NaCl (Tarchoune et al., 2013). Changes in the essential oil content were often accompanied by a simultaneous modification in the relative concentration of the essential oil constituents, resulting in a significant effect on the oil's compositional profile.

In conclusion, peppermint and spearmint are suitable for a sustainable environmentally friendly soilless production in the floating system under mild salinity conditions (10 mmol · L⁻¹ to 20 mmol · L⁻¹ NaCl), without any adverse effects on the fresh biomass yield, the nutritional characteristics or the essential oil content and composition. Indeed, spearmint benefits from a slight salinity stress increase in the NS, as the nitrate content decreased and the secondary metabolites increased. Both species may be grown in greenhouse production units that do not have access to high quality water. The essential oil and the phenolic composition of both peppermint and spearmint were not affected, indicating that these two species may profitably be grown even under 20 mmol · L⁻¹ NaCl without suffering from oxidative stress or from risking any market value loss.