**1. Introduction**

The world population has peaked at eight billion people, and recent estimates project an increase of more than two billion people by 2050. In the coming decades, the imperative for community policies will be to ensure food security, a daunting challenge when we consider that it is mainly dependent on agricultural production [1]. Currently, the agricultural sector is managing to bridge the gap between demand and production, but demographic growth will put further pressure on the entire sector. As if that were not enough, changing climate scenarios have thrown even more fuel on the fire, endangering the food supply for future generations.

The increase in the frequency and intensity of drought has led to an increase in desertification with consequent soil salinization, a concern that should not be underestimated considering that horticultural crops are widespread in regions with high levels of water salinity [2–4]. In an era of climate change, salinity is undoubtedly a constraining factor in agricultural cultivation [5]. More than 20% of irrigated agricultural soils have high levels of salinity, resulting from the natural erosive process (i.e., primary salinity) but mainly from anthropogenic activities (i.e., secondary salinity) such as intensifying agricultural practices, deforestation, and irrigation, promoting seawater infiltration [6]. If not remedied, negative salinity impacts will affect 50% of the world's agricultural land by 2050. NaCl salinity causes a rapid osmotic effect that has drastic consequences on plant water and nutrient availability, photosynthetic and transpiration rates, stomatal regulation and control mechanisms, and root functional activities [7–11]. If the stress continues, morphological

**Citation:** Ciriello, M.; Formisano, L.; Kyriacou, M.C.; Carillo, P.; Scognamiglio, L.; De Pascale, S.; Rouphael, Y. Morpho-Physiological and Biochemical Responses of Hydroponically Grown Basil Cultivars to Salt Stress. *Antioxidants* **2022**, *11*, 2207. https://doi.org/ 10.3390/antiox11112207

Academic Editor: Nafees A. Khan

Received: 9 October 2022 Accepted: 5 November 2022 Published: 8 November 2022

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and physiological changes could occur, leading to reduced yields. Taking into account the long-term economic unsustainability of water and soil desalination processes and the low availability of salt-tolerant genotypes, finding an appropriate use for salinized areas is one of the biggest challenges. One possible solution could be to allocate such regions to the cultivation of medicinal plants, appreciated in gastronomic, pharmaceutical, medical, and cosmetic fields due to their richness in secondary metabolites. In fact, although salt stress leads to the production of reactive oxygen species (ROS), which limits production performance, it prompts the plant to activate defensive mechanisms that culminate in the bioaccumulation of phenolic molecules with an antioxidant function. As observed by Valifard et al. [12] on *Salvia mirzayanii* and *Salvia acrosiphon* and by Perin et al. [13] on *Fragaria ananassa*, the increase in polyphenols in response to salt stress is positively related to the up-regulation of phenylalanine ammonia-lyase (PAL) enzyme activity. Although secondary metabolites do not play a specific role in growth processes in medicinal plants, these valuable species-specific molecules increase under stress conditions, improving plant quality traits. The growing interest in products of natural origin that are readily available and have no side effects has increased the demand for medicinal plants because of their beneficial antimicrobial and anti-inflammatory properties, making them inexpensive and renewable "ingredients" for producing natural preservatives, new types of drugs, and cosmetics [14,15].

Currently, in developing countries, about 80% of medicines are of plant origin, while in developed countries, the proportion is only 25%. Among medicinal plants, basil (*Ocimum basilicum* L.), which is exceptionally rich in essential oils, is among the most widespread and well known worldwide, so much so that it has earned the nickname "King of Herbs", with the existence of at least 18 cultivars selected and developed over the years [16]. Its fame is mainly attributable to its gastronomic role as tender and fragrant leaves, critical ingredients of tasty dishes typical of Italian gastronomic tradition [17]. Not surprisingly, as with other plants belonging to the Lamiaceae family, basil, in addition to its recognized culinary aptitude, is cultivated for its secondary metabolites. Most studies have focused primarily on Genovese basil's productive and sensory characteristics without considering the genetic diversity typical of the genus Ocimum. Over time, many basil cultivars that have genetically distinct phytochemical profiles have been selected for their shape, color, aroma, and flavor. Basil contains relatively high concentrations of carotenoids (lutein and *β*carotene) and polyphenols, which belong mainly to flavonol-glycoside classes (rutin, quercetin, and di-hydroquercetin) and phenolic acids (rosmarinic, chicoric, caffeic, chlorogenic, kaftaric, salvianic A, salvianolinic A, L and K acids), which, in addition to acting as stress mitigators, are beneficial for human health. However, although Genovese basil cultivated in soil is known to be tolerant of salinity (up to 100 mM NaCl) under, few studies have evaluated the effects of salt stress on the polyphenolic profile of non-Genovese basil for the pharmaceutical and cosmetic industries. The few contributions available in the literature have mainly focused on evaluating the effects of salinity on the yield and quality of Genovese basil while neglecting other non-Genovese types belonging to the genus Ocimum. Our work was aimed at characterizing three types of basil (*Ocimium basilicum* var thyrsiflora, *Ocimum basilicum* cv Cinnamon, and *Ocimum* × *Citriodorum*) under salt stress, both from a yield and phytochemical point of view. We evaluated the yield, morphophysiological response, antioxidant activity [FRAP (ferric ion reducing antioxidant power), DPPH (1,1-diphenyl-2-picrylhydrazyl), and ABTS (2,2 -azinobis- (3-ethylbenzothiazoline-6-sulfonate)], mineral profile (by ion chromatography) and phenolic profile (by ultrahigh performance liquid chromatography).
