**1. Introduction**

Salinity is among the major abiotic stresses impacting plant growth and productivity [1,2]. It affects approximately 1125 million hectares of agricultural land globally [3]. In China, salinity affects about 36.7 million hectares of land. By 2050, it could damage more than 50% of the agricultural land [4]. The intensity of the salt stress affects the plants' morphological, physiological, and metabolic changes. Soil salinity can inhibit plant growth by causing ion toxicity, osmotic and oxidative stresses, pigment degradation, and photosynthesis inhibition [5–7]. Ion toxicity and osmotic stress cause nutritional imbalances and oxidative stress by restricting plants from extracting water from the soil and from inside the plants themselves [8,9]. Additionally, salt stress causes oxidative stress by increasing reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), superoxide (O2 *•*−), and hydroxyl radicals (OH*·* ) [10,11]. Salt stress can severely disrupt the equilibrium between producing and scavenging reactive oxygen species (ROS) [12]. Plants require a certain threshold level of reactive oxygen species (ROS) to function normally; any variation in the ROS concentration can have detrimental effects on a plant's physiology [13]. Specifically, excessive concentrations of radical species cause damage to plant cell components, resulting in cell death [14].

**Citation:** Birhanie, Z.M.; Yang, D.; Luan, M.; Xiao, A.; Liu, L.; Zhang, C.; Biswas, A.; Dey, S.; Deng, Y.; Li, D. Salt Stress Induces Changes in Physiological Characteristics, Bioactive Constituents, and Antioxidants in Kenaf (*Hibiscus cannabinus* L.). *Antioxidants* **2022**, *11*, 2005. https://doi.org/10.3390/ antiox11102005

Academic Editor: Nafees A. Khan

Received: 20 September 2022 Accepted: 7 October 2022 Published: 10 October 2022

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Plants have varied defense strategies against salt stresses, involving morphological, physiological, and molecular responses [15]. Plants can produce osmolytes, including soluble sugars, proteins, and proline, which protect plant cells from the adverse effects of salt stress [16,17]. Protecting cellular membranes via enzymatic antioxidants against salt-induced ROS over-production and membrane lipid peroxidation leads to salt tolerance [18–20]. Under salt stress, antioxidants such as phenolic and flavonoid compounds might also act as ROS scavengers [21]. Plants also defend themselves against biotic and abiotic stressors by emitting volatile organic molecules [22]. Multiple classes of terpenes, phenylpropanoids, and benzenoids, as well as volatile fatty acid and amino acid derivatives, are among the volatile chemicals produced in response to stress [23]. Moreover, salt-stressed plants regulate salt-stress-related genes and have signal transduction factors [24].

Kenaf (*Hibiscus cannabinus* L.) is a major annual fiber crop native to east-central Africa and widely grown in the Asia–Pacific region. *Hibiscus cannabinus* cultivation has increasingly shifted to saline land due to an increased demand for food crops and reduced available arable land [25]. Although kenaf is mostly used for fiber, the seeds, leaves, and flowers may be useful in the food industry [26]. It is also used as a cosmetic ingredient and in folk medicine. *Hibiscus cannabinus* contains bioactive components such as phenolics, flavonoids, terpenes, citric acid, and fatty acid derivatives, which have a variety of pharmacological activities. For example, phenolic compounds have antiaging [27], antiproliferative [28], antityrosinase [29], and antioxidant properties [30]. Flavonoid-rich products also have a variety of biological actions, such as antibacterial [30], anti-inflammatory [31], antioxidant [32], and antidiabetic activities [33]. Moreover, phytol, an acyclic diterpene alcohol, can be used as a precursor in producing synthetic vitamins E and K1 [34]. Hydroxycitric acid has been demonstrated to lower blood insulin levels [35]. Omega-3 polyunsaturated fatty acids are also responsible for lowering the risk of cardiovascular disease and the fracture risk [36]. The essential oil composition and phytotoxic and fungitoxic activity levels of kenaf leaves were investigated by Kobaisy et al. [22]. The oil was effective against *Colletotrichum gloeosporioides*, *Colletotrichum fragariae*, and *Colletotrichum accutatum*, while also being phytotoxic to bentgrass and lettuce. In addition, aqueous extracts of kenaf leaves have been shown to protect rats' livers from carbon tetrachloride and paracetamol-induced damage [37]. Diet-induced hyperlipidemia was mitigated by a hydroalcohol extract of *H. cannabinus* leaves [38]. Kenaf extract induces a cytoprotective molecule in activated macrophages, resulting in a significant immunomodulatory effect [39]. Secondary metabolites of *H. cannabinus*, namely phenolics, flavonoids, and phenolic acids, correlate strongly with the antioxidant capacity, and these compounds prevent oxidative damage to cells by lowering ROS levels under salt stress [40].

Most previous studies have concentrated on the phytochemical properties of *H. cannabinus* under normal conditions [32,41]. However, the acumination and synthesis of bioactive and nutritional compositions depend on abiotic stresses [16,42]. Many plants subjected to salinity stress exhibit changes in the composition of the phenolics [43], flavonoids [44], and saponins [45]. These alterations are dependent on the degree and duration of the stress. Thus, plants stressed by salinity may have the potential to be polyphenol sources. Furthermore, *Hibiscus cannabinus* can potentially be used in phytoremediation to remediate salt-affected soils due to its suitability for cultivation in salinity-affected soils [46,47]. Despite these promising features, the physiological and biochemical responses of *H. cannabinus* to salinity conditions have been scarcely studied. The current study was, therefore, carried out to investigate the effects of six different salt concentrations (0, 50, 100, 150, 200, and 250 mM of NaCl) on the plant growth, physiological traits, bioactive components, and antioxidant capacity of *H. cannabinus*.

#### **2. Materials and Methods**

#### *2.1. Plant Material, Growth Conditions, and Salt Treatments*

China kenaf 21, a typical kenaf variety from the Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences, was chosen for this study. The kenaf seeds were placed on moist filter paper and were enabled to germinate for three days at a temperature of 25 ◦C in the dark after being soaked in sterile water for five hours. The germinated seeds were transferred to a <sup>1</sup> <sup>4</sup> -strength Hoagland nutrient solution (pH 6.0) comprising 5.79 mmol L−<sup>1</sup> Ca (NO3), 2, 4.17 mmol L−<sup>1</sup> MgSO4, 8.90 mol L−<sup>1</sup> MnSO4, 8.02 mmol L−<sup>1</sup> KNO3, 0.94 mol L−<sup>1</sup> ZnSO4, 1.35 mmol L−<sup>1</sup> NH4H2PO4, 0.20 mol CuSO4, 0.015 μmol L−<sup>1</sup> (NH4)2MoO4, 48.3 μmol L−<sup>1</sup> H3BO3, and 72.6 μmol L−<sup>1</sup> Fe-EDTA for continued growth [48]. After 5 days, the seedlings were transferred to a <sup>1</sup> <sup>4</sup> -strength Hoagland nutrient solution supplemented with 0 (control), 50, 100, 150, 200, and 250 mm NaCl solutions, with three replicates at each concentration level, replenished every two days. The seedlings were grown in a culture chamber with a 28/25 ◦C temperature regime, a photoperiod of 16 h/8 h (light/dark), relative humidity of around 60%, and a light intensity of 700 μmol m<sup>−</sup>2s−1. The plants were harvested 14 days after being subjected to salt stress because the plants treated with 200 and 250 mM of NaCl showed salt stress symptoms (leaf chlorosis and necrosis).

#### *2.2. Chemicals and Reagents*

Solar Bio-Science and Technology Co. (Beijing, China) supplied the 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2 -azinobis (3-ethylbenzothiazoline-6-sulfonic acid (ABTS), rutin, and gallic acid. Coolaber Technology Co. (Beijing, China) provided the vanillin, 2,4,6-tripyridyl-s-triazine (TPTZ), and Folin–Ciocalteau reagent. The ferrozine, iron sulfate heptahydrate, and other chemicals were purchased from Shanghai Macklin Biochemical Technology Co. (Shanghai, China). All reagents used in the assay were of the highest analytical grade.
