4. DISCUSSION
Salinity is considered a type of physiological stress that desiccates plant tissues to increase osmotic stress and check plant growth and yield [17,18,47]. However, different concentrations of soil salt have varying impacts on different crop species and varieties. Crops belonging to the glycophytic category of plants are sensitive to salt stress, and Ocimum genus plants belong to this category. Recently, various studies have been performed on plants in the Ocimum genus to evaluate the impact of salinity-induced toxicity on their growth, physiology, antioxidant system, and yield [48,49]. Metabolism perturbations and limited assimilative biochemical reactions due to salinity are reflected in the form of a loss in length, fresh and dry mass of the plant, and leaf area. The decrease in plant growth due to soil salinity is due to the inability of the plant roots to absorb water and nutrients from the root zone, mainly through Na+ accumulation in the root cells [50,51]. Salinity also causes nutrient imbalance in plants by disturbing the osmoticum [52], decreasing the rate of cell division and elongation, and ultimately reducing root and shoot length [53]. The length, fresh weight, and dry weight of plants are the outcomes of proper cell division and photosynthesis, leading to proper assimilation and accumulation of storage material ultimately resulting in proper plant growth [54]. Decreased water potential and oxidative stress by salinity could have adversely affected the enzymes of the carbon and nitrogen assimilation cycle, resulting in low root and shoot dry weight of affected plants [55-57]. A reduction in growth morphology by salt stress was also observed in different crop plants such as Solanum lycopersicum, Brassica juncea, Helianthus annuus, and O. basilicum plants [58-62].
Salinity affects the cell division machinery and decreases plant photosynthesis and transpiration by closing stomata, inhibiting the genetic expression of genes involved in chlorophyll biosynthesis, and enhancing the biosynthesis of chlorophyllase enzymes by osmotic stress [63,64]. Due to osmotic stress and the ion toxicity of salinity, the decline in photosynthetic pigments in antenna molecules of thylakoids could ultimately limit the maximum quantum yield of PSII [65,66]. Further decreased leaf area due to limited turgor expansion of cells seems to result in decreased leaf photosynthetic area, causing the photosynthesis rate to be low [50,51]. Earlier studies by researchers [67-69] have shown a positive correlation between the net photosynthetic rate and chlorophyll level of leaves. Salt stress damages PSII electron transport [70], blocking electron transfer from the primary acceptor to the secondary acceptor plastoquinone (QA → QB) and leading to a decreased maximum quantum yield of PSII [66,71]. Furthermore, excess salt is taken up and accumulates Na+ ions in shoots (stem, leaves, and flowers), passing through roots and damaging roots, shoots, and leaf cells by ion toxicity, inducing lipid peroxidation, and causing electrolytic leakage in plant cells [72,73]. Furthermore, earlier studies revealed that salinity reduced K+ accumulation; however, Zn treatment enhanced K+ uptake in plants [13,27,33]. Similarly, salinity promoted Na buildup, whereas Zn treatment decreased Na concentration. The decreased biomass could be attributed to increased Na+ buildup and decreased K, Zn, Cu, and Mn concentrations [36,52,54].
Salinity also alters plant metabolism by generating oxidative stress through the generation of excess reactive oxygen species (ROS) that trigger the antioxidant response of cytoplasmic and membranous enzymes such as CAT, POX, and SOD and the generation of molecules such as glutathione, ascorbate, and proline to counter free radicals generated in response to biotic or abiotic stresses that attack plants [74,75]. ROS in excess damages various cell organelles and molecules [76,77]. Salt-induced increases in proline levels reported in different crops [69,78,79] maintain cell osmoticum and prevent cell protein enzymes from desiccation by improving water potential [80,81]. The increased activity of the antioxidant system was suggested to provide salt tolerance or a sensitive response in plant genotypes [61,82].
Earlier studies concluded that ion accumulation and selectivity declines have been well established in wheat, sorghum, maize, barley, and rice under salt stress conditions [54,83-86]. The productivity of any plant is dependent upon the photosynthesis rate which ultimately depends upon the gaseous exchange by the stomata of the plant. Na ion accumulation in saline-stressed plants disturbs the K ion concentration in the guard cells resulting in stomatal closure that decreases productivity and ultimately low growth, dry weight, and yield [14,51,54].
In recent studies, basil plant species showed great selectivity for K+ absorption, which increased the K+/Na+ ratio in salt stress conditions [83,84]. Zn influences the structural integrity and permeability of stem cell membranes, which decreases excessive Na uptake in saline environments. Zn supplementation reduces Na+ accumulation and improves the K+/Na+ ratio of plants exposed to salinity. As a result of Zn shortage, cell membranes exhibit significant permeability or leaking of certain chemicals from the roots [87]. Zn deficiency can result in harmful ion accumulation, such as Na+ and Cl– [86].
The increased concentration of Na+ ions generated by salt stress inhibits root potassium (K+) absorption. Potassium is the most abundant cation in living cells and is required for normal root cell turgidity as well as the action of numerous enzymes [62]. Due to a paucity of K+ ions, the root cell’s growth and development were halted [85]. Excessive Na+ absorption into the root cytoplasm can inhibit the action of important enzymes. When the Na+/K+ ratio is high, it might harm the plant roots [86]. Under salt stress, basil plants had the highest Na+ concentration and the lowest K+ concentration, according to recent findings [49,62]. In earlier studies, maximum K+ levels were evaluated in ZnO-NPs-treated basil plants compared with control and salinity stress plant roots [62,87]. Plants of maize and cotton also produced comparable results [54,56]. In earlier studies, a link was found between an increase in harmful ions (Na+ and Cl–) and a decrease in the absorption of critical components required for growth, as seen by the high Na+/K+ ratio for salt-treated basil plants [62,88]. Salinity also causes a significant reduction in the fruit and seed yield of commercially important crops [14,53,54,89,90].
Zn is an essential mineral nutrient for plants that is scarcely available for plant growth in the soil [72]. Enzymes, including dehydrogenases, aldolases, isomerases, transphosphorylases, and RNA and DNA polymerases, all require zinc to function [91,92]. Moreover, it contributes to tryptophan production, cell division, membrane structure maintenance, and photosynthesis and functions as a regulatory cofactor in protein synthesis [84,92,93]. Zn is important for plant growth, but its excess causes growth inhibition in plants. Reduced growth and plant biomass, restriction of cell elongation and division, wilting, curling, and rolling of young leaves, chlorotic and necrotic leaf tips, and suppression of root growth are all symptoms of Zn-induced toxicity in plants [94].
Nanomaterials such as nanoparticles, nanobiochar, and nanofertilizers enhance the potential of plant resource use efficiency and reduce the environmental toxicity of different chemical salts [21,95]. In recent studies, various nanomaterials such as silicon (Si) nanoparticles and silicon fertilizers exhibited positive effects on the physiology and morphological traits of basil under salinity stress by increasing growth and development, chlorophyll level, and proline content in the leaves of basil under salt stress [96,97]. SiO2 NP application increased the fresh and dry weight of the leaf, chlorophyll level, and proline accumulation with increased antioxidant enzyme activity [96-98] and seedling growth under salt stress [99]. Si NPs have shown better physiological and biochemical responses under salt stress in various plants [100]. This resulted in improved photosynthesis, relative water content, photosynthetic pigments, and cell osmolites such as sugars and proline [101]. Proline content maintains cell osmoticum and excludes the toxic level of salts from the cell membrane, thus improving plant growth [102,103]. The application of Si NPs reduced MDA content (lipid peroxidation) and thus EL [101]. Exposure of onion seedlings to TiO2 NPs increased SOD activity. Seedling growth in onions was enhanced with the low-concentration application of TiO2 NPs [104]. However, the exact working mechanism of different element NPs is not yet understood in the case of desiccation stress (salt or drought).
Plants mainly uptake nutrients from the soil, and non-essential elements present in the soil hinder their uptake by overtaking the essential element channels. Therefore, the foliar spray of Zn as ZnO NPs helps provide the nutritional requirements of Zn in the plants [33]. In the agriculture sector, Zn doses proved potent to reduce salinity-induced toxicity on basil plants [62], but the foliar spray of NPs such as ZnO and other nutrients released in a controlled manner made the macromolecule delivery more selective and effective [21,88,105]. Foliar spray of ZnO NPs also enhances the expression level of genes encoding Rubisco- and chlorophyll-binding proteins, increases proline production and accumulation, and increases the antioxidant activity of plants by regulating the gene activation responsible for proline production and antioxidant activities [21,84,86,105].
Zn NPs (ZnO and ZnSO4) have been shown to promote seedling vigor, manifesting early flowering and higher leaf chlorophyll content [106-108]. ZnO NPs also effectively improved the stem and root growth and pod yield of the plants [106,109,110]. The role of different nanoparticles, including ZnO, in overcoming different abiotic stresses in plants, such as heavy metal toxicity and drought stress, has been reviewed [20,107]. In maize plants, it was suggested that ZnO and other nanoparticles mediated a reduction in salinity stress by reducing Na+ ion absorption by plant tissues, thus managing osmotic potential and Na+ toxicity [54,111,112]. Foliar spray of ZnO NPs efficiently absorbs the leaf surface inside, dilutes the toxic effects of salinity, and decreases Na+ ion accumulation by increasing the water potential and proline content, which minimizes electrolytic leakage and lipid peroxidation in plant cells [72,73]. ZnO NPs protect the photosynthetic machinery by hampering the activity of enzymes involved in the degradation of photosynthetic pigments such as chlorophyll and carotenoids [72,113]. ZnO NPs upregulate the genes that are involved in chlorophyll pigment biosynthesis, resulting in proper photosynthesis in plants [83,111]. ZnO NPs trigger the anti-oxidant activity of salt-stressed plants and help them mitigate the oxidative stress of salinity [85,86]. ZnO NPs induce the expression of genes that regulate carbon and nitrogen assimilation, resulting in improved growth and yield under saline conditions [83,105]. ZnO NP application also enhances the water uptake capability, leaf water potential, WUE, and transpiration rate in the plants grown under salt stress, maintaining the photosynthetic activity that results in greater biomass production [84].
Leaves of O. basilicum plants are the main economic product for farmers and are mainly cultivated for their leaves [87]. The essential oil constituents in basil leaves are responsible for the therapeutic properties of this plant species [114]. Under stress conditions such as salinity, the therapeutic values of these important phytochemicals decreased in the plants, making them less valuable. A study done by Ciriello et al. [87] revealed that Zn fortification helps enhance the antioxidant metabolite production in basil plants. Various studies revealed that foliar sprays of ZnO nanoparticles proved efficient in enhancing the photosynthetic activity and antioxidant and other phytochemical production-related activities in plants grown under saline soil [84]. ZnO NPs bind and activate the genes and proteins related to maintaining cell membrane leakage, lipid peroxidation, proline production, total soluble sugar content, stomatal conductance, transpiration activity, and chlorophyll production [21]. Essential oils are the key products of economically important plants such as mint, basil, mustard, soybean, sunflower, and flax [5,13,62,113,115,116]. Identification of the optimum dose or concentration of ZnO NPs foliar spray will be helpful to boost the large-scale cultivation of these plants and the oil content in their valuable parts under saline conditions [22].
Our results are in agreement with Tolay on basil, Ali et al. on barley, Rakgotho et al. on sorghum, and Singh et al. on rice exploring the impact of Zn and ZnO NPs on these plant species growing under saline conditions [62,84-86,88].
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