Morphological and physiochemical changes of jojoba under water pollution stress condition

Due to water limits and the rise in water exhaustion, water security is viewed as one of the major obstacles to sustainable agriculture in the world’s arid and semi-arid regions (Zarei et al., 2020). Egypt faces a significant challenge in securing its water supply, particularly with the ongoing construction and filling of the Renaissance Dam in Ethiopia, which could reduce Egypt’s annual share of the Nile’s water. The amount of water used per person each year has fallen below the critical level of 1000 m³, indicating water scarcity. The nation’s average water consumption per person is 663 m³, which indicates water poverty (<700 m³). By 2030, this number is predicted to drop even further to 582 m³ (Abd Ellah, 2020). As its population continues to grow, Egypt faces a significant challenge in creating and implementing a comprehensive and sustainable water resources plan to meet the fundamental needs of its people (Nasr et al., 2019). Unfortunately, many farmers in areas close to urban or suburban locations have no other choice but to use wastewater to irrigate their crops (Khurana and Singh, 2012; Eissa et al., 2018). The world’s largest water usage, agricultural irrigation, is perceived to be solved by the use of substandard water resources (Galavi et al., 2010; Talat, 2020). However, the use of wastewater, such as raw agricultural drainage water (RADW) and sewage drainage water, for irrigation without conducting a risk assessment study and management can seriously endanger water, soil quality and even human health (Mahmoud and Ghoneim, 2016; Abdel-Rahman et al., 2018; El-Hassanin et al., 2020). Wastewater contains heavy metal (HM) pollutants that can pose a significant health risk (Razzak et al., 2022). Effluent containing HMs can cause damage to plants and become toxic to humans and animals when consumed. These metals are poorly soluble in water and can accumulate in plants, especially when affected by stress (Wang et al., 2021; Rashid et al., 2023). HMs are a class of compounds that pose health risks and can be found in many inorganic contaminants, including polluted plants. These contaminants are not biodegradable, resulting in prolonged biological half-lives and undesirable side effects when accumulated in human body organs (Mitra et al., 2022). Organic fertilisers and pesticides are the primary sources of HM contamination of plants, along with contaminated soil, irrigation water and atmospheric pollution from industrial or motor vehicle emissions (EL-Baz et al., 2015; Salama et al., 2015; Alengebawy et al., 2021). Treated or recycled wastewater is used increasingly for irrigation due to decreasing fresh water and its nutrient composition, providing the soil with essential macro and micronutrients such as copper, zinc, iron, manganese, nitrogen and phosphorus, among others (Singh et al., 2012; Ganjegunte et al., 2018). Wastewater contains a significant amount of organic matter which can be used to improve soil quality and promote plant growth (Ofori et al., 2021). jojoba (Simmondsia chinensis (Link) Schneider) is a remarkable plant that grows in hot, arid and semi-arid climates, earning its nickname ‘green gold’. It is an evergreen perennial woody shrub that naturally grows in the Sonora desert of Mexico and the Southwestern United States. With separate male and female flowers, it is classified as a true xerophyte with roots that can reach depths of up to 9 m, allowing the plant to survive in high temperatures and dry conditions (Benzioni et al., 2006). jojoba seed oil has found favourable commercial uses in cosmetics and as fuel for aircraft and missiles, making it an intriguing alternative crop. The ‘oil’ content of the seed is substantial, comprising around 50% of its dry weight; it is composed of lipids with uniformly sized straight-chain liquid wax esters (Benzioni et al., 1992). jojoba is regarded as one of the most useful solutions for using poor-quality water resources for irrigation in Egypt and other developing countries (Aboryia et al., 2022). Because of tolerance to high temperatures, drought and salinity, lower probabilities of disease, lower need for fertilisers and plentiful financial income, jojoba cultivation can be done effectively in desert areas (Bafeel et al., 2016).

This study aimed to investigate the effect of irrigation with wastewater (RADW and treated sewage drainage water [TSDW]) in different concentrations on vegetative growth and chemical components of jojoba plants, as well as HM residues in the leaves were evaluated to determine its tolerance to irrigation with polluted water.

Freshwater scarcity is a major issue in Egypt due to the construction and filling of the Renaissance Dam in Ethiopia. It is predicted that by 2025 there will be a shortage of about 13.5 million cubic meters of freshwater annually. To tackle this problem, it is important to focus on reusing wastewater for plant irrigation. However, this can cause plant stress. Water stress occurs when there is an imbalance between the water supply and the plant water demand (Pomar and Moussa, 2016; Abd Ellah, 2020). Salt stress significantly limits crop growth and development, posing a threat to global food production. It mainly induces osmotic stress, ionic and nutritional imbalance and reactive oxygen species (ROS), resulting in a significant loss in plant growth (Dustgeer et al., 2021; Seleiman et al., 2022; Sultan et al., 2022). Salt stress leads to a reduction in crop productivity by decreasing photosynthetic efficiency (Alghamdi et al., 2021). Drought is a significant abiotic stress that greatly limits crop growth and global food production (Hassan et al., 2020; Memon et al., 2022). The reduced water availability causes severe changes in plant physiological processes, leading to significant yield losses (Hassan et al., 2021). Water scarcity results in a significant increase in abscisic acid accumulation in plants. The increased presence of abscisic acid in plants can cause oxidative stress, leading to lipid peroxidation, electrolyte leakage and chlorophyll degradation (Jiang et al., 2020). Salt and drought stress can induce the production of ROS in plants, which are regulated by different plant growth regulators. These ROS serve as the plant’s internal defence system, initiating the scavenging of ROS and reducing oxidative stress by enhancing the activities of antioxidant enzymes (Liang et al., 2019; Khan et al., 2022; Wongla et al., 2023). Plants have developed various strategies to cope with limited water availability. These strategies include minimising water loss, supplying water to essential members and maintaining the water content of cells during dry spells (Kern et al., 2020). Water shortage tolerance is the capacity of plants to detect and adapt to water stress. Plant water deficit tolerance is a multifaceted characteristic that can take the form of three main approaches: increasing water use efficiency to avoid drought stress and tissue damage, shortening their lifecycle to escape stress and managing water deficiency by preserving low water content and little activity to live with the least amount of loss (Bolger et al., 2014). The jojoba plant’s ability to withstand irrigation with wastewater as RADW and TSDW at various levels (25%, 50%, 75% and 100%) was investigated. The data showed that the jojoba plants did not experience severe effects or death when exposed to water contaminated with HMs and organic residues as agreed with Rasheed et al. (2019). These findings are also consistent with previous research which found the jojoba plant can grow in polluted soil with HMs (Nasr et al., 2019). These abiotic stresses disrupt plant physiological and metabolic functioning development processes (Jeandroz and Lamotte, 2017) and induce the production of ROS, lipid peroxidation accumulation of various osmolytes and significant yield losses (Batool et al., 2022). The accumulation of abscisic acid and jasmonic acid (phytohormones) in the leaves of plants under water stress activates the regulation of stomatal closure (Iqbal et al., 2014). Stomatal closure inhibits transpiration, preserves water content and suppresses carbon dioxide diffusion to the chloroplast (Ma and Bai, 2021). This reduction in carbon dioxide assimilation can harm photosynthesis activity and lead to the generation of ROS such as O₂^•− and H₂O₂ (Hasanuzzaman et al., 2021). The excessive production of ROS causes oxidative stress, damaging DNA, proteins and lipids, reducing plant yield and growth and ultimately leading to plant apoptosis (Afzal et al., 2023). At the physiological, cellular and molecular levels, plants have evolved a variety of dehydration resistance mechanisms to offset these detrimental consequences. Enzymatic (superoxide dismutase [SOD], catalase [CAT], peroxidase, glutathione reductase) and non-enzymatic (tocopherol, ascorbic acid, carotenoids, glutathione, phenolics, flavonoids) antioxidant components can reduce the detrimental effects of accumulated ROS, creating an essential defence system (Zandi and Schnug, 2022). Plants that produce more antioxidant molecules are better able to reduce the amount of ROS they produce (Laxa et al., 2019).

Furthermore, plants accumulate osmolytes such as proline, glycine betaine, sugars and sugar alcohols in their cells as a result of the fast formation of ROS, which acts as a stress signal (Mukherjee and Choudhuri, 1983). Osmolytes are essential for preserving the osmotic balance of plants and for balancing the composition and activity of several enzymes and macromolecules in the face of elevated ROS levels (Huang et al., 2009). Osmolytes are tiny, soluble substances that insulate cells from damage by maintaining the water content of cells, and protecting the integrity of the cell membrane (Sarma et al., 2023). Auxins, a type of phytohormone that the plant produces in response to its constant need for water, are important in changing the roots (Ilyas et al., 2021). The primary sites of auxin biosynthesis are developing leaves and leaf primordia. Auxins, however, go to the root tips under conditions of water stress via the phloem, cells or auxin-transport proteins such as PIN-FORMED (PIN) (Merelo et al., 2017). Plant roots become more active as a result of this movement when there is water stress. As a result of abiotic and biotic stresses, there is an increase in membrane leakage due to elevated lipid peroxidation. Recall that, other significant macromolecules such lipids, proteins, nucleic acids and pigments involved in photosynthetic processes are also impacted by ROS. Antioxidants can lower the likelihood of experiencing oxidative stress situations by mediating the scavenging of ROS. They comprise both enzymatic (SOD, CAT, ascorbate peroxidase [APX], etc.) and non-enzymatic components (Wu et al., 2019). Reusing wastewater for plant irrigation can reduce the need for freshwater resources and costly chemical fertilisers, but it has been found that it may also have negative effects on soil and plant physiochemical properties (Pomar and Moussa, 2016; Alnaimy et al., 2021). Nonetheless, it contributes to soil fertility and plant growth because it contains high quantities of nutrients (Tarchouna et al., 2010; Saliba et al., 2018). It’s also important to remember that using wastewater for irrigation can lead to soil salinisation, which poses potential risks. Higher levels of sodium compared to other cations, along with contaminated soil and plants containing HMs, can have long-term health implications. The extent of risk depends on the origin of the wastewater and the methods used for treatment (Abd-Elwahed, 2018; Ganjegunte et al., 2018; El-Hassanin et al., 2020). Low molecular weight non-enzymatic antioxidants, such as proline, ascorbic acid and glutathione, can neutralise ROS. On the other hand, enzymatic antioxidants, such as SOD, POD, GST and CAT, can convert ROS into H₂O₂, which is then further converted into water and oxygen. These two categories of antioxidants protect the cell structure from oxidative stress and can also deactivate harmful oxygen by-products. In the data reported herein, the amount of proline increased as the levels of waste water increased. When plants are under HM stress, proline helps to remove harmful ROS in Brassic juncea and Cajanus cajan. It also helps to regulate water potential, reduce metal uptake and maintain osmotic balance for cellular health (Kalaivanan and Ganeshamurthy, 2016). Several studies have shown that proline acts as an osmolyte under stressful conditions and can enhance antioxidant enzymes to mitigate the harmful effects of oxidative stress. Proline also helps to regulate plant growth and maintain osmotic balance, protecting cells from ROS when exposed to high levels of cadmium (Shak and Dubey, 1997; Islam et al., 2009). The EC in the leaves, which indicates IL, increased with high levels of RADW and TSDW. Salt-induced water stress leads to a reduction in chloroplast stroma volume, resulting in the generation of ROS and inhibition of the photosynthetic rate (Rasool et al., 2022). While carotenoids and Chlb considerably decreased at high salt concentrations, pigment levels in the leaves of jojoba plants significantly decreased at intermediate salt concentrations in Chla. This implies that at low and moderate levels of salt stress in jojoba leaves, stroma volume and photosynthetic rate were unaffected, suggesting that jojoba can withstand moderate levels of salt stress (Inoti, et al., 2016). It has been observed that using TSDW at a concentration of 100% led to a decrease in all vegetative characteristics compared to other concentrations (25%, 50% and 75% RADW and TSDW). This decrease was also noted in biomass production, LFW, LDW and SCC (Redha et al., 2021). The reason for this is that agricultural wastewater has a higher concentration of nutrients such as nitrogen, phosphorus, potassium, magnesium and calcium, with a lower concentration of HMs than TSDW, especially at a concentration of 100%. HMs pose a severe threat to global food production. Their concentration in agricultural soil is rapidly increasing due to human activities (Rehman et al., 2022). Plant contamination with HMs can occur due to various sources, such as irrigation with polluted water (Alia et al., 2015), pesticides (Shaheen et al., 2016), chemical fertilisers, motor vehicle emissions and industrial discharges (Abou-Arab et al., 2015). Additionally, certain products like fungicides, inorganic fertilisers and phosphate fertilisers may contain varying levels of cadmium, chromium, nickel, lead and zinc, depending on their sources (Tőzsér et al., 2017). There have been numerous investigations examining the plant species that can resist HMs, either by accumulating or translocating them, so they can be used in phytoremediation (Redha et al., 2021). Based on the results obtained, it was found that when using TSDW at a concentration of 100% compared to other concentrations of RADW and control, the concentration of HMs in the leaves of jojoba plants was high. This may be due to the jojoba plant’s ability to concentrate HMs in its root system. Similar trends have been documented in previous studies where HM concentrations, such as lead, were found to increase in roots instead of leaves (Qasim et al., 2016). Plants have developed various mechanisms to cope with the harmful effects of HMs. One such mechanism is the limited movement of HMs from roots to leaves, which helps plants tolerate the toxic effects of HMs that mostly damage the pigments of the photosynthesis system. Different plant species have different ways of absorbing, translocating and accumulating HMs. The concentration of HMs in the soil also affects these mechanisms (Radojčić Redovniković et al., 2022). HM toxicity in plants leads to the formation of ROS, which in turn causes the peroxidation of essential cellular components (Awad et al., 2021). To combat these harmful effects, plants have evolved a strong defence system consisting of enzymatic and non-enzymatic antioxidants. Exposure to HMs has been reported to increase the percentage of IL% due to the generation of ROS (Tamás et al., 2006; Ofori et al., 2021). Polyunsaturated fatty acids, which are the main components of membrane lipids, are particularly sensitive to oxidation. The lipid peroxidation reaction is initiated by −OH radicals and results in the formation of highly reactive lipid peroxyl radicals that can react with another fatty acid (Prisacaru, 2016; Radwan et al., 2010). The chain reaction can be terminated when two radicals combine to form a non-radical compound, and MDA is usually the end product of the process (Martemucci et al., 2022). Therefore, an increase in MDA concentration is generally considered to be the main biomarker for the severity of oxidative stress (Sharma and Dietz, 2009). In biological systems, oxygen molecules can produce oxygen free radicals when they take electrons from other molecules. Additionally, several intracellular processes can convert oxygen to O₂^•− or H₂O₂. Hydroxyl radicals (−OH) are responsible for most oxidative damage in biological systems, even though these molecules are not particularly reactive (Sharma et al., 2012). Therefore, the balance between the production of free radicals and antioxidants (DPPH) activity is crucial for plants to adapt to unfavourable environmental conditions. An increase in free radicals can inhibit the formation of photosynthetic pigments like chlorophyll and carotenoids, as the chloroplast membrane becomes oxidised (Malar et al., 2014). HMs can affect physiochemical processes and generate ROS instantly in humans, animals and plants, leading to the stopping of some proteins and glutathione (Abdulaal et al., 2017). To evaluate antioxidant capacity, different techniques have been used to estimate the antioxidants’ ability to quench ROS. The stable DPPH radical quenching is a widely used technique to assess antioxidant capacity (Shahidi and Zhong, 2015).

Under water stress, ion buildup and functional abnormalities observed during stoma opening and closure may also reduce the total chlorophyll content (Molazem et al., 2010; Nawaz et al., 2010). When salt stress occurs, genotypes that are susceptible to salt show a more pronounced reduction in chlorophyll concentration than cultivars that are more tolerant, like jojoba (Ali et al., 2013; Sharkey, 2020). When exposed to a salt stress of 100 mM NaCl, the chlorophyll-a level somewhat decreased, but at 200 mM NaCl, it increased. Conversely, the outcomes for carotenoids and chlorophyll-b showed significant drops at 200 mM NaCl salt stress but rises at 100 mM NaCl. These findings suggest that the genes linked to salt stress response to produce chlorophyll a took longer (day 30) to respond than the genes responsible for producing carotenoids or chlorophyll b (day 25) (Inoti et al., 2016). Water-stress-related genes were found in a recent study, which may help explain the mechanisms underlying jojoba’s remarkable salt tolerance. The genes linked to water stress found in jojoba may aid in defining the plant’s resistance to salt and offer a useful supply of genes that can help other species develop a tolerance to water stress (Al Dossary et al., 2024).

Increases in carotenoids and chlorophyll-(a, b) were evident in our results. This is because, in response to salt stress, the jojoba plant produces more transketolase as a defence mechanism to keep the amount of carotenoids and chlorophyll-(a, b) in the leaves of the plant constant. Because (S)-2-hydroxy-acid oxidase is activated during the conversion of glycolate to glyoxylate and is increased in jojoba under salt stress, it is expected that H₂O₂ will be generated extensively throughout this process. During the synthesis of CAT, the H₂O₂ level in jojoba is dramatically lowered as it is transformed to O₂^•−. As a result, the latter greatly increased under salt stress in jojoba in favour of H₂O₂ reduction. Both in the physiological and molecular analyses, APX increased dramatically. The crosstalk between the ‘ascorbate and alternate metabolism’ and ‘glyoxylate and dicarboxylate metabolism’ pathways is complemented by the high expression of APX in jojoba, where tartronate semialdehyde functions as a transitional step. The most significant stable non-radical ROS is H₂O₂. (Ślesak et al., 2007; Sofo et al., 2015). Along with other ROS, the concentration of H₂O₂ in cells is a reliable indicator of the degree of oxidative stress. Therefore, the primary enzymatic H₂O₂ scavenging mechanism in plants, the balance of APX and CAT activity is essential for the suppression of hazardous H₂O₂ levels in a cell (Sofo et al., 2015). Plants that collect ROS will have reduced light-harvesting efficiency and slowed down signal transduction and protein synthesis as a result of osmotic pressure fluctuations. ROS can be scavenged by APX and other peroxidases including CAT and SOD (Sofo et al., 2015). Molecular investigation of the gene encoding the enzyme that demonstrated an increase under salt stress supports the enhanced activity of the APX enzyme in jojoba under salt stress. Tartronate semialdehyde in the ‘ascorbate and alternate metabolism’ pathway produces APX. Research conducted by Hasanuzzaman et al. (2019) has shown that an increase in the formation of ROS leads to an increase in antioxidant capacity, which aids in the detoxification of H₂O₂. Previous studies (Bratovcic, 2020) have also found that the creation of antioxidants protects against the generation of free radicals, which act as superoxide generated in many cells under HM stress. The experiment demonstrated that as the amount of TSDW and RADW treatments with high concentrations of HMs increased, the production of antioxidant capacity (DPPH) gradually increased as well (Figure 8). The jojoba plants triggered their antioxidant capacity along with the accumulation of superoxide anion O₂^•− and H₂O₂, which was demonstrated through the parallel production of antioxidant capacity (DPPH) and H₂O₂ and O₂^•−. However, the level of HMs plays a crucial role in either energising or inhibiting antioxidant activity (Emamverdian et al., 2023). Generally, HM toxicity is registered to raise the activity of antioxidant defence systems in plants grown in polluted soil (Emamverdian et al., 2023).

Climate change has a significant impact on the environment and living organisms, and it can cause water scarcity. One way to adapt to these changes is by reusing wastewater to irrigate plants that are capable of withstanding it. The jojoba plant, S. chinensis (Link) Schneider, was studied to determine its ability to tolerate wastewater rich in HMs and organic residues. The results of the study showed that the physical and chemical properties of the jojoba plant were only slightly affected by using different concentrations of RADW and TSDW (25%, 50%, 75% and 100%). The jojoba plant has a robust antioxidant defence system that can mitigate the harmful effects of HM-contaminated water. It can also reduce the transfer of HMs from the roots to the leaves, which preserves the photosynthetic pigments and the photosynthetic system from damage, maintains the cell wall and reduces the production of ROS such as H₂O₂, superoxide anion (O₂^•−), MDA and IL%. Based on these findings, we recommend irrigating the jojoba plant with RADW and TSDW at a concentration of 100% and 75%, respectively. For future studies, it would be helpful to use some phytohormones external treatments to study their effect on water stress genes in the jojoba plant. The obtained information can also aid in the development of new economically important crop plants with improved salt tolerance through metabolic engineering.