Foliar application of silicon-based nanoparticles improve the adaptability of maize (Zea mays L.) in cadmium contaminated soils

Heavy metals (HMs) especially cadmium (Cd) absorbed by the roots of crop plants like maize have emerged as one of the most serious threats by causing stunted plant growth along with disturbing the photosynthetic machinery and nutrient homeostasis process. A trial was conducted for inducing Cd stress tolerance in maize by exogenous application of silicon nanoparticles (SiNPs) using five doses of SiNPs (0, 100, 200, 300, and 400 ppm) and three levels of Cd (0, 15, and 30 ppm) for maize hybrid (SF-9515). The response variables included morphological traits and biochemical parameters of maize. The results indicated that Cd level of 30 ppm remained the most drastic for maize plants by recording the minimum traits such as shoot length (39.35 cm), shoot fresh weight (9.52 g) and shoot dry weight (3.20 g), leaf pigments such as chlorophyll a (0.55 mg/g FW), chlorophyll b (0.27 mg/g FW), total contents (0.84 mg/g FW), and carotenoid contents (0.19 µg/g FW). Additionally, the same Cd level disrupted biochemical traits such as TSP (4.85 mg/g FW), TP (252.94 nmol/g FW), TSAA (18.92 µmol g−1 FW), TSS (0.85 mg/g FW), and antioxidant activities such as POD (99.39 min−1 g−1 FW), CAT (81.58 min−1 g−1 FW), APX (2.04 min−1 g−1 FW), and SOD (172.79 min−1 g−1 FW). However, a higher level of Cd resulted in greater root length (87.63 cm), root fresh weight (16.43 g), and root dry weight (6.14 g) along with higher Cd concentration in the root (2.52 µg/g−1) and shoot (0.48 µg/g−1). The silicon nanoparticles (Si NPs) treatment significantly increased all measured attributes of maize. The highest value was noted of all the parameters such as chlorophyll a (0.91 mg/g FW), chlorophyll b (0.57 mg/g FW), total chlorophyll contents (1.48 mg/g FW), total carotenoid contents (0.40 µg/g FW), TSP (6.12 mg/g FW), TP (384.56 nmol/g FW), TSAA (24.64 µmol g−1 FW), TSS (1.87 mg/g FW), POD (166.10 min−1 g−1 FW), CAT (149.54 min−1 g−1 FW), APX (3.49 min−1 g−1 FW), and SOD (225.57 min−1 g−1 FW). Based on recorded findings, it might be inferred that higher levels of Cd tend to drastically reduce morpho-physiological traits of maize and foliage-applied silver nanoparticles hold the potential to ameliorate the adverse effect of Cd stress on maize.


Introduction
Maize (Zea mays) belongs to the angiosperm family Poaceae (Gramineae) and is considered one of the three firstly domesticated food crops (Yang et al. 2015;Ahmad et al. 2021). Globally, its consumption stands out at 10.98 billion bushels per year making it the third most important cereal crop (Ahmad et al. 2021). The demand for maize has persistently increased owing to widespread consumption by humans and livestock (Qiao et al. 2022), which has necessitated boosting its yield in a sustainable and eco-friendly way.
Globally, HMs pollution has seriously increased due to the continuous development of industrialization and urbanization which has threatened the ecosystem functioning and deteriorated human health. In particular, the soil of coastal regions has been extensively polluted by HMs infiltrating through direct emission, surface runoff and atmospheric deposition (Yan et al. 2022). The HMs especially Cd is highly dangerous for human as it causes different physiological disorders and diseases including brain disorders, cancer and genetic toxicity . The Cd toxicity disrupts plant growth by lowering the photosynthesis rate due to photosynthetic pigments degradation. Additionally, Cd causes an imbalance in nutrient uptake which causes oxidative damage. Moreover, accumulation of oxygen increased which enhanced the peroxidation under Cd stress. Moreover, Cd toxicity restricts plant growth by damaging photosynthetic machinery and disrupting the nutrient homeostasis process. Similarly, a significant reduction in cell diameter, cell elongation rate, and thickness of the meristem have been reported by Cd stress (Rehman et al. 2015). However, plants tend to trigger the biosynthesis of antioxidants, osmolytes, chelating agents and non-enzymatic antioxidants to cope with Cd-induced drastic alterations (Zhao et al. 2021).
Silicon (Si) is an abundant element on the earth's planet and is considered beneficial for assisting to maintain plant diversity and regulating plants' response to abiotic stresses especially HMs toxicity in agricultural fields (Rehman et al. 2020). The Si is available in the form of mono-silicic acid and imparts tolerance in the plants against Cd, iron, aluminium, chromium, zinc, and manganese toxicities. There are different mechanisms through which Si confers tolerance in crop plants such as its application effectively decreasing the uptake of manganese to hamper its toxicity in plant tissues (Tripathi et al. 2017). Interestingly, Si applied as NP remained even more effective in defeating microbial infections, and nutrient toxicity, and triggered the photosynthesis rate in crop plants (Khan et al. 2017). The SiNPs applied as soil amendment have also proved their efficacy in terms of offering protection against HMs deposition in the soil solution (Iavicoli et al. 2017). Likewise, Si gets deposited within the layers of the cell wall and lignifies the cell wall which resulted in the stabilization of protein structure and prevented electrolytic leakage. The Si ions improve the antioxidant system by reducing the apoplastic flow of water which reduced abiotic stresses. Under drought and heat stresses, foliage applied SiNPs decrease the synthesis of ROS and thus enabling plants to survive spells of stress (Moradi et al. 2022). However to the best of our knowledge based on the literature review, there exist significant research gaps regarding nano-based Si effectiveness for mitigating Cd stress for maize, while dose optimization of SiNPs continues to remain an unexplored research aspect under varying levels of Cd stress. Therefore, it was hypothesized that maize hybrid might respond differently to varying doses of SiNPs, while various Cd stress levels could impart drastic impacts on the morphological and biochemical traits of maize. Thus, a study was conducted with dual objectives to evaluate the adverse impacts of Cd levels on maize growth in terms of morphological and physiological traits and to optimize nano silicon particles dose for mitigating the Cd toxicity.

Experiment site and location of experiment
A present pot experiment was performed to induce Cd stress tolerance in maize by exogenous application of Si NPs through alteration of its physiological and antioxidant activities in the Department of Botany, University of Central Punjab, Bahawalpur Campus, Punjab Group of Colleges Bahawalpur, Pakistan (29°23′11″N 71°39′12″E and having an altitude of 118 m) (Jamil et al. 2019).

Experimental detail
The study was performed by using five doses of Si NPs including control, 100 ppm, 200 ppm, 300 ppm and 400 ppm and three Cd levels such as control, 15 ppm and 30 ppm applied as a foliar spray on maize hybrid (SF-9515). The trial was executed in completely randomized design (CRD) with the factorial arrangement and three replications were maintained.
The seeds of maize hybrid (SF-9515 F1 Single cross, higher vigour and germination rate, early maturing, resistant to lodging, salt tolerant and high yielding) were purchased from Seyfert Seed Pvt. Ltd., District Bahawalnagar, Pakistan. For pot filling, the sand was collected and washed thoroughly to free it from nutrients and subsequently shade drying was performed. For adding equal sand to all pots, a digital balance was used to weigh 2 kg of sand. As far as the pots dimensions were concerned, their height was 18 cm while the outer diameter was 20 cm. After filing the pots, 5 seeds per pot were sown and water was applied in accordance with the field capacity of sand. The field capacity was measured and calculated by using the procedure of Ahmad et al. (2021). At the time of seed sowing, the recommended dose NPK at 120, 60, and 40 kg ha −1 (full dose of P and K were applied as basal dose, while N was applied in three equal splits) was incorporated in the sand. After the completion of seed germination, the thinning of plants from each pot was done and maintain three plants per pot. Thereafter, Cd levels as per treatments were applied to all pots except the pots that were reserved as control treatment. The foliar application of silicon nanoparticles in accordance with treatments was performed in three splits at one-week intervals. The maize plants attained the 8th leaf stage after 40 days of sowing and at that stage, all response variables were estimated using standard procedure.

Morphological traits
Two maize plants were removed from each pot and washed properly. Then, the plant shoot and root were separated with the help of a blade to measure the root and shoot length using a ruler. The harvested plants were blotted with soft paper to remove the moisture present on the surface of the plants and weighed using a digital balance. For recording the dry weight, plants were dried in an oven overnight at a temperature of 40 °C.

Leaf pigment determination
The leaves from harvested maize plants were removed and 0.5 g of leaf was added into pestle mortar with 80% Acetone for making a paste of leaves. Then, leaves paste was added into the cube of the spectrophotometer for estimating the absorption of the given supernatant at 663, 652, and 642 nm. The chlorophyll and carotenoids were estimated by following Arguedas et al. (2018).

Measurement of the chlorophyll
Measurement of the chlorophyll a (Eq. 1): Measurement of the chlorophyll b (Eq. 2): Total chlorophyll content was measured by the following equation (Eq. 3).

Total carotenoid contents
In this equation, it was vindicated that the volume of a sample of the plant was extracted and W indicates the weight of the plants.
Total carotenoid concentration is calculated by the following formula (Eq. 4).

Total soluble protein (TSP)
Total proteins in the form of soluble in plants were analysed in the laboratory by using the process of Assay (1951).

Process of extraction
The leaves were separated from harvested plants and 0.6 g chopped sample material was added to 10 mL of phosphate buffer having 7.0 and were thoroughly grounded. Thereafter, the grounded material was placed into the centrifugation machine at 5000 × g for 6 min. For protein determination used some supernatant. From each treatment, extract of the leaf was full in a holed test tube. Firstly, phosphate buffer was added to this buffer (pH 7.0) and then the solution was added into a test tube. The test tube was kept for 15 min at the normal temperature. After that, the folin phenol reagent was added at the rate of 0.5 mL and retained on starrier for 30 min at room temperature. Lastly, the sample was placed into the spectrophotometer to record the density at 620 nm. (1) Total proline (TP) The proline content was determined by following the method of BATES et al. (1973). Firstly, leaves (0.5 g) were added in the 10 ml of sulfosalicylic acid to dissolve the sample material and then grounded. Before the test, the sample was filtered with filter paper (Whitman number 2) and the sample was put into the test tube containing 2 ml of ninhydrin solution. Thereafter, by dissolving the 1.2 ninhydrin in 30 ml glacial acetic acid, about 20 ml of 6 M ortho-phosphoric acid was added by the method of the acid ninhydrin. After that, 2 ml of glacial acetic acid was added to a test tube under the temperature of 100 centigrade for 1 h. Then, 10 ml of the mixture was extracted and placed in the steamer and for 2 min the steam was passed. Then, values shown by the spectrophotometer were noted proline content was estimated by the slandered curve.

Total soluble amino acid (TSAA)
The quantity of amino acid was determined by the method of (Hamilton 1965). The leaves (0.5 g) sample was mixed with phosphate buffer (0.2 M) and then 1 mL of extract was added to a test tube. After that 1 mL of pyridine was added with 1 mL of ninhydrin solution (10%). The ninhydrin was dissolved in a distilled water test tube at the rate of 2 g then the ninhydrin solution was peppered. The sample in the test tube was heated in a boiling water bath for 30 min. then, the density of colours was noted by the spectrophotometer at 570 nm and slandered curve was formed.

Total soluble sugar (TSS)
For the extraction of soluble sugar, the method of (Yemm and Willis 1954) was followed. The oven-dried plant samples were crushed to fine powder and sieves with the 1 mm sieve of the micro mill. Then, 0.1 g material was added into the test tube containing 80% ethanol solution. Thereafter, the sample was placed into the incubator for 6 h at 60 °C temperature. After that,25 ml plant extract was taken in a test tube and 6 ml of enthroning reagent was added which was followed by heating and boiling of the material for 10 min then cooled at low temperature. After ice cooling for 10 min, the sample was transferred into the incubator for 20 min at room temperature. The sample was placed into the spectrophotometer and density was recorded at 625 nm.

Total soluble amino acid =
Graph readin of sample×Volume of sample×Dilution factor Weight of fresh tissues×1000

Peroxidase dismutase (POD)
The activity of Peroxide dismutase was analysed by the method of Chance and Maehly, in which the method measured the peroxidation of hydrogen peroxide. The peroxidase dismutase was measured by using an organic compound named guaiacol. Guaiacol is an electron donor (Chance and Maehly 1955).

Catalase (CAT)
Chance and Maehly (1955) method was used to determine the catalase. Catalase was measured by measuring the alteration rate of the water molecule and hydrogen peroxide from the oxygen molecule. The 3 ml solution sample solution comprised a phosphate buffer at the rate of 50 mm. Their pH is neutral (7.0) not acidic and does not have basic properties with 5.9 mM H 2 O 2 and an enzyme extract at the rate of 0.1 mL. The result was noted by the spectrophotometer as a decline in absorption at 240 mm. These are the consumption of H 2 O 2 after every minute.

Ascorbate peroxidase (APX)
To estimate APX content of maize, a method involving monitoring of reduction in absorbance of ascorbic acid was followed. The absorbance was noted at 290 nm.
The mixture of the sample containing 50 mM phosphate buffer with 7.6 pH. In 1 ml reaction and 0,1 mN of Na EDTA, 0.25 mM of ascorbic acid, 12 mM H 2 SO 4 . This description method was determined by the scientist (Cakmak et al. 1994).

Superoxide dismutase (SOD)
The superoxide dismutase was measured from the plant according to the Zhang method. Firstly took the plants and ground them with N 2 solution. If the plant is ground (convert into small pieces) then uniform by the phosphate buffer at the pH of 7.6. Then, note the result.

Cadmium in leaf and root
Cut the fresh leaves from three replication of plants. Two to three leaves were selected from every replication. Then, put all the leaves into the polythene bag along with the name tag of treatment and replication. The leaves were brought to a laboratory for analysis.
Firstly, took the plant leaves and placed them in the air for the air-dry process then placed them into the electric oven at a constant time and temperature of 65° C. When the sample is fully dried then crush it with pestle mortar to convert it into powder shaped then stored it in a zipper bag for further analysis.
Apply the digestion process proposed by the U.S. salinity lab staff, 1954. The leaves sample was digested by the diacid in a ratio of 1: 2. Took a conical flask and added sample (Roots and leaves) at the rate of 0.5 g. Added HNO 3 sample into a given conical flask where the sample was added and then kept overnight. On the next day, the conical flask was placed on the hot plate at the temperature of 150 °C. Keep heating until showed a yellow colour. When the sample turns yellow then added 2 ml of HCIO 4 and the sample is cool at room temperature. After cooling at room temperature heated the sample until colourless. The colourless is the endpoint. After the digestion, the volume of the sample was made in a 25 mL flask and filtered with Whatman filter paper. The filtered sample was stored in a clean and air tide bottle then apply AAS analysis was to detect metal.

Statistical analysis
The collected data were analysed statistically using the method of Fisher's Analysis of Variance technique and the least significant difference (LSD) test was applied to compare the treatments' means at a 5% probability level (Steel and Blaszczynski 1998).

Morphological parameters
The results showed that Cd levels and different treatments of SiNPs as well as their interactions significantly affected the growth attributes such as shoot, root and their fresh as well as dry weight of maize (Table 1).
Different levels of cd stress significantly affected the shoot and root length along with their fresh and dry weights of maize. The maximum shoot length (56.60 cm) and their fresh weight (14.34 g) and dry weight (5.15 g) were recorded for control treatment while the lowest shoot length (39.35 cm) and their fresh (9.52 g), and dry weights (3.22 g), were noted for the maximum Cd level of 30 ppm. In this way, Cd stress recorded a 43%, 55%, and 67% reduction in shoot length and their fresh and dry weights respectively compared to the control treatment. In contrast, higher Cd levels maximized the root length (87.63 cm) and their fresh (16.58 g) and dry weights (6.14 g). Among SiNPs foliar sprays, 400 ppm dose exhibited the highest shoot and root lengths along with their fresh and dry weights (compared to the control treatment (Fig. 1).

Leaf pigments
The Cd stress and foliar sprays of SiNPs had a significant influence on the leaf pigments attributes such as chlorophyll a, chlorophyll b, and total chlorophyll as well as carotenoid pigments of maize (Table 1). The lowest chlorophyll a (0.55 mg/g FW), chlorophyll b (0.27 mg/g FW), total chlorophyll (0.84 mg/g FW), and carotenoids (0.19 µg/g FW) were noted as the most severe Cd stress level 30 ppm under investigation and Cd-induced adverse effects gradually reduced with lower levels of Cd. Likewise, SiNPs applied at the rate of 400 ppm remained effective in by giving the maximum chlorophyll a (0.91 mg/g FW), chlorophyll b (0.57 mg/g FW), and total chlorophyll content (1.49 mg/g FW) along with carotenoid content (0.40 µg/g FW). Overall, this treatment exhibited higher chlorophyll a, chlorophyll b, total chlorophyll and carotenoid contents that were 116%, 38%, and 217% respectively greater than the control treatment entailing no foliar application of SiNPs (Fig. 2).

Biochemical attributes
The research findings elaborated that Cd stress levels and different treatments of SiNPs as well as their interaction significantly affected the biochemical attributes such as total soluble proteins, total proline and total soluble amino acid as well as total soluble sugar of maize (Table 1). The maximum soluble proteins (5.65 mg/g FW), proline (282.60 mmol/g FW) and soluble amino acid (21.07 µmol g −1 FW), as well as soluble sugar (1.76 mg/g FW), were recorded where Cd was not applied while the highest level of Cd decreased soluble proteins, proline, soluble amino acid, and soluble sugar by 17%, 119%, 11%, and 112% respectively than the control treatment.

Antioxidant activities
As per findings of this trial, Cd stress levels and different doses of foliar applied SiNPs as well as the interaction had a significant influence on the antioxidant attributes such as peroxidase dismutase, catalase, ascorbate peroxidase and superoxide dismutase contents of maize (Table 1). The Cd-free maize plants exhibited 17, 19, 23 and 9% higher peroxidase dismutase, catalase, ascorbate peroxidase, and superoxide dismutase respectively compared to the values recorded by the highest level of Cd stress under study. Contrastingly, the lowest peroxidase dismutase (99.39 min −1 g −1 FW), catalase (81.58 min −1 g −1 FW), and ascorbate peroxidase (2.04 min −1 g −1 FW), as well as superoxide dismutase (172.79 min −1 g −1 FW), were noted for Cd level 30 ppm.

Discussion
The silver nanoparticles hold a bright perspective in increasing the biomass of the p maize by reducing the stress of HMs (Tripathi et al. 2017;Khan et al. 2021;Zhang et al. 2014). The Si NPs are non-toxic partials that increase the silicon level in plants under HMs stress. In this trial, a higher dose of SiNPs remained outstanding by recording a significant increase in root and shoot lengths under varying levels of induced stress. It might be ascribed to a significant increase in Si content within the tissues of roots and shoot by foliarapplied SiNPs which might have suppressed further uptake of Cd and resultantly shoot and root growth was enhanced as inferred by .
The results also revealed that the highest level of Cd under investigation seriously reduced plant growth which might be attributed to the accumulation of Cd in the cell vacuole leading to the dis-functioning of the cells. Interestingly, SiNPs applied as foliar spray remained effective in mitigating the adverse effects of Cd stress which might be attributed to improved nutrient uptake triggered by Si as reported by Abd El-Mageed et al. (2020) andSeregin et al. (2004). Additionally, it was also inferred that silicon provided to the plant triggered the growth by reducing the adverse effect of HMs, while underlying mitigation mechanisms are still needed to be explored ( Rastogi et al. 2019). Moreover, it has been concluded that Cd stress caused maize plants to increase their root length for absorbing nutrients in high quantities which assisted crop plants to ameliorate Cd toxicity (Siddiqui et al. 2020). Furthermore, Dresler et al. (2015) have also opined that greater root length of maize under Cd stress also resulted in higher root fresh and dry weights and it was also inferred that Si application remained effective in boosting roots weight by improving the absorption of numerous macro-and micro-nutrients.
The results of our trial showed that foliage-applied SiNPs especially the dose of 400 ppm remained effective under Cd stress for boosting leaf pigments including chlorophyll a, b, total chlorophyll contents, and carotenoids. This increment might be attributed to Si-induced increment in the photosynthetic process owing to the higher uptake of nutrients by the xylem which increases the chlorophyll b content (Khan et al. 2019). Additionally, SiNPs treatment under Cd stress remained effective in boosting the carotenoid content and these findings are in concurrence with (Ling et al. 2017) who opined that Si application might serve as an effective strategy to improve the biosynthesis of carotenoids. Moreover, SiNPs showed a significant impact on the concentration of biochemical traits of maize such as soluble proteins, proline, soluble amino acid, and sugar content. Previously, it has been established that under heavy metals stress, protein integrity and persistent biosynthesis play an important role in maintaining the structure and function of cells. Likewise, all the plant's enzymes are formed by proteins which elaborate the critical role of protein synthesis under HM stress. The Cd stress noticeably reduced the protein concentration in the plants which led to stunted growth of crop plants. The result showed the Si NPs increased the proteins and decreased the Cd level. The Cd decreases the synthesis of proteins and reduces the work of proteins. The Si act as a barrier against cell injury and has been considered responsible to repair cell membranes (Khan et al. 2019).
The proline plays a vital role in plants and also offers protection against HMs stress. When Cd stress occurs the proline act as a barrier and reduced the Cd stress to promote plant growth. There is the highest value of total proline was recorded in the treatment where Si NPs were applied at the rate of 400 ppm while the lowest value of total proline was noted in the treatment where no Si NPs were applied. These findings are in line with those of (Hayat et al. 2012) who inferred that Cd stress reduced proline content and Si application remained effective in triggering the biosynthesis of proline and ultimately plants were enabled to survive HMs toxicity. Similarly, the maize plant showed a high level of amino acids when SiNPs were applied in a higher dose as a foliar spray, while Cd stress reduced the concentration of the amino acid. It has been inferred that reduction in the amino acids also hampered the biosynthesis of proteins and resultantly HMs toxicity imparted significant damages to cell membranes.
In this study, SiNPs had a pronounced effect on the sugar level of maize plants. Sugars are involved in critical metabolic processes and are also responsible for signal-regulating genes involved in the photosynthesis process and sucrose metabolism. The SiNPs under Cd stress also increased the antioxidant concentration in the plants which enable plants to cope with adverse effects imparted by HMs toxicity. Moreover, SiNPs applied as foliar spray proved their effectiveness by increasing the concentration of POD which protect plants against the negative effects of abiotic stresses like HMs toxicity. In the result, the highest value of peroxidase dismutase was recorded in the treatment where Si NPs were applied at the rate of 400 ppm while the lowest value of peroxidase dismutase was noted in the treatment where no Si NPs were applied. Different levels of Cd stress significantly affect the peroxidase dismutase of maize. The maximum value of peroxidase dismutase was recorded where Cd was not applied such as in the control treatment while the lowest value was noted in treatments where Cd was applied at the rate of 30 ppm (Thind et al. 2021).
Besides, SiNPs application in higher doses increased the CAT concentration which is an antioxidant enzyme which makes plants energy efficient under environmental extremities. Additionally, the CAT facilitates the catalyzation of the H 2 O 2 molecules into O 2 and H 2 O which improves photosynthetic efficiency. As a result, the highest value of catalase was recorded in the treatment where Si NPs were applied at the rate of 400 ppm while the lowest value of catalase was noted in the treatment where no Si NPs were applied. Different levels of Cd stress significantly affect the catalase of maize. The maximum value of catalase was recorded where Cd was not applied such as in the control treatment while the lowest value was noted in treatments where Cd was applied at the rate of 30 ppm. Similar findings were reported by (Lukačová et al. 2013) who inferred that Si application increased CAT concentr4ation under HMs stress. The Si NPs treatment increased the APX (Antioxidant enzyme Ascorbate peroxidase) concentration in maize leaves under Cd stress. As a result, the highest value of ascorbate peroxidase was recorded in the treatment where Si NPs were applied at the rate of 400 ppm while the lowest value of ascorbate peroxidase was noted in the treatment where no Si NPs were applied. Different levels of Cd stress significantly affect the ascorbate peroxidase of maize. The maximum value of ascorbate peroxidase was recorded where Cd was not applied such as in the control treatment while the lowest value was noted in treatments where Cd was applied at the rate of 30 ppm. The APX is increased by the Si NPs under the stress of Cd (Thind et al. 2021).
The SOD is an enzyme found in all living plant cells. This is responsible for the speed of chemical reactions and speeds up the chemical reaction in the cells. The SOD breakdown the harmful molecules in the cell to prevent cell damage under stress conditions in the experiment result the highest value of superoxide dismutase was recorded in the treatment where Si NPs were applied at the rate of 400 ppm while the lowest value of superoxide dismutase was noted in treatment where no Si NPs was applied. Different levels of Cd stress significantly affect the superoxide dismutase of maize. The maximum value of superoxide dismutase was recorded where Cd was not applied such as in the control treatment while the lowest value was noted in treatments where Cd was applied at the rate of 30 ppm. The concentration of SOD was increased by the SiNPs treatment under Cd stress. Different levels of Si NPs that were applied in the form of foliar spray highly significantly exaggerated the Cd in the leaf and root of maize and showed that more Cd contents in leaves and roots were noted in the controlled group in which foliar Si NPs were not applied while the lowest value of Cd content in leaves and roots was recorded where the Si NPs foliar was applied in the form of foliar at 400 ppm in the stress conditions of Cd (Shafeeq-ur-Rahman et al. 2020).

Conclusions
The research findings remained in line with the postulated hypothesis because different Cd levels restricted morphological and physiological attributes of maize while Si-based nanoparticles remained effective in mitigating the adverse effects of Cd stress. Based on the recorded findings of this trial, it might be inferred that severe Cd stress significantly reduced the morphological traits (shoot length, shoot fresh weight, and shoot dry weight), leaf pigments (chlorophyll a, b and total contents and carotenoid contents), and biochemicals (TSP, TP, TSAA, and TSS) and antioxidants concentration (POD, CAT, APX, and SOD). In contrast, Cd content remained higher in the root and shoot of maize plants, while a higher dose of SiNPs (400 ppm) surpassed the rest of the doses in terms of ameliorating the adverse effects of Cd stress. Thus, SiNPs foliar application holds bright perspectives in mitigating Cd toxicity; however, there is a dire need to conduct further in-depth studies to test further doses of SiNPs along with optimizing the time of application.
Author contribution ZA and MI planned and supervised the research; SA conducted the research work and wrote the introduction part; ZA wrote the manuscript; SA and ZA did the static analysis and graphical representation; MAI and HFA read the manuscript as proofreading and arranged it according to the journal style; AA provided reagents and assisted in the analytical work; and AH and AES did the editing and improved the English language quality of the manuscript.
Funding This work was funded by Institutional Fund Projects under grant No. (IFPIP: 439-130-1443), Ministry of Education in Saudi Arabia.

Data availability
The raw data are available and will be provided on demand.

Declarations
Ethical approval All authors approve this manuscript for submission to Environmental Science and Pollution Research.
Consent to participate All authors participate in the preparation of the manuscript.

Competing interests
The authors declare no competing interests.