Introduction

Plants often face a wide range of environmental stresses, which may be broadly categorized into biotic and abiotic types. Among the abiotic ones, heavy metals toxicity, water scarcity, and temperature extremes are the most common stresses that seriously impede plant growth and productivity1. Unlike animals, plants are sessile and must face and tolerate such environmental stresses, which strongly affect their physiological functions and overall development2. Heavy metal contamination of soils has been recognized as one of the critical environmental issues in recent years globally3. Toxic levels of heavy metals severely impact plant growth, morphology, and reproductive development4. In addition to disrupting plant metabolism, heavy metals alter nutrient cycling in soils by impairing microbial activity and altering biogeochemical processes5.

Amongst these, chromium (Cr) is considered one of the most toxic elements to plants, animals, and human beings6. Soil chromium contamination has become one of the fastest-growing ecological issues associated with widespread applications in industries. The hexavalent form, Cr(VI), is more toxic than the trivalent form, Cr(III), due to its higher solubility and mobility, which makes it more readily absorbed by plant roots6,7. Cr is extensively used in mining, electroplating, and especially in the leather tanning industry, which discharges large quantities of Cr(VI) into surrounding soils and water bodies8. In agricultural soils, particularly those prone to flooding, Cr(III) is the predominant form, though both species pose risks9.

Chromium contamination is a global problem10. Elevated Cr levels have been reported in agricultural soils near industrial zones in countries such as China, India, Pakistan, the United States, and various parts of Europe11,12. In Asia, leather and electroplating industries contribute significantly to soil Cr accumulation13. In the United States, Cr pollution has been documented near mining and chemical manufacturing sites14. According to global assessments, Cr is one of the top five toxic heavy metals affecting soil and water quality worldwide15.

Cr stress causes many physiological disturbances in plants, including oxidative damage and disruption of cellular membranes. Cr reduces the availability of essential nutrients, interferes with photosynthesis, and suppresses metabolic processes such as seed germination and energy production16,17,18. It also induces overproduction of reactive oxygen species (ROS), leading to oxidative stress, lipid peroxidation, and damage to cellular organelles.

These challenges have recently been tackled with novel solutions provided by advances in nanotechnology. Nanoscience approaches to farming have transformed crop protection and productivity with the development of nanopesticides, nanosensors, and nanofertilizers19,20. Among others, nanoparticles, particularly metal oxide nanoparticles, have shown great potential in alleviating abiotic stresses through improved nutrient use efficiency, enhanced plant defenses, and reduced metal toxicity21,22. Among these, silica nanoparticles (SiO2 NPs) have gained attention due to their biocompatibility and beneficial effects on plant health. SiO2 NPs have been reported to alleviate aluminum toxicity in Cicer arietinum by enhancing photosynthesis, promoting plant growth, and facilitating silicon uptake23,24. These particles also enhance antioxidant activity, reduce the accumulation of toxic metals, and improve stress tolerance by strengthening the plant’s defense mechanisms23. Although various other metallic nanoparticles such as zinc, iron, and selenium nanoparticles have been reported to alleviate heavy metal stress in plants25,26,27. The rationale behind selection of SiO2 NPs lies in their unique and advantageous physiological and defense related properties. Silicon (Si) is known to strengthen cell wall, improve photosynthesis and enhance tolerance against heavy metal stress28. In particular, SiO2 NPs due to their high stability, low toxicity and excellent biocompatibility are efficiently taken up and translocated with in plants tissues. Furthermore, SiO2 NPs can reduce metal uptake in roots and translocation towards shoots by making complexes with metal ions that immobilize their movement in aerial parts29. SiO2 NPs are also involved in production of phytochelatins and organic compounds which can bind to metal ions and sequester these toxic ions into less harmful forms30. SiO2 NPs can also boost antioxidant defense by stimulating activities of enzymatic antioxidants which are involved in reducing oxidative damage caused by excessive ROS generation under heavy metal stress31. SiO2 NPs can also prevent ions leakage by stabilizing cellular membranes and cell structures32. Given these unique properties, SiO2 NPs represent a promising and eco-friendly approach for mitigating Cr stress in plants. Despite this all, role of SiO2 NPs in alleviating Cr stress in marigold plant least explored.

Tagetes erecta L., commonly known as Mexican marigold, belongs to the Asteraceae (Composite) family and is widely cultivated as an economically important ornamental plant. The genus Tagetes includes approximately 40–50 species33. Marigold flowers are rich in carotenoids and phenolic compounds, giving them therapeutic potential against various human diseases34. Lutein is a xanthophyll pigment that gives the flower its characteristic orange-yellow color and is an important factor in eye health by protecting the retina from oxidative damage35. Due to environmental concern for Cr contamination and economic and medicinal importance of T. erecta, the present study was undertaken to investigate the physiological and biochemical responses of T. erecta under chromium stress. Specifically, the study evaluated the efficiency of exogenous application of silica nanoparticles (SiO2 NPs) in mitigating Cr-Induced toxicity through enhancement in growth, photosynthetic pigments, antioxidant defense system, and nutrient uptake. This study provides insights into the mechanistic role of SiO2 NPs in mitigating heavy metal stress and enhancing plant resilience under soil contamination.

Materials and methods

Preparation of silica nanoparticles (SiO 2 NPs)

Synthesis of SiO2 NPs was conducted by a sol-gel technique using sodium silicate (Na2SiO3) as a precursor. In the synthesis, 10 mL of Na2SiO3 was added to 500 mL of distilled water and continuously stirred by a magnetic stirrer for 4 h. At the same time, 0.5 M HCl was prepared in 1 L of distilled water. This HCl solution was added drop by drop in the sodium silicate mixture under continuous stirring until the pH dropped from 11.8 to 7, which indicated the neutralization and formation of gel. The solution was allowed to react completely over 4 h, then left undisturbed overnight for the formation of silica gel. The silica gel was washed several times with distilled water to eliminate residual acidity, poured into a petri dish, and air-dried at room temperature. The gel was then dried in an oven at 60°C for 12 h, after which the dried mass was ground into a fine powder to obtain SiO2 NPs (see Fig. S1 in supplementary file). To confirm successful synthesis and structural integrity of SiO2 NPs, multiple characterization techniques were employed including; SEM, EDX, UV-VIS spectrophotometry, FTIR and XRD.

Experimental design and layout

A two-factor completely randomized design (CRD) experiment was conducted with three replicates at the Botanical Garden, University of Education, Lahore, Pakistan, under natural environmental conditions during September 2023. The average relative humidity during the experimental period was 55%, with day/night temperatures of approximately 35/25°C. Seeds of Tagetes erecta L. were procured from Punjab Seeds Centre, Lahore, and surface-sterilized prior to sowing in pots (15 cm in height, 20 cm in diameter) filled with uncontaminated garden soil. Treatments applied in this experiment were as follows; T1 = Control, T2 = Cr-I (CrCl3; 50 mg kg−1), T3 = Cr-II (CrCl3; 100 mg kg−1), T4 = NPs-I (SiO2 NPs; 100 mg L−1), T5 = NPs-II (SiO2 NPs; 200 mg L−1), T6 = Cr-I + NPs-I, T7 = Cr-I + NPs-II, T8 = Cr-II + NPs-I, and T9 = Cr-II + NPs-II. Chromium (Cr) stress was induced by adding chromium chloride (CrCl3), purchased from Sigma-Aldrich, at concentrations of 50 and 100 mg kg−1 soil. This controlled pot-level application was conducted solely for experimental purposes to simulate Cr toxicity in a closed system. Silica nanoparticles (SiO2 NPs) were synthesized and applied via foliar spray at concentrations of 100 and 200 mg L−1 using a manual hand sprayer. Tween-20 (2%) was used as a surfactant for better adhesion and coverage. Spray applications were made in calm conditions and close to the target so that there could be minimum dispersion and only targeted delivery of the spray solution was given. This foliar application was done three times: first, at the 4-leaf stage and further at one-week intervals. Uniform volumes of spray solutions per pot were used until full leaf wetting was visibly confirmed to ensure uniform exposure with thorough foliar uptake. The mode of application was a foliar one, which was used instead of soil application for the reason of not allowing the nanoparticles to be fixed or accumulated in the soil, and thus, cells may absorb them more efficiently through stomata and cuticular absorption. All the growth, physiological, and biochemical measurements were recorded 28 days after the first foliar application. Phytoremediating grass Cynodon dactylon and lime amendments were given to the post-harvest soil to immobilize and reduce Cr bioavailability. Then, these soils were sealed and stored further to be detoxified. The contaminated soil thus had a secured disposition without causing its release into the environment.

Chlorophyll and carotenoid content determination

Chlorophyll and carotenoid contents were determined following the method described by Arnon36. Fully expanded, freshly harvested leaves from each replicate were weighed (0.5 g) and ground in a mortar and pestle with 10 mL of 80% acetone. The homogenate was filtered through Whatman No. 42 filter paper, and the filtrate was stored at 4 °C for 24 h. Absorbance readings were taken at 480, 645, and 663 nm using a spectrophotometer. Carotenoid content was calculated using the formula described by Lichtenthaler37.

Soil plant analysis development (SPAD) value measurement

SPAD values were measured using a SPAD meter. One mature leaf was selected from each pot, and readings were taken from three positions on the leaf (upper, middle, and lower regions). The mean of these three readings was recorded as the SPAD value for each sample38.

Chlorophyll fluorescence measurement

Chlorophyll fluorescence characteristics and rapid light response curves were measured using an OS30p + Chlorophyll Fluorometer (Opti-Sciences, Inc., Hudson, NH, USA). One leaf per replicate was dark-adapted for 15 min prior to measurement. After dark adaptation, maximum quantum efficiency of PSII photochemistry (Fv/Fm) was recorded without exposing the leaves to light39.

Relative water content (RWC) assessment

Leaves of similar size were selected, immediately weighed (fresh weight), and floated in distilled water at 25–26 °C for 3 h to achieve full turgidity. The turgid weight was recorded, after which the leaves were dried in an oven at 80 °C for 24 h to determine dry weight. Relative water content was calculated using the formula provided by Jones and Turner40.

Relative membrane permeability (RMP) determination

Relative membrane permeability was calculated as following the method mentioned in Mirrani et al.41.

$$\:RMP\:\left(\%\right)\:=\:\left[\right({EC}_{1}-{EC}_{0})/({EC}_{2}-{EC}_{0}\left)\right]\:\times\:\:100$$

Hydrogen peroxide (H2O2) determination

Hydrogen peroxide content was estimated using the method described by Velikova et al.42. Fresh leaves (0.5 g) were homogenized in 5 mL of 0.1% (w/v) trichloroacetic acid in a pre-chilled mortar and pestle. The extract was centrifuged at 12,000 g for 15 min at 4 °C. The supernatant (0.5 mL) was mixed with 0.5 mL of 10 mM potassium phosphate buffer (pH 7.0) and 1 mL of 1 M potassium iodide. Absorbance was measured at 390 nm using a spectrophotometer.

Malondialdehyde (MDA) content determination

MDA content, an indicator of lipid peroxidation, was determined following a modified version of Cakmak and Horst’s method43. Fresh leaf tissue (0.5 g) was homogenized in 3 mL of 1% (w/v) trichloroacetic acid (TCA) at 4 °C. The homogenate was centrifuged at 20,000 g for 15 min, and 0.5 mL of the supernatant was mixed with 3 mL of 0.5% (w/v) thiobarbituric acid (TBA) prepared in 20% TCA. The mixture was heated at 95 °C for 50 min in a water bath, then rapidly cooled in an ice bath. After centrifugation at 10,000 g for 10 min, absorbance was measured at 532 and 600 nm. MDA content (nmol g−1 FW) was calculated using the following formula:

$$\:MDA\:\left(nmol\right)\:=\:\left[\right({A}_{532}\:-\:A_{600})\:/\:1.56\:\times\:\:10^{5}]\:\times\:\:V/W\:\times\:\:10^{6}$$

Where V = extract volume (mL) and W = fresh weight of the tissue (g).

Assessment of total soluble protein content

Fresh leaf samples (0.5 g) from each replicate were homogenized in 10 mL of 50 mM phosphate buffer (pH 7.8) under chilled conditions. The homogenate was centrifuged at 6,000 g for 20 min at 4 °C, and the supernatant was stored in a deep freezer until analysis. Total soluble protein content was estimated using the Bradford’s method. The Bradford reagent was prepared by dissolving 100 mg Coomassie Brilliant Blue in 50 mL of 95% ethanol, followed by the addition of 100 mL of 85% phosphoric acid, and bringing the volume to 1 L with distilled water. For protein quantification, 0.1 mL of the plant extract was added to 5 mL of Bradford reagent, and absorbance was recorded at 595 nm using a spectrophotometer44.

Determination of catalase (CAT) activity

Catalase activity was assayed by adding 0.1 mL of leaf extract to 1.9 mL of 5.9 mM H2O2 and 1.0 mL of 50 mM phosphate buffer, pH 7.0. The reduction in absorbance was measured at 240 nm at an interval of 30 s for 120 s using a spectrophotometer45.

Determination of peroxidase (POD) activity

For POD activity, the reaction mixture consisted of 0.1 mL of leaf extract, 0.6 mL of 20 mM guaiacol, 0.7 mL of 50 mM phosphate buffer (pH 5.0), and 0.6 mL of 40 mM H2O2. Absorbance was measured at 470 nm at 30-second intervals for 150 s46.

Determination of ascorbate peroxidase (APX) activity

For the determination of APX activity, the 3.0 mL reaction mixture for every replicate contained 2.70 mL of 50 mM phosphate buffer at pH 7.0, 0.10 mL of 7.5 mM ascorbic acid, 0.10 mL of 300 mM H2O2, and 0.10 mL of enzyme extract. Absorbance was measured at 290 nm at 30-second intervals up to 60 s47.

Determination of superoxide dismutase (SOD) activity

The reaction mixture for SOD activity determination contained 0.3 mL of 130 mM methionine, 50 µM nitro blue tetrazolium (NBT), 100 µM EDTA-Na2, and 20 µM riboflavin each. To this, 0.05 mL of enzyme extract was added. The reaction mixture was exposed to 4000 lx light for 20 min, and absorbance was recorded at 560 nm48.

Assessment of inorganic mineral ion content and cr uptake

Dried shoot and root samples were ground to a fine powder using a mortar and pestle. To this, 0.1 g of powdered sample was taken into test tubes and 2 mL of concentrated H2SO4 was added. After incubation for 24 h, samples were heated on a hot plate, adding H2O2 gradually until the solution became colorless. Digested samples were diluted with distilled water to 50 mL. The concentrations of Na+, K+, and Ca2+ were analyzed using a flame photometer. For Cr uptake, the dried root and shoot samples were first ground to a fine powder and then digested in a mixture of nitric acid (HNO3) and perchloric acid (HClO4) in a 3:1 ratio using standard wet digestion protocols for heavy metal analysis. The digested samples were filtered and brought to a fixed volume with deionized water. The concentrations of Cr in the prepared root and shoot extracts were then determined using an atomic absorption spectrophotometer (Hitachi Polarized Zeeman AAS, Z-8200, Japan) under the instrument operating conditions optimized for chromium detection, as described by Shah et al.49.

Statistical analysis

Graphical representations were generated using R software (Version 2024.12.1 + 563). Data are presented as the mean ± standard error (SE) of three replicates. Statistical significance among treatment means was determined using Duncan’s Multiple Range Test (DMRT) at p ≤ 0.05. Different letters above the bars indicate statistically significant differences among means.

Results

Characterization of silica nanoparticles

Scanning electron microscopy (SEM) analysis

SEM images provide three-dimensional, high-resolution images important for applications in chemistry, materials science, and nanotechnology. An electron beam is applied on the surface of the nanoparticle; it releases an electron that is collected to make an image. The shape of SiO2 NPs depends on the method of synthesis. Results revealed that, the size of SiO2 NPs is 20–38.5 nm with more or less spherical shape (Fig. 1A).

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.
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Characterization of synthesized SiO2 NPs. (A) SEM micrograph of silica nanoparticle, (B) EDX analysis of silica nanoparticle, (C) UV-VIS spectrophotometry showing the optical absorption spectra of silica nanoparticles, (D) FTIR spectra of silica nanoparticle and (E) XRD graph showing the size of silica nanoparticles.

Energy-dispersive X-ray (EDX) spectroscopy

The elemental composition of nanoparticles can easily be determined by the utilization of the EDX technique in our synthesis, EDX analysis was used to validate the presence of silica in nanoparticles sample synthesized by the sol-gel method. The percentage of silicon is 29.52% and oxygen is 45.09% respectively. The highest peaks are of silicon and oxygen as compared to other additional components confirming the presence of silica (Fig. 1B).

Ultraviolet-visible (UV-Vis) spectroscopy

UV-Vis spectroscopy is commonly used to determine the size of nanoparticles because nanoparticle size influences their absorption properties while nanoparticles are smaller and their absorption spectra shift to shorter wavelengths. As in our results, the peak is found at 277 nm which proves that the size of silica nanoparticles lies within the range of 100 nm (Fig. 1C).

Fourier transform infrared (FTIR) spectroscopy

FTIR analysis is used to identify the functional groups of metabolites involved in the synthesis of nanoparticles. Figure 1D presents the FTIR analysis results for silica nanoparticles within the wavenumber range of 4000 –540 cm−1. The spectrum shows various peaks corresponding to different functional groups. Distinct absorption bands between 3500 and 2000 cm−1 indicate characteristic OH and N-H stretching of aldehydes. The peak at 3243 cm−1 is attributed to polyphenols, the OH groups of alcohols, or amines. Peaks in the region of 2500 to 2000 cm−1 signify C ≡ C and C ≡ N vibrations. Absorption bands between 1400 and 1000 cm−1 are due to methylene and C-N stretching vibrations. The peak at 1068 cm−1 reported in the literature for the formation of silica nanoparticles is shown here. The combined spectral features indicate the presence and participation of various functional groups on the surface of the silica nanoparticles regarding their composition and molecular interaction (Fig. 1D).

X-ray diffraction (XRD) analysis

The X-ray diffraction pattern of Fig. 1E gives useful information on the crystalline nature of the silica nanoparticles. It is characterized by well-defined broad peaks representing a nanocrystalline phase. The primary diffraction peaks are observed at 2θ values of 21.93°, 28.41°, 31.55°, 36.11°, 41.36°, 45.39°, 48.41°, 53.67°, and 56.78°, corresponding to the (101), (111), (102), (200), (201), (112), (202), (113), and (220) planes, respectively. These peaks confirm the crystalline structure of the silica nanoparticles, which agrees with the Joint Committee on Powder Diffraction Standards database, file No. 39-1425.

Application of the Scherrer formula allowed the average crystallite size for the synthesized silica nanoparticles to be estimated at 38.5 nm. This X-ray diffraction analysis provided essential information about the crystallinity and structure of the synthesized silica nanoparticles (Fig. 1E).

Effects of SiO2 NPs on growth parameters of T. erecta under cr stress

Chromium (Cr) stress significantly reduced the growth of T. erecta plants, which was effectively alleviated by the SiO2 NPs application (Fig. 2). The shoot length under Cr stress was reduced by 21% and 67% in Cr-I and Cr-II treatments compared with their controls. Contrarily, foliar application of SiO2 NPs (NPs-I and NPs-II) increased shoot lengths by 35% and 57%, respectively compared to their control. Application of SiO2 NPs in Cr-contaminated soil enhanced shoot growth of plants grown under Cr stress compared to those subjected to Cr stress alone (Fig. 3A). Similarly, root length was inhibited by 46 and 68% in the Cr-I and Cr-II treatments, respectively, compared to untreated controls. In contrast, NPs-I and NPs-II enhanced the root length by 9 and 35%, respectively, in non-stressed conditions, and similar trends were found in Cr-polluted soil (Fig. 3B).

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.
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Effect of SiO2 NPs on growth of T. erecta plants under Cr stress. Where; Cr 1 = CrCl3 50 mg Kg−1 of soil, Cr 2 = CrCl3 100 mg Kg−1 of soil, NP 1 = SiO2 NPs 100 mg L−1 and Np 2 = SiO2 NPs 200 mg L−1.

Fig. 3
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Effect of SiO2 NPs application on growth of T. erecta plants under Cr stress. (A) shoot length, (B) root length and (C) number of leaves. Graph bars presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

Cr toxicity also led to a significant decline in the number of leaves, with 32% and 40% reductions under Cr-I and Cr-II stresses, respectively, compared with the control. Treatments with NPs-I and NPs-II increased the number of leaves by 28% and 42%, respectively, over the control. Moreover, SiO2 NPs markedly improved leaf number in Cr-stressed plants, showing increases of 69% and 82% in Cr-I conditions, and 118% and 136% under Cr-II stress, respectively, compared with the corresponding Cr-only treatments (Fig. 3C).

Cr stress significantly reduced the biomass of T. erecta plants. Shoot fresh weight (SFW) declined by 18% and 68% under Cr-I and Cr-II treatments, respectively, compared with non-stressed control plants. Foliar application of SiO2 NPs improved SFW under both non-stress and Cr-stress conditions. Under non-stress conditions, NPs-I and NPs-II increased SFW by 37% and 63%, respectively, compared with untreated plants. In Cr-I contaminated soil, SFW increased by 32% with NPs-I and 43% with NPs-II compared with Cr-I alone. Similarly, in Cr-II contaminated soil, SFW increased by 232% with NPs-I and 293% with NPs-II relative to Cr-II only plants (Fig. 4A).

Fig. 4
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Effect of SiO2 NPs application on biomass of T. erecta plants under Cr stress. (A) shoot fresh weight, (B) shoot dry weight, (C) root fresh weight and (D) root dry weight. Graph bars presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

Shoot dry weight (SDW) was also adversely affected by Cr stress, decreasing by 34% and 80% under Cr-I and Cr-II, respectively, compared with control plants. Under non-stress conditions, NPs-I and NPs-II increased SDW by 24% and 43%, respectively. In Cr-I contaminated soil, SDW improved by 27% with NPs-I and 39% with NPs-II relative to Cr-I alone. In Cr-II contaminated soil, supplementation with NPs-I and NPs-II increased SDW by 148% and 175%, respectively, compared with Cr-II only plants (Fig. 4B).

Cr stress significantly reduced the root biomass of T. erecta plants. Root fresh weight (RFW) declined by 14% and 37% under Cr-I and Cr-II treatments, respectively, compared with the non-stressed control. Under non-stress conditions, foliar application of SiO2 NPs increased RFW by 42% with NPs-I and 60% with NPs-II relative to untreated plants. In Cr-I contaminated soil, RFW improved by 30% with NPs-I and 50% with NPs-II compared with Cr-I alone. Similarly, in Cr-II stressed plants, RFW increased by 74% and 79% with NPs-I and NPs-II, respectively, relative to Cr-II only plants (Fig. 4C).

Root dry weight (RDW) was also negatively affected by Cr stress, decreasing by 63% and 79% under Cr-I and Cr-II, respectively, compared with the control. Under non-stress conditions, RDW increased by 56% with NPs-I and 89% with NPs-II. In Cr-I contaminated soil, foliar application of NPs-I and NPs-II enhanced RDW by 48% and 65%, respectively, relative to Cr-I alone. In Cr-II stressed plants, RDW increased by 138% with NPs-I and 162% with NPs-II compared with Cr-II only plants (Fig. 4D).

Impact of SiO2 NPs on photosynthetic pigments of T. erecta under cr toxicity

Soil contaminated with Cr significantly reduced the photosynthetic pigments in T. erecta plants. Chlorophyll a content decreased by 20% and 30% under Cr-I and Cr-II treatments, respectively, compared with the non-stressed control. Foliar application of SiO2 NPs increased chlorophyll a under non-stress conditions by 10% with NPs-I and 25% with NPs-II relative to untreated plants. In Cr-I contaminated soil, chlorophyll a improved by 25% with NPs-I and 31% with NPs-II compared with Cr-I alone, whereas in Cr-II stressed plants, supplementation with NPs-I and NPs-II enhanced chlorophyll a by 50% and 71%, respectively (Fig. 5A).

Fig. 5
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Effect of SiO2 NPs application on physiological attributes of T. erecta plants under Cr stress. (A) chlorophyll a, (B) chlorophyll b, (C) carotenoid and (D) relative water contents. Graph bars presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

Similarly, chlorophyll b content declined by 39% and 54% under Cr-I and Cr-II treatments, respectively, compared with control plants. Under non-stress conditions, NPs-I and NPs-II increased chlorophyll b by 17% and 22%, respectively. In Cr-I stressed plants, chlorophyll b improved by 25% with NPs-I and 68% with NPs-II relative to Cr-I alone. In Cr-II contaminated soil, supplementation with NPs-I and NPs-II enhanced chlorophyll b by 162% and 181%, respectively, compared with Cr-II only plants (Fig. 5B).

Carotenoid content was also reduced by Cr stress, decreasing by 24% and 34% under Cr-I and Cr-II treatments, respectively, relative to control. Foliar application of NPs-I and NPs-II increased carotenoid content by 14% and 16%, respectively, under non-stress conditions. In Cr-I contaminated soil, carotenoids increased by 18% with NPs-I and 41% with NPs-II compared with Cr-I alone, whereas in Cr-II stressed plants, supplementation with NPs-I and NPs-II enhanced carotenoid content by 63% and 75%, respectively, relative to Cr-II only plants (Fig. 5C).

Effect of SiO2 NPs on relative water content (RWC) of T. erecta plants under cr stress

Cr toxicity significantly decreased the relative water content (RWC) of T. erecta plants, reducing it by 30% and 42% under Cr-I and Cr-II treatments, respectively, compared with control plants. Foliar application of SiO2 NPs under non-stress conditions increased RWC by 12% with NPs-I and 17% with NPs-II relative to untreated plants. In Cr-contaminated soil, SiO2 NPs further improved RWC, with increases of 47% and 63% under Cr-I + NPs-I and Cr-I + NPs-II, and 94% and 108% under Cr-II + NPs-I and Cr-II + NPs-II, respectively, compared with the corresponding Cr-only treatments (Fig. 5D).

Effect of SiO2 NPs on maximum quantum efficiency of PSII (Fv/Fm) and SPAD value of T. erecta plant under cr stress

The maximum quantum efficiency of photosystem II (Fv/Fm) in T. erecta plants was reduced by 27% and 29% under Cr-I and Cr-II treatments, respectively, compared with control plants. Foliar application of SiO2 NPs under non-stress conditions increased Fv/Fm by 9% with NPs-I and 14% with NPs-II relative to untreated plants. In Cr-I stressed plants, Fv/Fm improved by 25% with NPs-I and 35% with NPs-II compared with Cr-I alone. Similarly, in Cr-II contaminated soil, Fv/Fm increased by 30% and 35% with NPs-I and NPs-II, respectively, relative to Cr-II only plants (Fig. 6A).

Fig. 6
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Effect of SiO2 NPs application on (A) quantum efficiency of PSII (Fv/Fm), (B) SPAD value and (C) total soluble proteins in T. erecta plants under Cr stress. Graph bars presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

The SPAD value, indicative of chlorophyll content, decreased by 47% and 57% under Cr-I and Cr-II treatments, respectively, compared with control plants. Under non-stress conditions, foliar application of NPs-I and NPs-II increased SPAD values by 21% and 31%, respectively. In Cr-I stressed plants, SPAD increased by 28% with NPs-I and 38% with NPs-II relative to Cr-I alone, whereas in Cr-II contaminated soil, SPAD values improved by 45% and 53% with NPs-I and NPs-II, respectively, compared with Cr-II only plants (Fig. 6B).

Effect of SiO2 NPs on total soluble protien contents of T. erecta plants under cr stress

In the present study, Cr toxicity significantly reduced the total soluble protein (TSP) content in T. erecta plants, with decreases of 6% and 26% under Cr-I and Cr-II treatments, respectively, compared with control plants. In contrast, foliar application of SiO2 NPs under non-stress conditions increased TSP by 23% with NPs-I and 46% with NPs-II relative to untreated control plants. In Cr-contaminated soil, TSP improved by 18% and 30% under Cr-I + NPs-I and Cr-I + NPs-II treatments, respectively, and by 62% and 96% under Cr-II + NPs-I and Cr-II + NPs-II treatments, respectively, compared with the corresponding Cr-only plants (Fig. 6C).

Effect of SiO2 NPs on the antioxidant enzymes (CAT, POD, APX & SOD) activities in T. erecta plants under cr stress

Cr stress significantly influenced the antioxidant enzyme activities in T. erecta plants. Catalase (CAT) activity increased by 31% and 46% under Cr-I and Cr-II treatments, respectively, compared with control plants. Foliar application of SiO2 NPs under non-stress conditions enhanced CAT activity by 8% with NPs-I and 54% with NPs-II. In Cr-contaminated soil, CAT activity further increased by 5% and 19% under Cr-I + NPs-I and Cr-I + NPs-II, and by 5% and 7% under Cr-II + NPs-I and Cr-II + NPs-II, respectively, relative to the corresponding Cr-only treatments (Fig. 7A).

Fig. 7
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Effect of SiO2 NPs application on antioxidant enzymes activities in T. erecta plants under Cr stress. (A) Catalase, (B) Peroxidase, (C) Ascorbate peroxidase and (D) Superoxide dismutase. Graph bars presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

Peroxidase (POD) activity was also elevated by Cr stress, increasing by 27% and 48% under Cr-I and Cr-II, respectively, compared with control plants. Under non-stress conditions, foliar application of NPs-I and NPs-II increased POD activity by 55% and 69%, respectively. In Cr-I contaminated soil, POD activity improved by 11% with NPs-I and 44% with NPs-II relative to Cr-I alone, whereas in Cr-II stressed plants, POD activity increased by 9% and 20% with NPs-I and NPs-II, respectively, compared with Cr-II only plants (Fig. 7B).

Similarly, ascorbate peroxidase (APX) activity was elevated by 32% and 38% under Cr-I and Cr-II stress, respectively, compared with control plants. Foliar application of SiO2 NPs under non-stress conditions enhanced APX activity by 5% with NPs-I and 14% with NPs-II. In Cr-contaminated soil, APX activity further increased by 6% and 8% under Cr-I + NPs-I and Cr-I + NPs-II, and by 8% and 12% under Cr-II + NPs-I and Cr-II + NPs-II, respectively, relative to the corresponding Cr-only treatments (Fig. 7C).

Superoxide dismutase (SOD) activity increased by 15% and 37% under Cr-I and Cr-II treatments, respectively, compared with control plants. Under non-stress conditions, foliar application of NPs-I and NPs-II increased SOD activity by 34% and 54%, respectively. In Cr-contaminated soil, SOD activity was further enhanced by 26% and 42% in Cr-I + NPs-I and Cr-I + NPs-II treatments, and by 8% and 13% in Cr-II + NPs-I and Cr-II + NPs-II treatments, respectively, relative to the corresponding Cr-only plants (Fig. 7D).

Effect of SiO2 NPs on stress indicators (MDA, H2O2 & RMP) and proline contents of T. erecta plants under cr stress

Cr stress significantly increased oxidative stress markers in T. erecta plants. Malondialdehyde (MDA) content increased by 92% and 99% under Cr-I and Cr-II treatments, respectively, compared with control plants. Foliar application of SiO2 NPs under non-stress conditions slightly reduced MDA by 3% with NPs-I and 27% with NPs-II relative to control. In Cr-contaminated soil, MDA levels were further reduced by 14% and 34% under Cr-I + NPs-I and Cr-I + NPs-II, and by 18% and 24% under Cr-II + NPs-I and Cr-II + NPs-II, respectively, compared with the corresponding Cr-only plants (Fig. 8A).

Fig. 8
Fig. 8The alternative text for this image may have been generated using AI.
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Effect of SiO2 NPs application on (A) malondialdehyde, (B) hydrogen peroxide, (C) relative membrane permeability and (D) proline contents of T. erecta plants under Cr stress. Graph bars presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

Hydrogen peroxide (H2O2) content also increased under Cr stress, rising by 83% and 114% in Cr-I and Cr-II treatments, respectively, compared with control plants. Foliar application of NPs-I and NPs-II under non-stress conditions decreased H2O2 levels by 6% and 17%, respectively. In Cr-I stressed plants, H2O2 levels decreased by 19% with NPs-I and 30% with NPs-II relative to Cr-I alone, whereas in Cr-II stressed plants, H2O2 content was reduced by 21% and 37% with NPs-I and NPs-II, respectively, compared with Cr-II only plants (Fig. 8B).

Cr toxicity also increased relative membrane permeability (RMP) by 23% and 48% under Cr-I and Cr-II treatments, respectively, relative to control plants. Foliar application of NPs-I and NPs-II under non-stress conditions decreased RMP by 16% and 34%, respectively. In Cr-contaminated soil, RMP was further reduced by 19% and 35% under Cr-I + NPs-I and Cr-I + NPs-II, and by 45% and 54% under Cr-II + NPs-I and Cr-II + NPs-II, respectively, compared with the corresponding Cr-only treatments (Fig. 8C).

Proline content increased in response to Cr stress, with higher levels observed under Cr-II than Cr-I. Foliar application of SiO2 NPs further enhanced proline accumulation in both non-stressed and Cr-stressed plants. Specifically, NPs-I and NPs-II increased proline levels under Cr-I and Cr-II conditions, indicating a synergistic effect of SiO2 NPs in stress mitigation (Fig. 8D).

Influence of SiO2 NPs on nutritional content and cr uptake in T. erecta plants under cr stress

Cr toxicity adversely affected the uptake of essential minerals in T. erecta plants. Under Cr-I and Cr-II stress, the absorption of Na+ decreased by 12% and 90%, K+ by 16% and 39%, and Ca2+ by 15% and 28%, respectively, compared with control plants. Under no-stress conditions, mineral uptake was enhanced with foliar application of SiO2 NPs; both NPs-I and NPs-II increased Na+ by 41% and 51%, K+ by 35% and 52%, and Ca2+ by 16% and 36%, respectively. In Cr-polluted soil, SiO2 NPs prominently enhanced Na+, K+, and Ca2+ absorption at both Cr-I and Cr-II levels compared to their respective Cr-only treatments (Fig. 9A-C).

Fig. 9
Fig. 9The alternative text for this image may have been generated using AI.
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Effect of SiO2 NPs application on mineral ions of T. erecta plants under Cr stress. (A) calcium (Ca), (B) potassium (K), and (C) sodium (Na) ions. Graph bars presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

SiO2 NPs significantly affected Cr accumulation and translocation in T. erecta under Cr stress (Fig. 10). Under Cr-I treatment, NPs-I enhanced the root accumulation of Cr by 21%, while it reduced the shoot Cr content by 12%, leading to a 26% decrease in TF compared with Cr-I alone. Treatment with NPs-II further enhanced root Cr sequestration by 32%, decreased shoot Cr by 23%, and reduced TF by 39%. Similarly, under Cr-II stress, NPs-I increased root Cr content by 13%, decreased shoot Cr by 21%, and reduced TF by 34%. NPs-II exhibited a stronger effect, elevating root Cr by 24%, reducing shoot Cr by 26%, and decreasing TF by 43% relative to Cr-II alone (Fig. 10A-C).

Fig. 10
Fig. 10The alternative text for this image may have been generated using AI.
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Effect of SiO2 NPs application on chromium uptake in T. erecta plants under Cr stress. (A) Chromium in root, (B) chromium in shoot and (C) translocation factor. Data in line graph presented the average of three replicates. Error bars presented standard error. Graph bars sharing same letters are not significantly different at p ≤ 0.05.

Pearson’s correlation and principal component analysis (PCA)

Pearson’s correlation in Fig. 11 revealed that growth and biomass related parameters were positively correlated with chlorophyll pigments and essential mineral ions. This showed that SiO2 NPs application improved photosynthesis related attributes and modulated mineral ion uptake. This ultimately upheld growth of T. erecta plants. On other side, growth related parameters were negatively correlated with stress indicators of T, erecta plants. It is thus depicted that Cr stress imposed oxidative damage to plants in the form of elevated MDA level, lipid peroxidation and permeability loss. This ultimately hampered plant growth and development. From this it can be assured that SiO2 NPs were much effective in alleviation of Cr imposed toxicity in T. erecta plants.

Fig. 11
Fig. 11The alternative text for this image may have been generated using AI.
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Pearson’s correlation for all studied parameters of T. erecta plants treated with foliar spray of SiO2 NPs (100 and 200 mg L−1) in control and Cr stress (50 and 100 mg Kg−1 of soil). (Various abbreviations used are as follows; RL root length, RFW root fresh weight, RDW root dry weight, SL shoot length, SFW shoot fresh weight, SDW shoot dry weight, nL number of leaves, CAT catalase, POD peroxidase, APX ascorbate peroxidase, SOD superoxide dismutase, MDA malondialdehyde, H2O2 hydrogen peroxide, RMP relative membrane permeability, TSP total soluble proteins, Chl chlorophyll, RWC relative water content, K potassium, Na sodium, Ca calcium, SPAD soil plant analysis development, Fv/Fm maximum quantum efficiency of PSII, Cr chromium, TF translocation factor).

PCA (Fig. 12A & B) also validated these findings. It showed that all the studied variances in this work were explained in 8 principal components (PCs) out of which the first two had maximum contribution 92.01% in total (Fig. 12A). Figure 12B showed that all the treatments applied in this study were also successfully distributed in the first two PCs. As shown in the graph, Cr-1 and Cr-2 are separated well from the other treatments. This showed that both stress levels had great influence on all studied attributes of T. erecta plants. PCA of parameters (Fig. 12B) depicted that studied parameters were divided into two groups. Parameters of first group were aligned with PC1 and parameters of second group were aligned with PC2. Parameters of PC1 including growth, physiological and mineral ions attributes had positive correlation with each other but had negative correlation parameters of PC2 mainly stress indicators of T. erecta plants.

Fig. 12
Fig. 12The alternative text for this image may have been generated using AI.
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Principle component analysis (PCA). (A) Percentage of explained variables (parameters) in all principal components, (B) PCA biplot (various abbreviations used are same as described in Fig. 11).

Discussion

In plants, the presence of chromium can be utilized to ascertain the level of stress induced by toxic metals. Normal growth indicates a reduced amount of heavy metals in the plants, and these metals often have fewer harmful impacts on the growth of plants. In the current research, exogenously applied SiO2 NPs significantly minimize the effect of chromium on Tagetes erecta (marigold). We examined how the growth of T. erecta was affected by chromium stress against SiO2 NPs. Our results showed that the growth of T. erecta was hampered due to the presence of Cr in the soil. The height of root and shoot and the fresh and dry weight of roots and shoots were greatly affected by the presence of Cr ions in the region of roots. This Cr metal replaces the essential ions and enters the plant via roots to deactivate the plant’s regular function, resulting in lower growth. Wu et al.50 clearly mention that the growth of peas was inhibited when Cr stress was applied. Fawe et al.51 explained that when SiO2 NPs were applied to gramineous plants, they made a thin layer on the plant, and that layer protected them from environmental changes. In the present work, the damage that occurred due to Cr was mitigated by providing SiO2 NPs; they entered the plant through aerial parts and shielded the plant from damage caused by oxidation reactions, hence encouraging the development of T. erecta52. Emamverdian et al.53 also revealed that when bamboo (P. pygmaeus) was subjected to lead (Pb) stress, SiO2 NPs preserved its cellular structure and enhanced growth characters.

Chromium, a non-essential and toxic heavy metal, has shown deleterious effects on the growth and productivity of T. erecta in the research area. It interferes with nutrient uptake and causes physiological and biochemical imbalances, which ultimately reduce plant vigor54. Our observations confirmed that Cr exposure significantly stunted shoot and root elongation, reduced biomass accumulation, and hampered pigment synthesis, thereby impairing photosynthetic efficiency and overall productivity. These findings align with earlier studies reporting similar toxicity responses in other species such as maize55,56. Such stress-induced damages to T. erecta under chromium exposure underscore the need for effective mitigation strategies in areas where chromium contamination threatens ornamental and economic plant species57.

The rate of photosynthesis decreases as the plant is subjected to Cr stress58. The presence of photosynthetic pigments is crucial for enabling plants to effectively harness light energy to form required products. Our findings also describe that photosynthetic pigments were reduced because Cr disturbs the mechanism for the production of above-mentioned pigments when a plant undergoes chromium-induced stress. Then these pigments diminished their activity, resulting in a low photosynthetic rate as well as stunted growth59. The results were reversed by the addition of SiO2 NPs. Plants with SiO2 NPs had significant values of chl a, chl b, and carotenoid pigments. According to Rizwan et al.24, when plants were exposed to Cd, chlorophyll contents such as chl a, chl b, and carotenoids were significantly increased by the supplementation of SiO2 NPs in rice plants.

Plants of P. oleracea were subjected to Cr stress, and they experienced elevated levels of MDA and H2O2. In our work, the levels of MDA and H2O2 were also high due to the higher amount of Cr in the plants. Increased levels of MDA and H2O2 indicate that the plants are encountering oxidative stress. The higher values of MDA and H2O2 mean a higher degree of oxidative damage; an increment in these values proved not to be beneficial, rather it damages the cell membrane and eventually causes cellular death. The roots and leaves are the major organs that experience higher levels of MDA and H2O260. Conversely, MDA and H2O2 values significantly decrease in T. erecta by the application of SiO2 NPs. The outcomes of our research work are similar to the findings of Ahmad et al.61, where during dry scenarios, the incorporation of SiO2 NPs considerably decreased the MDA and H2O2 concentrations.

In the present work, Cr stress markedly decreases the total soluble protein in T. erecta. Ali et al.62 also investigated that an increase in the chromium concentration further dropped protein content, which as a result led to the creation of reactive species in spinach plants. On the contrary, total soluble protein content was augmented by the supplementation of SiO2 NPs in T. erecta. Moreover, Fatemi et al.63 also revealed that the level of soluble protein was elevated when SiO2 NPs were applied in Pb-stressed plants.

When plants encounter stress conditions, they generate ROS at a greater rate, and the plant activates its antioxidant mechanism to alleviate or scavenge the ROS. Although chromium has a detrimental impact on the plant, the plant uses its antioxidant machinery to deal with stress situations59. Our results show that the level of enzymes that convert toxic substances into non-toxic molecules to avoid damage such as CAT, SOD, POD, and APX increases as the plant experiences stress. They tend to accumulate in tissues of the plant to protect the cellular machinery from heavy metal stress. Antioxidant enzymes rapidly activate themselves to detoxify the ROS produced from the Cr stress. However, this strategy is not sufficient to tolerate stress conditions. Exogenously applied SiO2 NPs further elevated the activity of antioxidant enzymes to minimize the impact of Cr stress on the plant. Garg et al.64 proved by their experiment on maize plants that the activity of all antioxidant enzymes was significantly greater in the presence of SiNP treatments of maize plants. Our findings are supported by Fatemi et al.63, who showed that the stress of lead (Pb) on E. sativa seedlings was decreased by increasing the activity of SOD, POD, and CAT under the influence of externally applied SiO2 NPs.

A very effective stress marker in plants is proline. Proline is a substance related to indicating the extent of stress in plants65. The results of the present research indicate that the proline level of T. erecta under stress was elevated as compared to the untreated control. The high rise in the values of proline is due to the stress. Under stress, the plants produce proline that is efficiently associated with different signaling molecules to reduce the stress of heavy metals66. Our results are very similar to Adhikari et al.67 findings; as a result of stress, plants accumulate more than enough proline as an adaptive mechanism to cope with stress situations. In the same pattern, we observed the high values of proline in T. erecta when SiO2 NPs were applied. SiO2 NPs improve the proline content by activating the enzymes that protect the plant from oxidative damage. Our results are in line with Zhao et al.65 who reported that SiO2 NPs elevated the proline under Cd stress in rapeseed seedlings.

Absorption of essential nutrients by the plant is hampered due to the presence of Cr stress. Roots of the plant contain Cr 100 times more compared to Cr content in shoots. The uptake of cations for normal functioning of cells is diminished by the Cr accumulation in the rhizosphere region. In the present study, Cr toxicity decreases the translocations of cations from soil to the roots of the plants. Even when present in very low quantities, it can seriously hinder the absorption of nutrients and water, resulting in a shortage of ions from roots to the shoots of plants27. Treatments of SiO2 NPs interestingly augmented the uptake of nutrients into the plant. These findings are confirmed by Fraceto et al.68, who reported enhanced absorption of nutrients enabled by SiO2 treatment. The results of this study also suggest that SiO2 NPs, particularly at higher concentrations, promote Cr immobilization in root tissues while effectively limiting its upward translocation to aerial parts. The observed decline in TF highlights the potential of SiO2 NPs to mitigate Cr toxicity by restricting its systemic movement within the plant, thereby potentially reducing Cr-Induced oxidative stress and physiological damage in shoots. This effect was more prominent under the higher Cr stress level (Cr-II), indicating the protective role of SiO2 NPs becomes more pronounced under elevated heavy metal stress.

SiO2 NPs likely mitigate Cr toxicity via multiple interacting mechanisms. SiO2 NPs can adsorb or make complex with Cr species and alter Cr speciation and local bioavailability. This reduces the pool of free, reactive Cr available for plant uptake30. Studies have shown that Si can bind heavy metals and form less mobile complexes69, which is consistent with observed reductions in shoot Cr following SiO2 NPs treatments. This reduced translocation following SiO2 NPs treatments can be attributed to Si mediated physical reinforcement of cell wall and regulation of metal transporters70. Studies have shown that Si treatment alter expression of metal transporter gene including the members of NRAMP, ZIP, HMA and ABC gene families70,71,72. Besides this, SiO2 NPs can also elevate the level of thiol-based chelators (glutathione and phytochelatins) that assist vacuolar sequestration/compartmentation of heavy metals73,74. SiO2 NPs consistently up-regulate enzymatic antioxidants (SOD, CAT, POD, APX) as observed in current study75. This further improves redox homeostasis (GSH/GSSG) which reduces reactive oxygen species (ROS) accumulation and lipid peroxidation caused by heavy metal stress76. Various studies have shown that Si application limit MDA and H2O2 accumulation while boosting antioxidant enzyme activities77. This also provide mechanistic insight for preserved membrane integrity, photosynthetic pigments and growth under Cr stress in current study.

Among all treatments, SiO2 NPs-II (200 mg L−1) proved to be the most effective in mitigating chromium-induced stress in T. erecta. This treatment led to marked improvements in growth metrics, photosynthetic pigment concentrations, nutrient uptake, enzymatic antioxidant activity (CAT, SOD, POD, APX), and osmolyte (proline) accumulation. SiO2 NPs at 200 mg L−1 not only minimized the oxidative damage (reduced MDA and H2O2) but also improved cellular protection and physiological balance more effectively than SiO2 NPs-I (100 mg L−1). Therefore, due to their consistent and broad-spectrum protective effects in relation to physiological, biochemical, and growth parameters, SiO2 NPs are recommended as the most effective and reliable treatment for improving plant resistance under chromium stress conditions.

In the current study, the foliar application of SiO2 NPs was highly effective in mitigating Cr toxicity and improving physiological performance in T. erecta. The SiO2 NPs are generally considered less toxic and more stable than metallic nanoparticles. However, their long-term environmental fate and indirect effects on soil and associated ecosystems remain uncertain despite their relatively low toxicity. Therefore, future studies should consider environmental risk assessments and support the development of regulatory guidelines to ensure safe and sustainable use in agriculture at large scale.

Conclusion

Chromium stress causes overproduction of reactive oxygen species in Tagetes erecta by inducing oxidative damage, enzymatic dysfunction, and finally cell death. Growth, photosynthesis, uptake of nutrients, and metabolism of proteins are affected negatively due to these disruptions. This study showed that exogenous application of SiO2 NPs effectively alleviated Cr-Induced phytotoxicity through enhancing the antioxidant defense system, including SOD, POD, CAT, and APX; it enhanced the accumulation of osmolytes like proline; it improved photosynthetic pigment content; and it restored nutritional balance in T. erecta under Cr stress. These results clearly indicate that SiO2 NPs significantly enhanced physiological and biochemical resilience in T. erecta against Cr-induced toxicity. Indeed, plants treated with SiO2 NPs had shown better growth, higher biomass accumulation, and reduced oxidative damage compared to nontreated, Cr-stressed ones. These findings suggest that SiO2 NPs not only alleviate heavy metal stress but also promote healthier plant development under contaminated soil conditions. Thus, the application of silica nanoparticles is recommended as a practical and efficient approach in view of improving the tolerance of marigold (T. erecta) to Cr stress. This approach can be valuable in improving the sustainability and productivity of ornamental plant cultivation in heavy metal-contaminated areas.