Introduction

Environmental degradation, caused by various organic and inorganic contaminants, has become a major global issue, endangering the ecosystem worldwide1. Increasing atmospheric concentrations of heavy metals-based particulates (PM) have posed a major concern to the population and the environmental health, and continue to rise globally2. Generally, these particles are categorized according to their diameter; for example PM 0.1, PM 2.5, and PM 10 have diameters of less than 0.1 μm, 2.5 μm, and 10 μm, respectively. Among heavy metals (HMs), Pb is considered the most persistent and hazardous metal in the environment, and its accumulation particularly in food crops, poses a serious health concern for humans3. Elevated Pb concentration in the environment and agricultural sector results in reduced crop yield and poor quality of agricultural products, which ultimately poses significant health risk to both humans and animals through the food chain4. Globally, it is estimated that 0.8 M tonnes of Pb particles have been released into the environment in the past half century5.

Aerial deposition of Pb is increasing through both natural and human-induced factors, including volcanic eruption, weathering, forest fire, dust emission, fertilizers, and pesticides, which lead to the deposition oflead to the deposition of Pb in the atmosphere6,7. Pb dust and fumes are released due to recycling activities in developing countries, raising major concerns regarding metabolic changes, nutritional value, and potential health implications8. Pb is highly toxic to human health even at low concentrations as it can cause severe damage to nervous and cardiovascular systems, as well as kidney failure9. Most Pb poisoning cases arise from the consumption of edible portion of crops grown under in Pb-polluted environments, along with the inhalation of air containing nano-sized Pb particulates10. Pb is present in trace quantities in almost all food crops11 and the maximum weekly intake of Pb in human food is approximately 25 µg/kg of body weight, as established by health authorities12.

Foliar Pb uptake through aerial depositions can raise Pb accumulation in plant tissues, resulting in obstruction of plant growth13, reduced photosynthetic activity, and imbalanced water and nutrients supply9. Exposure of plants to higher Pb levels lead to loss of cell membrane permeability, suppression of stress-responsive enzyme activity, wilting of older leaves, and stunted growth, ultimately resulted in plants death14. The Pb also induced oxidative stress through the generation of the reactive oxygen species (ROS) that caused lipids peroxidation, alterations in antioxidant enzyme activity, and cell membrane damage15. In response to this toxicity, plants have developed various defense mechanisms, such as scavenging of ROS and regulating antioxidant enzyme activity which ultimately alleviate oxidative stress in plants4,16.

Despite substantial sources of Pb, particle size plays a crucial role in effective absorption and transportation of Pb in plants, as NPs possess higher reactivity due to smaller size and greater surface area in comparison to bulk materials17. Absorption mainly depends upon the solubility, particle size, weather conditions, and the availability of the chemicals on plant surfaces1. Although leaves are the primary site for Pb absorption, little is known about Pb uptake by other non-leaf tissue in shoots, particularly in grain crops. The grain-filling period of the crop is a critical stage for grain formation and the accumulation of Pb into the grains with filling substances18. The application of PbO-NPs induces adverse effects on agricultural plants both biochemical and physiological levels19,20. Although research on the foliar uptake of metal by plants has been constructed21,22, but investigations regarding induced injuries in plants and health risk assessment under foliar application of PbO-NPs in rice remains unclear.

Rice (Oryza sativa L.) is an important cereal crop and contributes 50% of the calories consumed by half of the world’s population, with its demand is projected to rise by 28% in 205023. Rice is considered a staple food in Pakistan, cultivated on approximately 3.53 × 106 ha with an average yield of 3.950 tons ha− 124. It is a rich source of carbohydrates, proteins, fats, and essential nutrient such as calcium, iron, riboflavin and niacin, which are vital for human body growth and development25. Therefore, it is crucial to understand the response of rice plants to high Pb concentrations, particularly regarding its foliar absorption and distribution within plants. Considering the novel aspects and existing knowledge gaps, the current study focused on assessing uptake and toxicity of Pb in rice along with associated health risks under different levels (0, 10, & 50 mg/plant) of foliar-applied PbO-NPs.

Materials and methods

Synthesis of the lead oxide nanoparticles

PbO-NPs were synthesized through using Pb acetate (Analytical grade, Sigma Aldrich). About 0.5 g of Pb acetate was mixed with the 100 mL of coconut water, which acted as capping agent, reducing agent as well as stabilizer26. After completion of the dissolution of Pb acetate, the suspension containing PbO-NPs was centrifuged at 3000 rpm for 10 min (Fresco 17 microcentrifuge), resulting in two separate phases. Then supernatant was collected and it was heated on the hot plate to form a paste at 90 °C. The solid phase was dried at 90 °C in an oven (Model: 101-OAB Digital Lab Thermostatic Electric Incubator). Both components were then calcined at 450 °C for 2 h in a muffle furnace.

Soil collection and soil analysis

For experiment, the soil was prepared by sieving with 2 mm sieve to ensure the uniform particles size and remove unwanted materials after collection from natural texture agricultural fields. Standard methods were used to determine the soil Physiochemical parameters. The soil texture was sandy clay27 with soil pH (7.89) and EC (1.94 dSm− 1). Soil organic matter (1.32 ± 2.4 g kg− 1), available phosphorous (6.01 mg kg− 1), total nitrogen (0.091%), and extractable potassium (mg kg− 1) were determined using the method of Walkley and Black28, . The action exchange capacity CEC (10.64 cmol (+) kg− 1) was measured following the procedure proposed by Gillman and Sumpter29. Furthermore, Zn (4.24 mg kg− 1), Fe (55.75 mg kg− 1), and Mn (5.71 mg kg− 1) were determined using the method described by Mehlich30, , while available Cd (0.048 mg kg− 1) was analyzed according to the method by Park et al.31.

Plant materials, transplantation of plants, and growth condition

The current pot experiment was conducted in Khanqha Dogran city, Sheikhupura district, Punjab, Pakistan (31.8630°N and 73.6639°E) under controlled conditions, with a humidity of 65–68% and temperature ranging from 35 to 40 °C during seedling transplanting, and 20 to 25 °C at the maturity stage of rice crop. The duration of light was 10–12 h. Mature and Healthy seeds of rice (cv. Kisan Basmati-1509) were obtained from the Ayub Agricultural Research Institute, Faisalabad. The seeds were sterilized with the hydrogen peroxide (H2O2 15% v/v) solution, rinsed with distilled water to eliminate impurities and incubated for consecutive 3 days at 25 °C in darkness just before sowing as nursery. Twenty-five days old rice nurseries were collected from the research area of institute. The collected nurseries were transplanted into plastic pots containing 2.5 kg soil and pots labeled according to the respective treatments. Following a completely randomized design (CRD), 3 seedlings were shifted into each pot at transplanting time and maintained 3 replications of each treatment. Foliar applied Pb treatments included a control (without Pb foliar application), 10 and 50 mg/plant based on previous studies review26. The control was uncontaminated, whereas PbO-NPs10 and 50 comprised 10 and 50 mg/plant of applied PbO-NPs respectively. Three older leaves with adaxial surfaces were treated with PbO-NPs. Before the PbO-NPs application by foliar, a polyethene sheath was placed over soil surfaces to prevent any contamination. The rice leaves were treated with PbO-NPs for 28 days once a week. Recommended quantities of NPK (120:60:60) kg ha− 1 were applied to maintain nutrient availability. Nitrogen was applied in four split doses, with urea as the source; potassium was supplied as SOP, and DAP was used as the source of phosphorous. Irrigations were applied according to crop requirements and maintain appropriate moisture level in the pots.

Morphological attributes

Healthy plants were harvested after the trial treatments. Morphological attributes were examined at the time of harvest using measuring scale and weighing balancing. After harvesting for the morphological assessment, the plant shoots and roots were separated and cleaned with 2% hydrochloric acid followed by distilled water in order to remove aerial deposition. The harvested plant sample were then oven-dried (Model: 101-OAB Digital Lab Thermostatic Electric Incubator) at 70 °C for 48 h to determine their dry weight.

Determination of SPAD values and photosynthetic pigments

To determine the chlorophyll contents of in freshly developed plant leaves, the leaves were immersed in 80% v/v acetone at 4 °C in the dark overnight32. The leaves were ground the next day, and then measurements were recorded at (663 nm, 645 nm, and 480 nm) using a UV spectrophotometer (Labman LMSPUV1900 double beam UV–Vis spectrophotometer). SPAD values were measured in situ using the atLeaf CHL PLUS meter. Gas exchange parameters, such as photosynthetic and the transpiration rate, carbon dioxide assimilation, and stomata conductance, were determined using an IRGA (Infrared Gas Analyzer).

Analysis of antioxidant enzyme activity

To prepare the antioxidants enzymes extract, fresh samples were ground in pestle and mortar in a phosphate buffer solution having pH (7.8). Then the mixture was centrifuged in order to obtain supernatant for analysis of antioxidants enzymes activity. Superoxide dismutase (SOD), Peroxidase (POD) activities were measured according to Zhang33. For POD (POD; EC1.11.1.7) activity, the extract was added to phosphate buffer solution and 300 mM of hydrogen peroxide (H2O2) and the reaction was measured using spectrophotometer. To measure the SOD activity, solution was prepared with the enzyme extract, L-methionine, NBT, riboflavin, along with EDTA-NA2 and calculated using spectrophotometer (Labman LMSPUV1900 Double Beam UV–Vis spectrophotometer). Catalase (CAT) (CAT; EC.1.11.1.6) activity was measured following Aebi34, and ascorbate peroxidase (APX; EC 1.11.1.11) activity was determined according to the method of Nakano and Asda35.

Determination of oxidant content and ROS staining

Malondialdehyde (MDA) contents were ascertained through the method recommended by Zhang and Kirkham36. Electrolyte leakage (EL) was measured following protocol of Dionisio-Sese and Tobita37. The initial EC was recorded by extracting samples for 2 h at 32 °C, followed by repeating the procedure for 20 min at 121 °C. The H2O2 contents were estimated according to protocol given by Jana and Choudhuri38. Briefly, for the homogenization of the leaf sample, phosphate buffer solution (PBS 50 M, pH 6.5) was used, and the mixture was centrifuged at 10x for 20 min. After adding 20% v/v H2SO4 to the supernatant and centrifuging for an additional 15 min, the optical density was measured at 410 nm. In pepper leaves, the accumulation of O2− and H2O2 was detected using NBT and 3, 3-diaminobenzidine (DAB) staining techniques, as described by Rathinapriya et al.39. After treating the leaves with 100% ethanol at 65 °C to remove pigments, the staining results were examined using microscope (Eclipse E200MV R, Nikon Corporation, Japan). Each treatment was replicated 3 times.

Determination of metal content in plant

Shoots, grains, and roots of each dried plant sample were cut in pieces by the grinder and transferred to a flask. Dry plant samples (0.5 g) of grains, shoots, and roots were placed into a 10 mL flask containing HNO3–HClO4 (3:1, v: v) and acid digestion was performed following the of Rehman etl40. with minor modifications. An atomic absorption spectrophotometer (novAA 350, Analytik Jena, Germany) has been operated to determine the correct quantity of Pb in grains, shoots, and roots of the samples.

Health risk assessment

The health risk index (HRI) of Pb was calculated for each treatment to assess the potential risk to human health. The initial step involved estimating the daily intake of metals (DIM), which was determined by using following equation:

$${\text{DIM}}\,=\,{{\text{C}}_{{\text{grain}}}} \times {\text{IR}}/{\text{BW}}$$

Here in above equation the: C grain is (Pb concentration in rice grains in mg/kg of fresh weight), IR is (daily ingestion rate of rice kg/person/day), BW is (average body weight in kg).

The HRI was calculated given as following:

$${\text{HRI}}\,=\,{\text{DIM}}/{\text{RfD}}$$

Here in above equation the: RfD is (oral reference dose of Pb as 0.0003 mg/kg/day).

The daily intake of rice was taken to be 0.385 kg/person/day, and the average weight of both adults (18–60 years) and children (7–17 years) was assumed to be 65 kg and 35 kg, respectively. An HRI value above 1 implies the risk of a health hazard, and the value below 1 means it is safe.

Statistical analysis

Statistical analysis was done using SPSS software. One-way ANOVA was carried out to assess the statistical difference among the various treatments. Post-hoc Tukey test was applied to see the significant difference among treatments. Additionally, OriginPro, (Version 2023b, Origin lab corporation, Northampton MA. USA) was use to visualize data patterns for Pearson correlation analysis and principal component analysis.

Results

In the current study, the results were obtained by examining the effects of the foliar application of lead nanoparticles (PbO-NPs) on Pb accumulation, physiological and biochemical changes, and nutrient profiling.

Growth attributes response of rice to the foliar application of lead oxide nanoparticles

Reduced root and shoot lengths, spike length, fresh and dry weights, and rice yield were observed under PbO-NPs treatments (10 and 50 mg kg− 1). A significant reduction in root length, ranging from 28.49 to 70.54% was observed under PbO-NPs-10, and PbO-NPs-50 respectively. Significant reductions in shoot length were also observed under PbO-NPs-10 and PbO-NPs-50 with decreases of 25.47% and 59.16% respectively, compared with non-contaminated plants. The lowest spike length was observed under PbO-NPs-50 at 32.51% and 10.08% at PbO-NPs-10 compared with the control. Foliar application of PbO-NPs induced a notable reduction in shoot fresh and dry weight, with PbO-NPs-50 recording decreases of (69.46/46.08%) followed by PbO-NPs-10 (21.67/8.86%) compared with non-contaminated treatment. Similarly, the root fresh/dry weights were reduced by 36.48/68.41%, and 22.49/30.72% under PbO-NPs-50 and PbO-NPs-10 respectively, compared with the non-stressed control plants. Significant decrease in yields of 63.28% and 33.49% were observed for PbO-NPs-50 and PbO-NPs-10 treatments, respectively relative to control. The minimum number of tillers per plants was recorded by PbO-NPs-50 (72.01%) while PbO-NPs-10 (36.12%) showed a smaller reduction compared with the uncontaminated control as shown (Figs. 1 and 2).

Fig. 1
Fig. 1
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Effect of foliar application of lead oxide nanoparticle (Ck-0, NPs-10, NPs-50 mg L− 1) on (A) shoot length, (B) root length, (C) root fresh weight and (D) root dry weight of rice plants. Values are means of 3 replicates Error bars represent S.E. Different lettering indicate the significant difference at (P < 0.05).

Fig. 2
Fig. 2
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Effect of foliar application of lead oxide nanoparticle (Ck-0, NPs-10, NPs-50 mg L− 1) on (A) shoot fresh weight, (B) shoot dry weight, (C) number of tillers/plant (D) spike length and (E) grain weight of rice plants. Values are means of 3 replicates Error bars represent S.E. Different lettering indicate the significant difference at (P < 0.05).

Chlorophyll contents and gas exchange parameters response of rice to the foliar application of lead oxide nanoparticles

Distinctive responses of gas exchange traits, including photosynthetic rate and transpiration rate, stomatal conductance, and water use efficiency along with chlorophyll contents of rice crops, were observed under the foliar application of different PbO-NPs concentrations. The results indicated that plants exposed to PbO-NPs stress showed a significant reduction in Chl a and Chl b, total Chl, SPAD values, and carotenoids in rice crops compared with un-contaminated treatment. Under PbO-NPs-50, plants showed a significant reduction in Chl a (80.74%), Chl b (74.87%), total Chl (82.42%), SPAD values (48.27%) and carotenoids (83.63%). The decrease in Chl a, Chl b, total Chl, SPAD values, and carotenoid contents under PbO-NPs-10 was 18.77%, 23.59%, 22.22%, 16.18%, and 28% respectively, compared with the control (Fig. 3). Plants treated with different concentration of PbO-NPs exhibited a significant reduction in photosynthetic rate by 32.95%, 80.70% under the PbO-NPs-10, PbO-NPs-50 respectively, compared with the control. The lowest transpiration rate was observed under (72.67%) followed by PbO-NPs-50 followed by PbO-NPs-10 (18.87%) compared with un-contaminated control. A significant reduction in stomatal conductance, ranging from 16.79% to 61.2% was recorded under PbO-NPs-10 and PbO-NPs-50 respectively compared with the control. Similarly, water use efficiency was reduced under PbO-NPs-50 (77.89%) and PbO-NPs-10 (27.07%) in comparison with the corresponding un-contaminated control as shown (Fig. 4).

Fig. 3
Fig. 3
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Effect of foliar application of lead oxide nanoparticle (Ck-0, NPs-10, NPs-50 mg L− 1) on (A) chlorophyll a, (B) chlorophyll b, (C) total chlorophyll (D) carotenoids and (E) SPAD Values of rice plants. Values are means of 3 replicates Error bars represent S.E. Different lettering indicate the significant difference at (P < 0.05).

Fig. 4
Fig. 4
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Effect of foliar application of lead oxide nanoparticle (Ck-0, NPs-10, NPs-50 mg L− 1) on (A) photosynthetic rate, (B) transpiration rate, (C) water use efficacy and (D) stomata conductance of rice plants. Values are means of 3 replicates Error bars represent S.E. Different lettering indicate the significant difference at (P < 0.05).

Antioxidant enzyme activity and oxidative stress response of rice to the foliar application of lead oxide nanoparticles

The results of antioxidant activities showed that PbO-NPs stress affected all enzyme activities. POD activity significantly decreased under PbONPs-50 and PbO-NPs-10 by 82.34% and 43.01% respectively. PbO-NPs also significantly reduced SOD activity by 73.08% and 44.67% under 50 and 10 mg/kg PbO-NPs stress conditions, respectively. In comparison with the non-stressed control, the application of PbO-NPs reduced CAT activity in rice plants by 62.24% and 30.35% under PbO-NPs-50 and PbO-NPs-10 respectively. A notable reduction in APX activity was observed under PbO-NPs-50 (55.02%) while PbO-NPs-10 showed a reduction of 12.73% compared with the respective control as shown (Fig. 5).

Fig. 5
Fig. 5
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Effect of foliar application of lead oxide nanoparticle (Ck-0, NPs-10, NPs-50 mg L− 1) on (A) POD, (B) SOD, (C) CAT and (D) APX of rice plants. Values are means of 3 replicates Error bars represent S.E. Different lettering indicate the significant difference at (P < 0.05).

The results revealed that Pb stress increased MDA concentration compared with the control plants by 76.10% and 45.53% in plants treated with PbO-NPs-50 and PbO-NPs-10 respectively. PbO-NPs also increased the H2O2 content by 70.6% and 31.29% under 50 and 10 mg/kg treatments, respectively, compared with the corresponding treatments without PbO-NPs. A notable increase in electrolyte leakage was observed under detected under the application of PbO-NPs-50 (21.85%) while PbO-NPs-10 showed a rise of (10%) compared with their respective control as shown (Fig. 6A-E).

Fig. 6
Fig. 6
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Effect of foliar application of lead oxide nanoparticle (Ck-0, NPs-10, NPs-50 mg L− 1) on (A) MDA, (B) H2O2, (C) electrolyte leakage, Values are means of 3 replicates Error bars represent S.E. Different lettering indicate the significant difference at (P < 0.05). (D) leaf staining with nitro-blue tetrazolium (NBT) and (E) 3,3-diaminobenzidine (DAB) of rice plants.

Metal contents in different parts of plants

The foliar application of PbO-NPs resulted in considerable Pb accumulation in leaf tissues under PbO-NPs-50 and PbO-NPs-10 treatments. The roots of rice accumulated comparatively lower Pb levels (4.09 ug/kg and 2.33 ug/kg respectively for PbO-NPs-50 and PbO-NPs-10) compared with relevant shoots. This indicated that major amount of Pb, accumulated in shoots with relatively lower translocations towards the root. In addition, PbO-NPs-50 increased Pb levels by 9.15 ug/kg and 15.42 ug/kg compared with PbO-NPs-10 (3.04 ug/kg and 6.82 ug/kg), in the shoots and paddy respectively as shown (Fig. 7).

Fig. 7
Fig. 7
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Effect of foliar application of lead oxide nanoparticle (Ck-0, NPs-10, NPs-50 mg L− 1) on (A) Pb in grain, (B) Pb in shoot, (C) Pb in roots of rice plants. Values are means of 3 replicates Error bars represent S.E. Different lettering indicate the significant difference at P < 0.05.

Human health risk assessment

The HRI values indicated the degree of risk associated with the consumption of metals from contaminated crops as shown (Table 1). The results revealed significant Pb accumulation in rice plant tissues. When PbO-NPs were applied foliar, the risk assessment indices for NPs-I and NPs-II treatments varied significantly. Children had values of the DIM of between 1.34 × 10− 3 and 1.82 × 10− 1, and adults of between 7.22 × 10− 4 and 9.81 × 10− 2. According to these values, the resulting HRI was greater than 1 in higher PbO-NPs treatments for both age groups, suggesting a potential non carcinogenic health risks associated with the intake of Pb-contaminated rice grains. It is worth noting that HRI is a non-carcinogenic risk index and, therefore, should not be compared with carcinogenic thresholds (e.g., 10− 4). As Pb is primarily associated with non-carcinogenic effect in the dietary exposure assessment, our findings highlight a significant concern regarding food safety and public health. According to the parameters of the health risk assessment, the potential risks increased linearly with the applied PbO-NPs levels. This is because potentially toxic substances have polluted the environment more frequently, and have increased public awareness, raising people’s concerns about food safety (Fig. 8).

Table 1 The HRI values indicated the degree of risk associated with consuming metals from contaminated crops.
Full size table
Fig. 8
Fig. 8
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The potential health risks increased linearly with PbO-NPs at (10, NPs-50 mg L− 1) and Microsoft PowerPoint software used to draw this figure.

Discussion

HMs stress is one of the major environmental constraints that severely affects the growth, development, and productivity, ultimately disturbing the food chain and posing health risks to humans worldwide43. PbO-NPs caused morphological, physiological, and biochemical alterations in rice plants due to their highly toxic and potentially carcinogenic nature. The present study was conducted to investigate the adverse impacts of foliar applied PbO-NPs on rice crop productivity, as well as their potential impacts on human health following the consumption of Pb contaminated rice grains.

The results of present study showed a notable reduction in root-shoot length and spike length, fresh and dry weight, and paddy yield under the higher dose of PbO-NP-50. Pb stress caused a severe reduction in fresh-dry matter production in rice plants accompanied by a significant reduction in root-shoot length spike length, and grains (Figs. 1 and 2). The decrease in plant growth attributes is linked to the higher uptake of Pb concentration in the growth medium which may result in disturbed photosynthetic activity, an imbalance in nutrient uptake, and generation of ROS, which adversely impact plant metabolism. A previous study by Li et al.44 reported a significant decrease in rice tillering, shoot fresh dry weight, overall plant height which ultimately reduced rice yield due to aerial Pb deposition. Additionally, Shahid et al.19 also observed a significant reduction in shoot-root dry weights, shoot length, and number of tillers under the Pb foliar application on rice crops.

Furthermore, the physiological attributes such as photosynthetic activity, chlorophyll contents, SPAD values, and gaseous exchange parameters were disrupted under Pb toxicity. Leaves are considered major plant organs because of their role in capturing light and generating food through the photosynthesis process45. Pb toxicity can hinder the absorption of essential growth minerals, leading to alterations in enzyme activities and consequently reducing the production of photosynthetic pigment46. Pb significantly affects the chlorophyll contents of leaves, which is one of the primary indicators of metal toxicity in plants. As a result, a modification in plant physiological attributes can directly influence plant health as well as plant’s response to changing environment conditions47. In this study, exposure to the higher concentration of PbO-NPs resulted in a significant reduction in Chl a, Chl b, total Chl, and carotenoids as shown (Fig. 3). In addition to chlorophyll contents, the application of PbO-NPs also considerably reduced in the photosynthetic and transpiration rates, stomatal conductance, and water use efficiency. Our findings are linked with Li et al.44 who reported that to exposure to PbO-NPs disrupted photosynthetic activity, decreased chlorophyll and carotenoid contents along with other physiological traits in maize crops. Likewise, a previous study by Xiong et al.48 also reported a considerable reduction in chlorophyll, carotenoids, and other gas exchange traits due to exposure to foliar PbO-NPs concentration in lettuce.

The direct interaction of Pb leads to the excessive production of ROS in plants, which induces oxidative damage, breakdown of membrane, and biomolecules, and structural damages49. ROS production also adversely affects the functions of protein and DNA through lipid peroxidation, hindering plant growth and yield, and ultimately causes severe cellular damage that may result in plant death13,50. Pb altered the redox homeostasis in plants through several indirect mechanisms including variations in metal-related enzyme activity and displacement of essential elements from cellular biomolecules, which promote ROS production14. Consequently, any disturbance of redox homeostasis in plants results in oxidative stress ultimately leading to cell death. This study observed higher ROS in plant tissues under the foliar application of higher PbO-NPs concentration as shown (Fig. 6A–C). Similar findings were observed in a previous report that foliar application of Pb induced oxidative stress through reduced activities of SOD, POD, and CAT while promoting H2O2 production in spinach crops26. To combat oxidative damage, plants have developed an efficient defense system, consisting of glutathione and its corresponding metabolizing enzymes, proteins, and peptides. The tolerance to oxidative stress is supported by antioxidant enzymes such as SOD, POD, and CAT, which collectively serve as indicators of oxidative stress51. Moreover, SOD converts O2 into H2O2 whereas CAT breaks down H2O2 into H2O and oxygen molecules, similarly, other antioxidant enzymes contribute to the scavenging of ROS in plants52. According to Li et al.20, exposure to PbO-NPs significantly reduced antioxidant enzyme activity as Pb triggered the overproduction of ROS and induced oxidative stress in maize crops.

The Pb concentration in the root portion following foliar application indicated its translocation from the upper parts to roots via the phloem reloading mechanism, where most of the Pb is bound to the cell wall in the root section and also competes with Ca2+ for cations absorption by the organic compounds through carboxylic groups53. Pb deposited on the leaf surface penetrates internal tissues and is subsequently translocated to the roots in the form of Pb-carbonates54. Pb contents were reduced in the root portion with less translocation towards the root from the shoot53. In addition, Pb toxicity induced change in leaves, reduced the diameter of xylem vessels, and disturbed the function of xylem and phloem in vascular bundles resulting in the thinner leaf blades of soybean plants46. The results of the present study are consistent with previous studies reports indicating that both the shoots and roots contain higher concentrations due to Pb aerial deposition and uptake, ultimately reducing rice crop productivity53. Previously, Shahid et al.19 concluded that the majority Pb accumulated in leaf tissues, with lower amounts translocated to the root portion under foliar Pb application in spinach. Likewise, Gao et al.55 reported the Pb accumulation in the edible portion of cabbage. This increase in Pb contents in plant tissues may enter the food chain and pose significant health issues in humans. According to the parameters of the health risk assessment, the potential risks increased linearly with the applied levels of PbO-NPs as shown (Figs. 7, 9 and 10). This is because potentially toxic substances have increasingly polluted the environment, raising public awareness and concerns about food safety56. Several recent studies have focused on the risks to human health risk associated with consuming contaminated crops and vegetables57. As a result, it’s important to monitor the health risks associated with consuming grain crops contaminated with metals near urban and industrial areas. This is due to the fact that cultivating crops may increase human exposure to hazardous metals through foliar absorption, deposition, and subsequent consumption of crops contaminated with heavy metals58. The obtained DIM and HRI values suggested that exposure to Pb by rice grains was above the non-carcinogenic level (HRI > 1), especially among children, which points to a possible health threat posed by the diet. Being RfD-based, our findings relate to non-carcinogenic effects and should not be compared to carcinogenic limits (10− 4). These results, together with the observed oxidative stress and reduced antioxidant defense, indicate that the Pb contamination of rice causes a considerable risk to the food safety. The results indicate the possibility of dietary intake exceeding the safe reference dose (RfD = 0.0003 mg/kg/day) due to the consumption of rice contaminated with PbO-NPs, particularly in children. The HRI values exceeded 1, suggesting a potential health risk from the long-term consumption of such contaminated rice. Overall, our findings suggest that foliar application of PbO-NPs can reduced plant morphological and physiological growth, thereby lowering stress resistance against Pb toxicity in rice plants. By integrating foliar application of lead oxide nanoparticle to staple food contamination, our outcomes advance the risk assessment pathway and urgently opting the food safety guideline and sustainable cultivation practices (Fig. 11).

Fig. 9
Fig. 9
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Pearson correlation for the studied rice crop different parameters.

Fig. 10
Fig. 10
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Principal component analysis for studied rice crop different parameters.

Fig. 11
Fig. 11
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Proposed mechanism of PbO-NPs toxicity and Microsoft PowerPoint software used to draw this figure.

Conclusion

The current study evaluates the effects of foliar applied PbO-NPs on rice plants, including associated physiochemical changes and potential health implications. Experimental results revealed that foliar-applied PbO-NPs adversely effected rice and dry weights, as well as pigment contents. Foliar application promoted the production of ROS and altered lipid peroxidation in rice plants, ultimately disturbed plant defense mechanism and antioxidant enzyme activity. Additionally, results indicated that a considerable amount of Pb (15.42 ug/kg) accumulated in the grain through the foliar pathway, posing serious health hazards due to the consumption of Pb-enriched rice. This suggests that higher rates of foliar applied Pb caused a substantial reduction in plant growth and productivity. Consequently, it is recommended that the cultivation of crops to area having the higher atmospheric Pb deposition to be closely monitored to prevent food-chain contamination and potential health risks. Further investigations are needed for better understanding the relationship between physiological and molecular response of various plants species by foliar application PbO-NPs.