One-year-old saplings of P. zhennan saplings (the root systems of each sapling were carefully removed from the soil), sourced from S. Lee’s gardening farm (103°47’E, 30°55’N), were transplanted into 20-L plastic pots (15 cm inner diameter, 17 cm outer diameter, 30 cm height) containing 10 kg of farm-derived homogenous soil. The soil consisted of 1.78% organic matter, 1.47g·kg^–1 total N, 10.59g·kg^–1 total carbon, 0.1233g·kg^–1 available N, 84.003mg·kg^–1 available P, 0.1371mg·kg^–1 Cd, and had a pH of 7.7. The saplings were cultivated in a greenhouse at the Chengdu Institute of Biology, Chinese Academy of Sciences (104°4’E, 30°37’N, 478.5m altitude) beginning on March 1, 2021.

After three months of growth, the plants underwent randomized treatment combinations of two Cd levels (absent and high at 15mg/kg) and nutrient additions (N, P, and NP). Eight treatment conditions were established: CK (no nitrogen, phosphorus, and Cd); N (adding N, no P, and Cd); P (adding P, no N, and Cd); NP (adding N and P, no Cd); CdH (adding high Cd, no N, and P); CdHN (adding high Cd and N, no P); CdHP (adding high Cd and P, no N); and CdHNP (adding high Cd, N, and P), with at least 12 replicates, three biological replicates, and four saplings each. Based on the Cd pollution condition in Sichuan soil and combining the results from Tie, 2021 (Xiang, 2021) which indicated that P. zhennan seedlings are not suitable for growth when the soil Cd content exceeds 8 mg/kg, the impact becomes particularly significant when the applied Cd level reaches 20 mg/kg or higher. Therefore, the Cd treatment is set at 15 mg/kg. The Cd was dissolved in the form of CdCl₂·2.5H₂O (precipitated pure) solution and applied three times to irrigate the potting soil. The control treatment involved the addition of an equivalent volume of deionized water to the plants. The N fertilizer was applied as ammonium nitrate (NH₄NO₃, 34.99% N) at a dose of 60 mg/kg, totalling 600 mg N (He et al., 2015), and P fertilizer is added as sodium dihydrogen phosphate (NaH₂PO₄, 25.5% P) at a dose of 60 mg/kg, totalling 600 mg P (Olatunji et al., 2018). For each treatment, reagents are dissolved in 200 mL of water and mixed before being added, divided into three applications, with nutrient addition treatment conducted every 15 days (Tariq et al., 2018).

The plants were gathered at the end of the experiments and various measurements were taken. The increase in plant height (IPH) and ground diameter (IGD) were measured or computed. The leaf, stem, and root dry weights (LDW, SDW, and RDW), as well as the aboveground biomass (AB) and total biomass (TB), were weighed for each sapling. The dry weights of the 1st, 2nd, and 3rd roots were dried and weighed. The harvested roots were washed and scanned using a root scanner, and their length (RL), the average diameter (ARD), surface area (RS), and volume (RV) were analyzed using WinRHIZO Pro 2012.

To determine the concentrations of Chla (chlorophyll a), Chlb (chlorophyll b), TChl (total chlorophyll), and Caro (carotenoids), leaf tissue was extracted in 80% chilled acetone and quantified at wavelengths of 470, 646, and 663 nm, as described by (Lichtenthaler, 1987). Photosynthetic parameters, including Pn (net photosynthetic rate), Gs (stomatal conductance), Ci (intercellular CO₂ concentration), and Tr (transpiration rate), were measured using the LI-COR 6400 portable photosynthesis system (LI-6400, LI-COR Inc, USA) between 9:00-11:30 am, according to the method described by (Yang et al., 2010).

We exclusively utilized leaf samples to determine oxidative stress markers and antioxidant analysis. The leaf tissues were carefully harvested from the plants under study and were immediately processed for the analysis of oxidative stress markers and antioxidant enzyme activities. This choice was made considering that leaves are primary sites of ROS production under metal stress and provide crucial insights into the defense mechanisms of plant. To measure the O₂ ^·− (superoxide radicals), a method described by (Zhang et al., 2011) was used. Fresh samples were homogenized in 2mL of 65mmol·L^–1 phosphate buffer (pH 7.8), and the reaction mixture included 1mL of supernatant, 1mL of 65mM phosphate buffer, 1mL of 10mM hydroxylamine hydrochloride, 1mL of 17mM sulphanilamide, and 1mL of 7 mM α-naphthylamine. After incubation at 25°C for 20min, the mixture was extracted with an equal volume of n-butanol, and the absorbance was measured at 530nm, using NaNO₂ ^– as the standard curve to calculate the O₂ ^·− activity. The relative conductivity (RC) and MDA (malondialdehyde) were measured following the method described by (Yang et al., 2022).

The POD activity was quantified using the guaiacol colorimetric method at 470 nm following the procedure of Micro Peroxidase Assay Kit (EC 1.11.1.7, Beijing Solarbio Science & Technology Co. Ltd., Beijing, China) according to (Doerge et al., 1997). POD activity was calculated as an absorbance change of 0.005 per minute per unit.

The CAT activity was determined at 240 nm following the procedure of Catalase Activity Assay Kit (EC 1.11.1.6, Beijing Solarbio Science & Technology Co. Ltd., Beijing, China) according to (Johansson and Borg, 1988).

The SOD activity was measured using the reduction of nitroblue tetrazolium (NBT) photoreduction method as described by (Yang and Miao, 2010). The experiment involved creating a reaction mixture with 50 mM Tris-HCl buffer (pH 7.8), 0.1 mM EDTA, and 13.37 mM methionine. To this 5.7 ml mixture, 200 μl of 0.1 mM riboflavin (in 50 mM Tris-HCl and 0.1 mM EDTA, pH 7.8) and 0.1 ml of enzyme source were added. The reaction started when the test tubes were placed under fluorescent lights and stopped after 30 minutes by removing them from the light. Tubes kept in the dark acted as controls. A light-exposed control without protein showed maximum NBT reduction, indicating peak absorbance at 560 nm.

The APX activity was determined by following the method using the spectrophotometer described by (Nakano and Asada, 1981). To prepare the leaf extract, approximately 0.1 g of fresh leaves with veins removed were ground in an ice bath using 1.5 mL of PBS. The mixture was then centrifuged at 8000 r/min for 10 minutes at 4°C, and the resulting supernatant was retained on ice for analysis. For enzyme assays, 0.05 mL of this supernatant was combined with 0.13 mL of distilled water and 2.8 mL of PBS in a cuvette. The enzymatic reaction commenced upon adding 0.02 mL of 200 mmol·L^-1 H₂O₂ solution, and the absorbance at 290 nm was measured immediately, with readings taken every 15 seconds over a 2-minute period.

Proline content in fresh leaf samples (~200mg) was quantified using a method described by (Bates et al., 1973). Samples were homogenized with 5mL of 3% sulfosalicylic acid. 1mL of this homogenate was added to acid-acetic ninhydrin reagent and glacial acetic acid, incubated at 100°C for 1 hour, then cooled and subjected to toluene extraction. Absorbance was read at 520nm.

Soluble protein concentration was gauged following (Bradford, 1976) using Coomassie Brilliant Blue G-250. Fresh samples (100mg) were mixed with phosphate buffer (pH 7.8), and protein concentration was inferred from a standard curve with absorbance read at 595nm.

GSH levels in fresh leaves (~100mg) were determined using a kit from Nanjing Jiancheng Bioengineering Institute, following (Yang et al., 2010). Leaves were ground in liquid nitrogen and homogenized with PBS (5mg fresh leaves:50μL PBS) before centrifugation (10,000 rpm, 30 min, 4°C) and assay performance.

The Cd amounts in the plant samples were identified directly by ICP-MS (Thermo Fisher Scientific, XSeriesll, Germany) as described by (McBride and Spiers, 2001). The Plant tissues were collected and dried at 80-100°C until a constant weight was achieved. The dried tissues were then ground to a fine powder using a mortar and pestle. 0.5 g of the powdered sample was weighed into a digestion tube, and 2 mL of concentrated nitric acid (HNO₃) was added. The mixture was then heated on a hot plate until the volume was reduced to approximately 1mL, and the solution turned clear. 1mL of hydrogen peroxide (H₂O₂) was added, and the mixture was further heated until the solution turned clear again. Deionized water was added to the digestion tube to bring the final volume to 50mL. The solution was mixed well and filtered through a filter paper or membrane filter to remove any insoluble particles. The Cd levels in the filtered solution were measured using an ICP-MS instrument.

The digestion process involving HNO₃ and H₂O₂ is designed to ensure the complete breakdown of plant cellular structures, thereby releasing Cd from all compartments, including the apoplast. This method allows for the solubilization of Cd bound in both the apoplastic and symplastic pathways of the plant tissues. Consequently, the Cd levels measured by ICP-MS represent the total Cd content, encompassing Cd from all cellular compartments.

Plant NO₃ ^–N was determined using the direct extraction method. The plant leaves were ground with deionized water to extract the supernatant. From this extraction, 0.1 mL of supernatant was taken and mixed with a 5% salicylic acid-sulphuric acid solution and 8% NaOH. This mixture was then cooled in an ice water bath until it reached room temperature. Afterward, its absorbance was measured at a wavelength of 410 nm.

For NH₄ ⁺-N, the ninhydrin colorimetric method was employed. Fresh leaves, with their main veins removed, were weighed to 0.1 g and then finely chopped. To this, 1 mL of 10% acetic acid solution was added, and the mixture was ground into a homogenate. After centrifugation, 0.5 mL of the supernatant was collected and combined with 1.5 mL of distilled water, 3 mL of ninhydrin reagent, and 0.1 mL of 1% ascorbic acid. This solution was then heated in a boiling water bath for 15 minutes. After a cooling period of 15 minutes, its absorbance was measured at a wavelength of 570 nm.

In this study, statistical analyses were conducted using SPSS 23.0. Before analysis, all data were checked for normal distribution and homogeneity of variance, and Ln-transformed if necessary. Duncan’s multiple range test was used to determine differences among treatments, while an independent sample t-test was used to compare two Cd levels. Correlation analysis was performed using Origin Pro software (version 2.6.1) to investigate the relationship between growth morphology and Cd stress in P. zhennan. Differences were considered significant at p<0.05. Principal component analysis (PCA) was conducted using Canoco 5 software to compare the effects of adding nutrients on P. zhennan under Cd stress. To evaluate the impact of nitrogen, phosphorus, and mixed addition on the stress resistance Cd stress, mentioned growth and physiological indicators were measured and analyzed using treatment effect (Treatment effect = (T − CK)/(CK) *100) analysis. According to (Cisse et al., 2021), the fuzzy subordinate function analysis as described in their study, utilized the following calculation formula: U(Xi) = (Xi - Xmin)/(Xmax - Xmin). In this formula, U(Xi) represents the value of the subordinate function analysis for a specific parameter, Xi denotes the value of that parameter, Xmin indicates the minimum value of the parameter, and Xmax represents the maximum value of the parameter.

Table 3 reveals the influence of Cd, N, and P on the photosynthetic parameters of P. zhennan seedlings. The N treatment, in the absence of Cd, significantly decreased Chla by 51.74%, Chlb by 62.28%, and TChl by 53.20%. However, it only slightly influenced the Pn, Gs, Ci, Tr, and Caro by 7.6%, 7.9%, 2.0%, 3.6%, and 37.8% respectively. The absence of Cd also saw the P and NP treatments significantly enhance the Tr by 58.02% (p<0.05) compared to the CK treatment. In contrast, Cd stress significantly reduced the photosynthetic Pn by 21.04%, accompanied by decreases in Chl a, b, and TChl levels when compared to the CK treatment. Finally, the CdHN treatment resulted in decreased levels of Pn, Gs, Ci, and Tr while increasing the levels of Chla, Chlb, TChl, and Caro compared to CdH.

The Cd levels of P. zhennan in various organs under N, P, and Cd treatments are depicted in Supplementary Table S8 , Figure 5 calculated at the end of the experiment. The values of the Cd contents were in the following order: 2nd roots > 1st roots > 3rd roots > stems > leaves, indicating that under Cd-free stress, the 2nd roots had the highest Cd content and the leaves had the lowest. When N was added without Cd, Cd was more readily absorbed and accumulated in many organs, and the levels of the stem (61.96%) and third roots (57.44%) were significantly different. Similar to this, the addition of NP without Cd treatment markedly enhanced Cd accumulation in the stem, first root, and second root by 22.17%, 100%, and 24.86% respectively. However, P addition without Cd stress significantly reduced Cd accumulation in the second root.

Upon CdH treatment, the Cd content was the highest in the roots and the lowest in the leaves. Interestingly, both CdHN and CdHP treatments augmented Cd accumulation in the stem and leaf, while slightly decreasing it in the roots. However, CdHNP treatment substantially mitigated Cd accumulation in leaves compared to CdH.

The correlation analysis depicted that, the osmoregulatory substances including Pro, SP, and GSH had a significant positive correlation (*p<0.05, **p<0.01, ***p<0.001) with antioxidant enzymes activities including POD, SOD, CAT, and APX ( Figure 6 ). On the other hand, a significant negative correlation was shown among antioxidant enzymes (POD, SOD, CAT, and APX) with SDW, LDW, RDW, AB, and TB. However, the root architecture RL and RV showed a negative correlation with the 1^st RDW, 2^nd RDW, and 3^rd RDW and photosynthetic activities including stomatal conductance, transpiration, and intracellular CO₂.

PCA based on the growth, dry weight, photosynthesis, physiology, and root biomass factors of different root orders of P. zhennan seedlings under cadmium-free and added cadmium stress was conducted ( Figure 7 ). The results of PCA convincingly demonstrated the impact of mixed addition of N, P, and NP on P. zhennan seedling growth and physiology under both Cd-free and added Cd stress. The cumulative contribution of PC1 and PC2 was 88.99%. Pn indicated a significantly negative correlation with CAT, GSH, POD, LDW, 2nd RDW, RV, and a significant positive correlation with SDW, AB, TB, LDW, and RV. Furthermore, no noticeable correlation across each indicator under CdH and CdHN treatments was found.

To evaluate the impact of nitrogen, phosphorus, and mixed addition on the stress resistance Cd stress, various growth and physiological indicators were measured and analyzed using the fuzzy subordinate function. The results are summarized in Supplementary Table S9 , where higher mean values of the comprehensive evaluation indicate better plant growth and greater resistance to abiotic stress. The comprehensive evaluation values for each treatment under Cd-free stress were found to be in the order of P > NP > CK > N, indicating that phosphorus had the most positive effect on promoting the growth and development of P. zhennan. Under Cd stress, the order of comprehensive evaluation values was found to be CdHP > CdHNP > CdH > CdHN. This suggests that CdHP and CdHNP treatments can alleviate the negative effects of Cd stress, with CdHP having the most significant effect.

The preference of plants for different forms of N for optimal growth is an evolved characteristic, with some favoring NH₄ ⁺-N and others NO₃ ^–N (Guo et al., 2022). Yet, an excess of both NH₄ ⁺ and NO₃ ^– can pose harmful consequences. For instance, a 75:25 ratio of NO₃ ^–-N and NH₄ ⁺-N is the most beneficial, while any surfeit of NH₄ ⁺-N impeded growth (Huang et al., 2016). Our investigation into P. zhennan seedlings revealed higher NH₄ ⁺-N content in leaves ( Supplementary Figure S1 ) when N and NP were applied with the NO₃ ^–-N content correspondingly lower compared to the control group. These results suggest that P. zhennan seedlings demonstrate a preference for NH₄ ⁺-N and a diminished ability to utilize NO₃ ^–-N. Indeed, high NO₃ ^–-N concentrations may have toxic effects that inhibit growth.

The N supplementation has been shown to enhance plant growth and stress resistance, including to Cd, but excessive N lead to issues such as plant lodging and root strength reduction (Haynes and Goh, 1978; Mizuta et al., 2020; Aye and Masih, 2023; Raza et al., 2023). It has impacted plant root systems directly and indirectly by altering soil N availability (Galloway et al., 2004), which is pivotal for root dynamics (Vogt et al., 1995). An increase in soil N has been linked to decreased fine root biomass (Galloway et al., 2004; Hendricks et al., 2006). In our study, we observed that the N addition inhibited P. zhennan growth and induced negative responses under Cd stress ( Figure 2 ), contradicting some previous findings and calling for further investigations into the underlying mechanisms. Excess N application decreased leaf LDW, stem SDW, AB, and TB significantly and reduced IPH and IGD moderately, pointing towards potential nutrient imbalance (Galloway et al., 2004; Atkinson and Urwin, 2012; Sinclair and Rufty, 2012). Under Cd stress, N application (CdHN treatment) further decreased PH, SDW, AB, and TB, in line with prior research suggesting Cd stress disrupts nitrogen metabolism in plants (DalCorso et al., 2010). This differential response of growth parameters to Cd stress and N application indicates a possible unknown stress-response mechanism in plants which needs further exploration. The possibility of an unknown self-defence mechanism triggered by Cd stress and N application offers an intriguing avenue for future research. The observed negative response could be due to N toxicity (Goyal and Huffaker, 1984), though the detoxification response and underlying mechanism are yet to be elucidated. Our findings have significant ecological implications, suggesting a complex interplay of nutrients and stressors that may redefine our understanding of plant responses to environmental stressors.

Studies show that Cd inhibits plant growth even in the presence of N fertilizers (Yao et al., 2012). Confirming this, our research found that the groups treated with high concentrations of Cd (CdH, CdHN, and CdHNP) exhibited the lowest plant heights. Yet, P treatment under Cd stress has been reported to increase plant dry matter mass (Chia et al., 2015; Gong et al., 2019; Maqbool et al., 2022; Wang et al., 2022), possibly due to P’s ability to form insoluble phosphates with Cd, thus limiting its toxicity. Intriguingly, N application under Cd stress was found to enhance Cd absorption in leaves, while P application concurrently with fertilizers decreased it. This points to a synergistic interaction between N and P applications, as well as N and limited Cd, in mitigating the toxic effects of N application.

Plants have been observed to naturally compartmentalize toxic elements like Cd, predominantly in roots to reduce their translocation to aerial parts (Shi et al., 2019). Our study concurs with these findings showing Cd accumulation in 2nd roots > 3rd roots > 1st roots > stems > leaves under CdH conditions. The N addition, in the absence of Cd, led to a rise in Cd accumulation in several organs, possibly due to N synergy in Cd uptake and translocation (Tie et al., 2019; Jia et al., 2020). This effect might also be associated with N’s role in enhancing root hair formation and surface area available for nutrient uptake including Cd (Zhou et al., 2020). Meanwhile, the addition of NP without Cd treatment notably increased Cd accumulation in the stem, 1st root, and 2nd root. This increase may be attributed to the enhanced nutrient uptake and translocation capacity under NP treatment (Dai et al., 2018), and potentially the promotion of root exudates aiding Cd uptake (Jia et al., 2020). In contrast, the addition of P alone significantly reduced Cd accumulation, possibly due to the competitive inhibition of Cd uptake by P and the formation of less absorbable Cd-P complexes (Tariq et al., 2017). Moreover, especially under the addition of NP, the concentration of Cd in roots of different root sequences under CdH was reduced, thereby reducing its impact on the normal function of roots and improving the ability of P. Zhennan to resist Cd pollution ( Figure 5 ).

Our investigation reveals that both N and Cd act as stressors, adversely impacting P. zhennan. Notably, N supplementation appears to intensify the Cd stress effect. When combined, the detriments to P. zhennan exceed those imparted by either Cd or N alone, yet the observed inhibitory effect falls short of a simple additive expectation. Intriguingly, the co-supplementation of N and P seems to mitigate the stress effects imposed by cadmium and nitrogen, underscoring the potential benefits of balanced nutrient strategies for stress resistance.

This study confirms that elevated Cd levels in both CK and CdH treatments reduce chlorophyll content adversely affecting photosynthetic activity (Chen et al., 2020; Zhou et al., 2020). However, we found that N and P supplementation can counteract these deleterious effects, bolstering chlorophyll and carotenoid content, and thereby enhancing photosynthetic and transpiration processes. It has been demonstrated that phosphorus application can amplify stomatal conductance and water use efficiency (Liu and Guo, 2013). Our study aligns with these findings, with P and combined NP fertilizers enhancing stomatal conductance in P. zhennan leaves. However, contrary to expectations our results showed a decrease in water use efficiency. In the presence of Cd, no significant difference was observed in transpiration rate between CK and CdH treatments suggesting that fertilization ameliorates transpiration rate, while Cd stress impairs photosynthetic rate and water use efficiency. Our study concurs with (El Rasafi et al., 2022), that Cd stress can lower the net photosynthetic rate and stomatal conductance of leaves. The data underscore the assertion that Cd stress predominantly hampers plant photosynthesis and that non-stomatal factors primarily constrain leaf transpiration rates.

The detrimental accumulation of ROS under stress, leading to potential plant death, is well-documented (Ayala et al., 2014). Our results extend this understanding by demonstrating that N supplementation can both exacerbate and ameliorate ROS accumulation, depending on the presence of Cd. We further observed that P and NP treatments consistently suppress ROS generation, which together with the observed reduction in MDA content (El Rasafi et al., 2022), suggests effective stress mitigation.

While the role of plant antioxidants in mitigating Cd stress is well recognized (Yang et al., 2020), inconsistency prevails across studies examining the response of the antioxidant system to stress. Certain findings substantiate that the optimal P ratio enhances plant biomass, curtails oxidative stress, and augments antioxidant enzyme activity (Jiang et al., 2020; Maqbool et al., 2022). Our research echoes these observations noting an initial increase and subsequent decrease in POD activity with escalating Cd concentration in celery seedlings. Conversely, the SOD, CAT, and POD activities in Dendrobium officinale show an upward trend with rising Cd concentration, an outcome in alignment with typical plant stress responses (Jiang et al., 2020). Intriguingly, the CAT and POD activities in pepper seedlings initially recede and then advance, while the SOD activity consistently declines with an upward Cd content trajectory (Al-Khayri et al., 2022). A similar downward trend in SOD and POD activities is observed in Salix viminalis with increasing Cd content. However, the application of NP enhances the activities of all three antioxidant enzymes, deviating from our observations. The Cd exposure reportedly diminishes SOD activity and elevates MDA content in soybeans, while P application inversely impacts these parameters (Dixit et al., 2001). In contrast, our findings demonstrate an upsurge in these enzyme activities upon NP addition signifying an escalation in oxidative stress due to Cd exposure. The elevated MDA levels further underscore this oxidative stress manifestation, indicative of lipid peroxidation (Jia et al., 2020; Maqbool et al., 2022). However, the amalgamated NP application mitigates Cd stress, increases plant biomass, and substantially reduces MDA content.

Finally, we investigated the impacts of N, P, and NP supplementation on soluble protein, proline, and GSH levels under Cd stress. The observed increase in protein levels with N, P, and NP treatment, while proline and GSH levels remained relatively stable, aligns with existing literature (Hasanuzzaman et al., 2020). Interestingly, under Cd stress, these stress-responsive molecules were found to increase, indicating activation of plant defense mechanisms (Jia et al., 2020). However, Cd and P supplementation resulted in lower stress-responsive molecule levels, hinting at potential uptake competition (Grossiord et al., 2020; Jang et al., 2020). This point certainly merits further investigation.

Plants employ adaptive strategies such as root morphology modulation and metabolite exudation to augment tolerance to heavy metal stress (Liu et al., 2020). This research demonstrates that the P. zhennan root system encompassing RL, ARD, RS, and RV, exhibits significant growth under P treatment amidst Cd stress, indicating the potential of P to bolster root growth under heavy metal stress. This growth is particularly noticeable in the 1st root, which is crucial for nutrient absorption and supporting plant growth (Kong et al., 2010; Long et al., 2013). Interestingly, the 2nd and 3rd roots are associated with Cd enrichment, thereby enhancing stress tolerance via MDA level reduction, chlorophyll content increase, and promotion of transpiration and photosynthesis. Conversely, the N application appears to dampen these root growth parameters, intensifying stress and modifying root morphological characteristics. The root is the primary organ for Cd accumulation in plants (Dai et al., 2018). Generally, exposure to both N and P causes a decrease in the root-to-shoot ratio, reflecting higher biomass allocation to above-ground structures due to N and P stimulatory effect on shoot growth, coupled with Cd inhibitory effect on root growth. Nevertheless, under Cd stress, our study shows a significant increase in the root-to-shoot biomass ratio of P. zhennan seedlings (p<0.05), which tends to decline under N and P application. However, this response can fluctuate based on the plant species (Liu et al., 2018), Cd concentration, nutrient levels, and environmental conditions.

Our findings demonstrate the complex interplay between N, P, and Cd on root growth. Consistent with previous studies, excessive N supply decreased root dry weight, while P treatment enhanced root growth (Tariq et al., 2017; Tariq et al., 2019). Cd stress combined with high N levels had a synergistic negative effect on root growth (Li et al., 2017), whereas phosphorus alleviated Cd toxicity by activating Cd uptake and stimulating detoxification mechanisms (Jia et al., 2020). However, the combined N and P treatment mitigates Cd’s impact on root growth, suggesting contrasting or interactive effects of N and P under Cd stress. Further research is needed to elucidate the molecular and physiological mechanisms underlying these interactions and optimize nutrient management strategies for plants under heavy metal stress.