Introduction

The harmful effects of heavy metal (HM) pollution threaten the balance of the ecosystem (Khan et al. 2015). The presence of HM in soils affects plant growth and development, which could lead to a food security crisis if not properly managed (Ahmad et al. 2022; Zulfiqar et al. 2022b). Cadmium (Cd) is considered the most dangerous heavy metal for humans because of its great ambivalence (Ahmad et al. 2012). Due to urbanization and industrialization, increasing levels of Cd are found in the environment, especially in agricultural soils, as they threaten the food chain (Cheng et al. 2022; Li et al. 2023). The availability of Cd in the environment is due to industrial activities including mining and smelting, application of inorganic fertilizers and irrigation with untreated wastewater, as well as natural sources including mother rock weathering (Nawaz et al. 2021; Zulfiqar et al. 2022b; Li et al. 2023; Sabiha-Javied et al. 2023). Cd is not an essential element; however, plants absorb it when available in soil (Shahid et al. 2017). Cd high toxicity is well pronounced on plants by reducing all vital functions including germination (Aprile et al. 2019), growth (Ci et al. 2010), respiration (Huybrechts et al. 2019), transpiration (Haider et al. 2021), water relations (Rady and Hemida 2015), nutrient uptake (Qin et al. 2020), photosynthesis (Nazar et al. 2012), and biomass accumulation (Ahmad et al. 2012). Several studies reported its detrimental effects on other plants at different growth stages (Peralta-Videa et al. 2002; Daud et al. 2015; Shamshad et al. 2018). The entry of Cd into plant cells induces oxidative stress, resulting from the increased production of reactive oxygen species (ROS), which leads to cell damage and reduced plant productivity (Lamhamdi et al. 2013). Plants have antioxidant systems that protect them from oxidation damage through detoxification mechanisms, which are highly compartmentalized and complex in plant cells (Chen et al. 2023).

Germination is the first process where a seed plant embryo begins to grow and develop into a seedling (Lei et al. 2021). It requires energy to occur and initiate a regulation of enzymatic reactions which activates cell metabolism (Seneviratne et al. 2019; Zhang et al. 2021). Seed germination is blocked if any of these functions are negatively impacted (Lamhamdi et al. 2013). One of the explanations recognized for HM’s impact on the physiology of plants is that they induce many nutrient disruptions (Qin et al. 2020). Storage material degradation is one of the most important processes that regulate the germination of seeds after imbibition with water (Liu et al. 2022). Several studies reported the toxic impact of HM on seed germination (Cokkizgin and Cokkizgin 2010; Stanković et al. 2010; Lei et al. 2021), however, few reported on Cd (Ahmad et al. 2012; Ling et al. 2015; Baruah et al. 2019). For instance, Ahmad et al. (2012) evaluated the impact of different Cd concentrations on seed germination and seedling growth of different wheat cultivars. According to the authors, Cd was toxic to seed germination and shoot and root growth, and these effects were enhanced at higher concentrations. In another study, Baruah et al. (2019) investigated the toxic effect of increasing levels of Cd on the germination and growth of wheat, tomato, and pea seedlings, and reported that tomato seeds were highly sensitive to Cd, followed by pea seeds and finally wheat seeds. The results of this study also indicated that wheat has a greater potential for contamination of the food chain if grown in polluted soil due to Cd’s high transfer and mobility.

Durum wheat is used to satisfy most dietary needs of large numbers of people. Its nutritional value is of great importance, as it is one of the few crops that is extensively grown as a staple food (Rehman Arif et al. 2020; Pehlivan Karakas et al. 2021; Zheng et al. 2023). In addition, this crop is cultivated in different zones of the world on diverse soils with variable toxic metal levels (Aprile et al. 2019). Cd is one of the HMs that accumulate in the grain of durum wheat to levels that exceed the standards set by the Codex Alimentarius Commission (FAO/WHO 2019). To the best of our knowledge, many studies investigated the effect of Cd on wheat (Ahmad et al. 2012; Abbas et al. 2017; Nawaz et al. 2021), however, fewer on durum wheat (Aprile et al. 2019). In light of the growing concern surrounding HM pollution and its potential impact on food security, this study focuses on Cd and its effects on durum wheat germination. Our hypothesis is that Cd significantly inhibits the germination and early growth of durum wheat, with varying impacts on different cultivars (Karim, Nachit, and Louiza). Therefore, the present paper aims at understanding the growth changes that occur during seed germination under HM stress. In this work, by studying the morphological effects of Cd during the germination phase of durum wheat seeds with assessment of the tolerance of three cultivars (Karim, Nachit, and Louiza) to Cd stress, we present the extent of Cd uptake by wheat seedlings and its influence on germination.

Materials and methods

Plant material and experimental site

The experiments were performed using seeds of three varieties of durum wheat (Triticum durum L. cv. Karim, cv. Nachit, cv. Louiza), provided by the National Institute for Agronomic Research (INRA), Rabat, Morocco. The experiments were conducted at the Faculty of Science and Techniques of Mohammedia, within the laboratory of Biochemistry, Environment & Agri-food (LBEA) in Mohammedia, Morocco.

Seed germination

The study aims to investigate the influence of increasing concentrations of Cd (0, 0.125, 0.25, 0.375, 0.5, 0.675, 0.75, 0.875, 1 g/L) on germination, biomass production, and shoot and root length. The Cd solutions were prepared by dissolving CdCl2, H2O in ultrapure water, and adjusting the pH of the solution to 5.5 with HNO3.

Wheat seeds with a homogeneous aspect were surface sterilized with 5% (v/v) sodium hypochlorite for 10 min then rinsed several times with distilled water. Forty seeds of each variety were placed in Petri dishes that contained two filter papers to maintain moisture. The dishes were treated with 6 mL of sterilized water for the control and the same volume of Cd aqueous solutions for the other treatments. Petri dishes were incubated in the darkness at 25 ± 2 °C and were arranged using a completely randomized design (CRD) in a factorial layout with three replications. After 7 days, the number of germinated seeds was determined.

Growth parameters’ measurement

After 7 days of germination, the shoot (SL) and root (RL) length were measured with a ruler based on the length of the shoots and the average of the main axial roots. Seedling samples were weighed to determine fresh weight (FW) while dry weight (DW) measurements were obtained after drying samples in an oven at 70 °C for 48 h. Vigor index (VI), growth inhibition (GI), and tolerance index (TI) parameters were calculated to assess the effect of Cd on the development and plant growth as follows:

  • VI was calculated using the following formula (Baruah et al. 2019):

  • \( VI = \left( {{\text{radicale}}\;{\text{length}} + {\text{plumule}}\;{\text{length}}} \right) \times {\text{germination}}\;{\text{percentage}} \)

  • GI was determined for shoot and root using the following formula (Ahmad et al. 2013):

  • \( {\text{GI}} = \frac{{{\text{Mean}}\;{\text{Shoot}}\;{\text{or}}\;{\text{Root}}\;{\text{length}}\;{\text{in}}\;{\text{control}} - {\text{Mean}}\;{\text{shoot}}\;{\text{or}}\;{\text{Root}}\;{\text{length}}\;{\text{in}}\;{\text{Cd}}\;{\text{solution}}}}{{{\text{Mean}}\;{\text{Shoot}}\;{\text{or}}\;{\text{Root}}\;{\text{length}}\;{\text{in}}\;{\text{control}}}} \)

  • TI was determined using the following formula (Iqbal and Rahmati 1992):

  • \( {\text{TI}} = \frac{{{\text{Mean}}\;{\text{root}}\;{\text{length}}\;{\text{in}}\;{\text{metal}}}}{{{\text{Mean}}\;{\text{root}}\;{\text{length}}\;{\text{in}}\;{\text{control}}}} \times 100 \)

Statistics analysis

Statistical analyses were conducted using IBM SPSS Statistics Data Editor. The one-way general linear model ANOVA was used to assess the impact of each treatment. The Tukey test was performed for post hoc testing to identify significant differences between treatment groups. A significance level of 0.05 was used for all tests. To further explore the data, a Pearson (n-1) correlation and principal component analysis (PCA) were conducted using XLSTAT-software v.2016. The correlation analysis examined relationships between variables, while PCA reduced data dimensionality and identified underlying patterns.

Results

Effect of Cd on seed germination percentage

Cd had a significant negative effect on wheat seed germination (Fig. 1). However, the inhibition of germination varied between cultivars. Seed germination inhibition was observed for Karim, Nachit, and Louiza at 0.625, 0.875, and 0.625 g/L of Cd, respectively. Karim and Louiza seed germination decreased from 0.125 to 0.625 g/L and were significantly (p < 0.05) reduced starting from 0.625 of g/L with a germination percentage around 69% which continued to decrease with the increase of Cd concentrations to reach 26.67 and 5% respectively under 1 g/L of Cd. In the case of Nachit, 0.875 g/L of Cd significantly (p < 0.001) reduced the germination percentage with a value of 24.17%. This reduction was maximum at 1 g/L of Cd with 21.67% of seeds germination.

Fig. 1
figure 1

Effect of Cd on seed germination percentage of different wheat cultivars. The bars labeled by different letters indicate significant differences assessed by Tukey’s test after performing a one-way ANOVA (p < 0.05)

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Effect of Cd on root and shoot length and fresh and dry weights

The results showed that the negative effect of Cd was more evident in the early stages of seedling development compared to the germination stage where a significant (p < 0.05) inhibition was noticed in SL at 0.125 g/L of Cd for Karim and Louiza, respectively, and at 0.5 g/L of Cd for Nachit compared to the control (Table 1). Furthermore, a significant (p < 0.05) decline was recorded in RL at 0.25 g/L of Cd exposure for Karim, and 0.125 g/L of Cd for Nachit and Louiza compared to the control. The inhibition is enhanced at concentrations starting from 0.375 g/L of Cd for Karim, from 0.25 g/L of Cd for Nachit, and from 0.25 g/L of Cd for Louiza. Moreover, at 0.625, 0.875, and 0.375 g/L, Cd drastically inhibited RL for Karim, Nachit, and Louiza, respectively. Based on the highest Cd concentration, both SL and RL were negatively impacted in this order: Louiza < Nachit < Karim. On the other hand, Cd showed no significant effect on DW and FW for all the studied wheat cultivars compared to the control (Table 1).

Table 1 Effect of Cd on shoot and root length and fresh and dry weight of different wheat cultivars
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Effect of Cd on wheat tolerance indices

The effect of Cd on VI was found to be statistically highly significant (p < 0.001) according to Fig. 2. There was a notable variation among wheat cultivars in response to Cd, and this variation was also statistically significant (p < 0.001). At a concentration of 0.125 g/L of Cd, VI exhibited a drastic reduction. Specifically, for Karim, the decrease was 37.4%, for Nachit, it was 39.5%, and for Louiza, it was 54.94%. As the Cd concentrations increased to 0.375, 0.5, 0.625, 0.75, 0.875, and 1 g/L of Cd, Karim experienced a progressive decrease in VI with values of 74.82, 80.12, 89.30, 97.28, 99.03, and 99.1%, respectively. Similarly, Louiza exhibited a decline in VI with values of 87.32, 90.47, 91.75, 96.11, 99.26, and 99.97% for the corresponding Cd concentrations. In the case of Nachit, the VI showed a dose-dependent decrease where it notably declined at Cd concentrations of 0.875 and 1 g/L of Cd, reaching values of 99.47 and 99.56%, respectively (Fig. 2).

Fig. 2
figure 2

Effect of Cd concentrations on vigor index of different wheat cultivars. The bars labeled by different letters indicate significant differences assessed by Tukey’s test after performing a one-way ANOVA (p < 0.05)

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The response of wheat cultivars TI to increasing levels of Cd stress exhibited variability (Fig. 3). TI progressively decreased for all cultivars with increasing concentrations of Cd. Notably, significant reductions were observed for Nachit (41.37%) and Louiza (44.85%) at a low concentration of 0.125 g/L of Cd, and for Karim (37.70%) at 0.25 g/L of Cd. Despite a decrease in TI at the lowest Cd concentration, Nachit demonstrated superior tolerance to Cd stress under 0.375, 0.5, 0.625, and 0.75 g/L, with tolerance rates of 30.35, 26.16, 25.05, and 21.97%, respectively. In comparison to Nachit, Karim, and Louiza displayed lower TI of 19.36, 11.38, 6.17, 1.91%, and 8.15, 5.16, 4.00, 2.91%, respectively, during the same range of Cd concentrations. At high concentrations, Karim, Nachit, and Louiza exhibited minimal tolerance compared to the control (100%), with TI values of 1.27 and 1.09% for Karim, 0.66 and 0.65% for Nachit, and 1.43 and 0.12% for Louiza at 0.875 and 1 g/L of Cd, respectively. Overall, Nachit cultivar showed the highest TI values, followed by Karim and then Louiza.

Fig. 3
figure 3

Effect of Cd concentrations on tolerance index of different wheat cultivars. The bars labeled by different letters indicate significant differences assessed by Tukey’s test after performing a one-way ANOVA (p < 0.05)

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The impact of Cd stress on root growth inhibition (RGI) was more pronounced compared to its effect on shoot growth inhibition (SGI) (Fig. 4). SGI was significantly increased for Karim and Louiza cultivars at the same concentration of Cd, while no significant change was recorded for Nachit cultivar in comparison to the control. On the other side, both Nachit and Louiza cultivars recorded significant increase of RGI at 0.125 g/L of Cd stress, while no significant effect was noticed for Karim cultivar. Increased significant (p < 0.001) values of SGI were observed at higher concentrations of 0.875 and 1 g/L of Cd compared to the control. Indeed, Karim and Louiza exhibited significant (p < 0.001) improved values of SGI starting from 0.125 g/L of Cd, with maximum values occurring at 0.875 and 1 g/L of Cd compared to the control. On the other hand, RGI was significantly affected in all cultivars starting from 0.125 g/L of Cd for Nachit and Louiza (p < 0.001), and at 0.25 g/L of Cd for Karim (p < 0.001). Among the cultivars, the order of increased SGI values was Nachit < Karim < Louiza, whereas for RGI, it was Karim < Nachit < Louiza.

Fig. 4
figure 4

Effect of Cd concentrations on shoot and root growth inhibition of different wheat cultivars. The bars labeled by different letters indicate significant differences assessed by Tukey’s test after performing a one-way ANOVA (p < 0.05)

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Pearson correlation and principal component analysis

The correlation matrix reveals several noteworthy relationships among the variables (Table 2). One particularly important finding is the strong significant (p < 0.001) correlations observed between GP and six of the measured parameters. GP showed a significant positive correlation with SL (r = 0.826, p < 0.001), VI (r = 0.719, p < 0.001), TI (r = 0.655, p < 0.001), and RL (r = 0.644, p < 0.001) and a significant negative correlation with RGI (r = − 0.812, p < 0.001), and SGI (r = − 0.655, p < 0.001). Three significant positive correlations were recorded among the growth parameters. The strongest significant correlation (r = 0.862, p < 0.001) was observed between SL and RL, followed by the correlation between FW and DW (r = 0.620, p < 0.001), and then the correlation between SL and FW (r = 0.519, p < 0.05). These parameters, especially RL and SL showed strong correlations with the tolerance parameters where they recorded a significant positive correlation with VI (r = 0.986, p < 0.001 and r = 0.931, p < 0.001, respectively) and TI (r = 0.973, p < 0.001 and r = 0.858, p < 0.001, respectively) and a significant negative correlation with SGI (r = − 0.973, p < 0.001 and r = − 0.858, p < 0.001, respectively) and RGI (r = − 0.808, p < 0.001 and r = − 0.970, p < 0.001, respectively). Among tolerance traits, VI and TI showed a significant positive correlation (r = 0.968, p < 0.001) and recorded a significant negative correlation with SGI (r = − 0.968, p < 0.001 and r = − 1.000, p < 0.001, respectively) and RGI (r = − 0.886, p < 0.001 and r = − 0.801, p < 0.001, respectively). In addition, SGI and RGI showed a positive significant correlation (r = − 0.801, p < 0.001).

Table 2 Correlation matrix (Pearson (n-1)) of measured variables
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Principal component analysis (PCA) took into account all the analyzed parameters of the different treatments and showed that first and second components had a high cumulative variance (96.23% for Louiza, 90.27% for Karim, and 87.04% for Nachit) (Fig. 5). The biplot of the PCA analysis revealed that elevated Cd concentrations (0.75, 0.875, and 1 g/L of Cd) were positively associated with both SGI and RGI but especially with RGI for all wheat cultivars. In contrast, high Cd exposure led to a significant reduction in several parameters, including GP, SL, RL, FW, VI, and TI for all cultivars. Interestingly, the negative effect of Cd on Louiza growth and tolerance parameters (Fig. 5c) was observed even at medium concentrations (0.375, 0.5, 0.625 g/L) compared to Karim (Fig. 5a) and Nachit (Fig. 5b). Furthermore, the analysis demonstrated that Louiza DW was most reduced at higher Cd concentrations, whereas Karim and Nachit DW did not exhibit comparable susceptibility to the higher Cd concentrations in terms.

Fig. 5
figure 5

Principal component analysis of the studied traits of wheat under different Cd concentrations for three cultivars a Karim, b Nachit, and c Louiza. DW: Dry weight, FW: Fresh weight, GP: Germination percentage, RGI: Root growth inhibition, RL: Root length, SGI: Shoot growth inhibition, SL: Shoot length, TI: Tolerance index, VI: Vigor index

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Discussion

Cd is a hazardous HM known to have severe effects on wheat and can cause various diseases in humans (Li et al. 2023). Therefore, further investigation into the toxicity of Cd in this essential crop is warranted to enhance our understanding of the underlying processes. Significant variations were observed among the studied cultivars (Karim, Nachit, and Louiza) treated with different concentrations (ranging from 0.125 to 1 g/L) of Cd. Our study revealed that the presence of Cd has suppressive effects on both seed germination and seedling growth. The same findings were reported by previous studies (Ali and Nas 2018; Safari et al. 2020) where Cd application reduced germination percentage and index, hampered root and shoot length, decreased tolerance levels, and negatively impacted the dry mass of both roots and shoots. In a recent study, Shedeed and Farahat (2023) analyzed the effects of two Cd concentrations (80 and 100 mg/L) on wheat seed germination and found that Cd decreased germination rates, increased radicle phytotoxicity, and reduced seedling biomass. In our study, Cd showed similar effects on wheat seeds. Furthermore, it significantly inhibited germination (p < 0.05) for Karim and Louiza at 0.625 g/L (69.17%) and for Nachit at 0.875 g/L (24.17%). It was explained that the inhibitory effect of Cd on seed germination and growth is mainly due to reduced water uptake, which hinders the growth of the seed embryo (Huybrechts et al. 2019; Haider et al. 2021). Moreover, the deactivation of starch degradation in the endosperm, results in impaired transfer of soluble sugars to the embryo axis of the seed, ultimately exacerbating nutrient deficiency in the embryo (Rahoui et al. 2015). There have been reports of a reduction in alpha-amylase activity as a result of a decrease in starch release from cotyledons under the influence of Cd stress (Kalai et al. 2016). FW and DW were not significantly affected, in our experiment, by the increasing concentrations of Cd, which was also reported in the study of Mahmood et al. (2009) where the biomass yield was not impacted by Cd stress. However, Cd significantly reduced shoot and root length for all cultivars. Several studies support our results showing that Cd adversely affected the normal growth of wheat and other plants by reducing seed germination and decreasing root and shoot length (Guilherme et al. 2015; Zhang et al. 2019; An et al. 2022; Lu et al. 2022; Zhou et al. 2023; Zhao et al. 2023). Root growth inhibition was more pronounced than shoot growth inhibition. It was explained that Cd stress has a more noticeable impact on durum wheat roots causing alterations in root structure, including changes in root morphology, disruption of the integrity of cell membranes and walls, reduced cell number, and altered cell shapes (Popa et al. 2020; Sabella et al. 2022). Additionally, it is known that the suppression of root elongation has been demonstrated as an initial and distinct manifestation of Cd toxicity (He et al. 2017). This inhibition may be also attributed to the fact that Cd stress can induce water deficit in seedlings (Seneviratne et al. 2019). Among the cultivars, the order of inhibition for shoot length was Nachit < Karim < Louiza, whereas for root length, it was Karim < Nachit < Louiza, which revealed that there parameters were differentially affected by Cd concentrations for the different cultivars. Furthermore, Cd application has been shown to inhibit seedling growth by disrupting the breakdown of storage proteins through interactions with proteolytic enzymes (Han et al. 2023). This, together with changes in protein patterns and reduced root respiration, leads to increased nitrite production and impaired cellular functions, further contributing to the retardation of seedling growth (Seneviratne et al. 2019; Popa et al. 2020; Zulfiqar et al. 2022a). This reduction in root size led to diminished viability of root cells and an increase in cell death, accompanied by the gradual production of ROS (Thind et al. 2021; Hasan et al. 2022). Furthermore, the decline in seedling growth can be attributed to the physiological limitations caused by Cd stress, which leads to a reduction in plant vigor and overall growth, depending on Cd dose (Rady and Hemida 2015; Carvalho et al. 2023). These findings are consistent with our results, as the vigor index showed a significant decrease (p < 0.001) under increased Cd concentrations; however, this effect showed discrepancies between different cultivars. Indeed, Nachit cultivar showed higher resistance toward Cd stress and higher germination rates for most treatments (0.125–0.75 g/L) compared to Karim and Louiza cultivars. This could potentially be attributed to intervarietal genetic variances, which confer either resistance or susceptibility to Cd stress (Ahmad et al. 2012). TI clearly revealed the response of the studied cultivars toward increasing Cd concentrations that not only reduced tolerance, but also inhibited plant growth. Nachit cultivar showed the highest TI values, followed by Karim and then Louiza. The diverse reactions of the different wheat cultivars to Cd stress indicate the existence of inherent resistance in some cultivars, while others may lack such resilience. Similarly to other parameters, the lower TIs observed in wheat cultivars under Cd stress can be linked to modifications in key physiological mechanisms. These include reduced uptake of water and nutrients, impaired enzymatic activities crucial for metabolism and energy production, hindered hormonal regulation, and elevated production of ROS (He et al. 2017; Qin et al. 2020; Haider et al. 2021; Zulfiqar et al. 2022a). These alterations occur during the growth stages of wheat plants and collectively contribute to the decreased tolerance toward Cd stress (Amirahmadi et al. 2020).

Conclusion

Our findings indicate that germination percentage, as well as root and shoot length of early stages of growth, serve as reliable indicators of Cd toxicity in wheat. Cadmium exhibited detrimental effects on wheat growth parameters even at low concentrations, with the highest level of inhibition observed at the highest applied concentration (1g/L). Nachit cv demonstrated superior performance, as evidenced by its leading TI values, while Louiza cv showed the lowest. Significant correlations are found among various growth and tolerance parameters. Notably, Nachit emerged as the most tolerant cultivar, suggesting its potential suitability for successful cultivation in Cd-contaminated soil. These findings emphasize the intricate relationship between Cd exposure and wheat development, calling for further investigations to unveil the underlying mechanisms of the differential tolerance of the studied cultivars to Cd toxicity also Cd contamination’s implications for food security and environmental health.