Irrigation water pH regulates cadmium accumulation, growth performance, and forage quality trade-offs in Sorghum (Sorghum bicolor L.)

Abstract

Background

Cadmium (Cd) contamination is a serious challenge to agricultural sustainability, threatening soil health, crop productivity, and the safety of food and feed chains globally. Irrigation water chemistry can significantly alter Cd mobility, plant uptake, and forage quality in arid and semi-arid regions. Sorghum (Sorghum bicolor L.) is an important forage valued for its high biomass production and stress tolerance. However, the role of irrigation water pH in regulating Cd accumulation, soil chemistry, and forage quality in sorghum remains poorly understood.

Methods

A pot experiment was conducted in which sorghum cv. ‘Master BMR’ was grown in soil contaminated with 10 mg kg⁻¹ Cd (cadmium sulfate, 3CdSO₄·8 H₂O). Irrigation water was adjusted to pH levels of 5.0, 6.0, 7.0, 8.0, and 9.0 using HCl/NaOH over a 120-day season. Cadmium concentrations in plant tissues, morphological traits, soil chemical properties, forage quality parameters, translocation factors (TF), and bioconcentration factors (BCF) were evaluated to determine the effects of irrigation water pH on Cd behavior and plant performance.

Results

Increasing irrigation water pH reduced Cd concentrations in roots, stems, leaves, and panicles. Root BCF was highest under acidic irrigation and lowest at pH 9. Alkaline irrigation was associated with lower tissue Cd concentration but higher ADF and NDF values and lower crude protein content. Better plant growth and biomass production were observed under moderately acidic to neutral irrigation conditions (pH 6–7). Multivariate analyses revealed strong interactions among Cd accumulation, soil chemistry, forage quality, and growth-related traits across the pH gradient.

Conclusions

Irrigation water pH strongly influenced Cd behavior and forage quality in sorghum. Alkaline irrigation water lowered Cd concentrations in plant tissues but reduced forage nutritional value by increasing fiber fractions and decreasing crude protein. Considering Cd accumulation, plant growth, and forage quality together, pH 6–7 represented the most favorable overall compromise under the present pot-experiment conditions, although some aboveground Cd concentrations remained relatively high at pH 6. These results indicate that a near-neutral pH in irrigation water could mitigate heavy-metal risks in Cd-contaminated forage production systems. Nonetheless, further validation across various soils, sorghum varieties, levels of Cd contamination, and field conditions is necessary prior to making broad management recommendations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12870-026-09805-y.

Introduction

Heavy metal contamination of agricultural soils represents a growing global concern, with cadmium (Cd) recognized as one of the most hazardous contaminants due to its high mobility within the soil–plant system and severe adverse effects on soil health, plant physiology, and crop productivity [1, 2]. Cd enters food and feed chains through contaminated agricultural soils and has no known biological function in living organisms. However, it causes serious physiological, biochemical, and metabolic disorders in plants [1]. The tolerable weekly intake (TWI) for Cd is 2.5 µg kg⁻¹ body weight per week [3]. Higher dietary exposures have been reported in several vulnerable populations in various countries [4–6]. Globally, ~ 235 million ha of cultivated land have been affected by heavy metal pollution [7].

Approximately 19.4% of China’s agricultural land is contaminated with heavy metals, and nearly one-third of this contamination is attributable to Cd accumulation [8]. Agricultural soils in Türkiye (particularly in Eastern Anatolia) are similarly vulnerable to heavy metal contamination [9]. The FAO/WHO does not specify a universal permissible Cd limit for agricultural soils. Instead, it provides commodity-specific maximum levels for Cd in food and feed, highlighting the need to interpret soil Cd risk alongside plant tissue Cd concentrations and forage safety [10]. The former Soil Pollution Control Regulation stated pH-dependent Cd threshold values of 1.0 mg kg⁻¹ for soils with pH < 6 and 3.0 mg kg⁻¹ for soils with pH > 6 [11]. However, the current contaminated-sites framework uses pathway-specific generic pollutant limit values for Cd [12]. Several studies have reported elevated concentrations of Cd, Pb, and Cr in agricultural lands along major transportation corridors [9, 13, 14]. Anthropogenic activities such as industrial emissions, intensive use of phosphate fertilizers, and irrigation with wastewater are considered the principal sources of Cd enrichment in Turkish soils [15].

Sorghum (Sorghum bicolor L.) is an important C4 cereal crop widely cultivated in arid and semi-arid regions [16] for its high biomass production and adaptability to unfavorable environmental conditions [17–19]. Sorghum has attracted attention for phytoremediation due to its high Cd tolerance compared with wheat and maize [20–23]. Heavy metal accumulation in plant tissues can substantially alter physiological and nutritional processes, ultimately compromising crop quality [24]. Nonetheless, management strategies that reduce Cd accumulation without compromising forage quality remain critically needed.

Soil pH is a primary regulator of Cd mobility, controlling metal solubility, adsorption–desorption equilibria, and speciation [25–28]. Recent studies have emphasized that soil–water chemical interactions strongly regulate nutrient (ion) mobility, adsorption–desorption equilibria, and ion availability within tropical agricultural systems [29]. Increased proton activity under acidic conditions enhances Cd desorption from soil particles, thereby increasing the concentration of soluble Cd²⁺ ions available for plant uptake. In contrast, alkaline conditions generally reduce Cd mobility through precipitation and adsorption processes [30, 31]. Increasing soil pH can substantially reduce Cd accumulation in agricultural crops by decreasing Cd bioavailability in the rhizosphere [32, 33]. Nevertheless, direct modification of soil pH in contaminated agricultural lands is often expensive, labor-intensive, and slow because soils have considerable buffering capacity that resists rapid pH changes [34]. Modifying the pH of irrigation water may be a rapid and cost-effective alternative.

Several strategies have been proposed to reduce Cd bioavailability, including liming [32], biochar application [35], organic amendments [36], microbial inoculation [37], and nutrient management [38]. Recent studies have demonstrated that soil amendments that modify rhizosphere chemical conditions can substantially affect soil microbial activity and nutrient dynamics in agricultural systems [39]. However, many require long-term management or substantial investment, making short-term, irrigation-based strategies particularly attractive in irrigation-dependent systems [40, 41]. Despite the importance of pH in controlling heavy metal mobility, little information is available regarding the role of irrigation water pH in regulating Cd accumulation, plant growth, and forage quality in sorghum cultivated under Cd-contaminated conditions.

Cd contamination and related heavy-metal toxicity also pose risks to livestock health through contaminated forage [42]. Management practices aimed at reducing Cd accumulation may alter forage nutritional quality [43]. Forage quality parameters such as crude protein (CP), acid detergent fiber (ADF), and neutral detergent fiber (NDF) are critical indicators of feed digestibility and nutritional value for livestock production systems. Changes in soil chemical conditions and Cd stress responses may influence carbohydrate metabolism, protein synthesis, and structural fiber accumulation within plant tissues, potentially creating trade-offs between reduced Cd accumulation and forage nutritional quality. However, integrated studies that simultaneously evaluate Cd accumulation, soil chemical responses, plant growth, and forage quality characteristics under different irrigation water pH levels remain scarce.

Therefore, it was hypothesized that increasing the pH of irrigation water would reduce Cd bioavailability and Cd accumulation in sorghum tissues, but might also affect plant growth performance and forage quality traits. The objective of this study was to investigate the effects of irrigation water with different pH levels on Cd accumulation, soil chemical properties, morphological characteristics, and forage quality traits of sorghum grown under Cd-contaminated conditions. The study aimed to evaluate whether modifying the pH of irrigation water could serve as a practical, short-term management strategy to reduce Cd accumulation in forage crops grown on contaminated soils.

Materials and methods

Experimental site and plant material

This pot study was conducted during the 2022 growing season in the experimental garden of the Faculty of Agriculture at Bingöl University, Bingöl, Türkiye (38°54′0″ N, 40°29′18″ E). The region is characterized by a semi-arid continental climate with hot and dry summers. Average daily air temperatures fluctuated between 18.4 °C and 34.7 °C during the experimental period. Similarly, total precipitation was 42 mm, confirming the hot, arid conditions characteristic of the experimental area. All pots were kept in the open experimental garden under natural sunlight. Sorghum (Sorghum bicolor L.) cv. ‘Master BMR’ was used as the plant material due to its high biomass production and tolerance to environmental stress conditions. The seeds were procured from the local market in Bingöl, Türkiye.

Surface soil (0–20 cm depth) used in the experiment was collected from the Bingöl University campus area, air-dried, homogenized, and passed through a 4-mm sieve prior to use. Plastic pots were filled with 10 kg of prepared soil. The soil is classified as a Typic Calcixerept (USDA Soil Taxonomy) with a loamy sand texture (72% sand, 18% silt, 10% clay). Selected physicochemical properties of the experimental soil were determined before the initiation of the experiment (Table 1). The low soil organic matter level (0.38%) is characteristic of surface soils in semi-arid parts of Eastern Türkiye, where limited plant inputs, rapid oxidation due to elevated temperatures, and low clay content inhibit organic matter production.

Table 1.

Chemical properties of the soil used in the experiment

pH	EC	Lime	O.M	P	K	Ca	Mg	Available Cd
	µS cm− 1	%	%	mg kg− 1	mg kg− 1	mg kg− 1	mg kg− 1	mg kg− 1
6.88	190.30	1.96	0.38	21.20	354.50	7912.0	302.50	0.06

Cadmium contamination and experimental design

Soil was contaminated with Cd by applying cadmium sulfate [3CdSO₄·8H₂O] to achieve a final concentration of 10 mg Cd kg⁻¹ soil. The Cd solution was uniformly mixed into the soil prior to transplanting to ensure homogeneous distribution within each pot. The selected Cd concentration (10 mg kg⁻¹ soil) represents a moderately severe contamination level consistent with previous controlled studies on sorghum [22, 23, 44]. Cd concentrations in most agricultural soils are usually much lower (0.1–0.5 mg kg⁻¹ globally), and Cd levels above 1–3 mg kg⁻¹ are generally considered elevated or contaminated [11]. Nevertheless, 10 mg kg⁻¹ has been used to simulate mining/e-waste-affected contamination under controlled conditions [45, 46].

The experiment was arranged in a completely randomized design with five irrigation water pH treatments. The treatments consisted of pH 5.0, 6.0, 7.0, 8.0, and 9.0. Each treatment consisted of five replicates, resulting in a total of 25 experimental units.

Plant cultivation and irrigation treatments

Sorghum seeds were initially germinated in seedling trays under greenhouse conditions. Uniform seedlings 10–15 cm in height were transplanted into Cd-contaminated pots. Each pot received basal fertilization of 120 mg N kg⁻¹ soil (urea), 60 mg P₂O₅ kg⁻¹ soil (triple superphosphate), and 60 mg K₂O kg⁻¹ soil (potassium sulfate) before seedling transplantation. The fertilizers were thoroughly incorporated into the soil. Plants were irrigated daily with water adjusted to the designated pH levels throughout the experimental period (~ 120 days). Irrigation water pH was adjusted using hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions. The pH of irrigation water was monitored using a digital pH meter before each irrigation event. The use of HCl and NaOH for pH adjustment can introduce Cl⁻ and Na⁺ ions into the soil, which may affect ionic equilibrium and electrical conductivity, as well as alter pH. Therefore, applied volumes were carefully regulated to minimize secondary ionic effects, with changes in soil ionic composition primarily attributable to pH-induced shifts in ion solubility. Care was taken to avoid water drainage from the pots to minimize Cd leaching and maintain consistent Cd availability within the root zone.

Morphological traits

Plants were harvested manually at the end of the 120-day growth period and separated into roots, stems, leaves, and panicles. Morphological traits included plant height (cm), stem diameter (mm), panicle length (cm), shoot dry weight (g), and root dry weight (g). Plant height was measured from the soil surface to the tip of the panicle using a measuring tape. Stem diameter was measured using a digital caliper. Plant samples were washed with tap water, then distilled water, to remove soil particles and contaminants. Samples were oven-dried at 65 °C for approximately 48 h until a constant weight was reached, and then weighed.

Determination of cadmium concentration

Dried plant tissues were ground using a laboratory mill prior to chemical analysis. Approximately 0.5 g of each dried sample was digested with concentrated nitric acid (HNO₃) using a microwave digestion system, following the method described by Miller [47]. The samples were filtered and diluted with deionized water after digestion.

Cadmium concentrations in roots, stems, leaves, panicles, and soil extracts were analyzed using Atomic Absorption Spectroscopy (AAS; Perkin-Elmer Analyst 800, Cd hollow cathode lamp, wavelength 228.8 nm). Instrument calibration was performed using certified cadmium standard solutions (Merck) at concentrations of 0, 0.5, 1.0, 2.0, and 5.0 µg L⁻¹, yielding a correlation coefficient (R²) of at least 0.999. Certified reference material (BCR-281, rye grass) was used for quality assurance. Cadmium concentration in each tissue was expressed as mg kg⁻¹ dry weight, calculated as:

Cadmium translocation and bioconcentration factors

Cadmium translocation factors (TF) and root bioconcentration factor (BCF) were calculated using tissue-specific Cd concentrations. Translocation factors were calculated using Eq. 1.

1

Where separate TF values were calculated for stem/root, leaf/root, and panicle/root.

Root bioconcentration factors (BCF) were calculated by using Eq. 2.

2

Soil analyses

Soil samples collected after plant harvest were air-dried and sieved prior to analysis. Soil pH and electrical conductivity (EC) were measured in 1:2.5 (w/v) soil–water suspensions using a pH/EC meter, following Thomas [48] and Rhoades [49]. The pH meter (Hanna HI2210) and EC meter (Hanna HI8733) were calibrated daily using standard buffer solutions (pH 4.01, 7.01, 10.01) and a 1413 µS cm⁻¹ EC standard, respectively, in accordance with the manufacturer’s instructions. Lime content (%) was determined using a Scheibler calcimeter [50], while soil organic matter (%) was determined using the modified Walkley–Black method [51]. Available phosphorus (P) was determined using the 0.5 M NaHCO₃ extraction method [52]. Exchangeable potassium (K), calcium (Ca), and magnesium (Mg) contents were measured by 1 N ammonium acetate (NH₄OAc, pH 7.0) extraction, following Helmke and Sparks [53]. The available Cd concentration in soil was determined using the DTPA extraction method described by Lindsay and Norvell [54]. DTPA-extractable Cd offers an operationally defined estimation of potentially bioavailable Cd, rather than direct determination of actual rhizosphere Cd speciation. Consequently, mechanistic interpretations of Cd bioavailability should be regarded as assumptions derived from DTPA-extractable Cd and require direct speciation characterization.

Forage quality traits

Forage quality traits were determined using near-infrared reflectance spectroscopy (NIRS; FOSS NIRSystem 5000, FOSS Analytical, Denmark) with WinISI calibration software. Dried stem, leaf, and panicle samples were ground separately in a laboratory mill to pass through a 1-mm sieve, yielding homogeneous material for analysis. Ground samples were scanned using a calibrated NIRS instrument operating in the near-infrared wavelength range, following standard forage analysis procedures. The calibration equations were validated against reference wet-chemistry analyses, including Kjeldahl nitrogen determination for crude protein, expressed as N × 6.25 [55]. Acid detergent fiber (ADF) was determined according to AOAC Official Method 973.18 [56]. Neutral detergent fiber (NDF) was determined according to Van Soest et al. [57].

The measured forage quality parameters included crude protein (CP, %), acid detergent fiber (ADF, %), and neutral detergent fiber (NDF, %). Crude protein content was estimated based on total nitrogen concentration determined by NIRS calibration equations and expressed as a percentage of dry matter. Acid detergent fiber represents the cellulose and lignin fractions of plant cell walls and was used as an indicator of forage digestibility, whereas neutral detergent fiber represents total structural carbohydrates, including hemicellulose, cellulose, and lignin, and was used as an indicator of forage intake potential. All forage quality analyses were performed separately on stem, leaf, and panicle tissues to evaluate tissue-specific responses to irrigation water pH under cadmium stress. Results were expressed on a dry matter basis (%).

Statistical analysis

The collected data were subjected to one-way analysis of variance (ANOVA) [58] following a completely randomized design to evaluate the effects of irrigation water pH treatments. Normality and homogeneity of variance were evaluated using Shapiro–Wilk [59] and Levene’s tests [60], respectively. Treatment means were compared using Fisher’s least significant difference (LSD) test at P < 0.05. Pearson correlation analysis was conducted using standardized trait values, and the results were visualized as a correlation heatmap. Principal component analysis (PCA) was performed to investigate multivariate relationships among Cd accumulation, soil chemical, morphological, translocation, bioconcentration, and forage quality traits. Eigenvalues, percentage variance explained, and PCA biplots were generated using standardized data matrices. Hierarchical clustering heatmap analysis of Z-score-standardized data was additionally performed to identify integrated response patterns among irrigation water pH treatments and measured traits. Euclidean distance and complete linkage clustering methods were used for hierarchical clustering analyses. Linear regression analyses were conducted to evaluate the relationships between irrigation water pH and selected Cd accumulation, soil chemical, and forage quality parameters. Regression coefficients (R²) and significance levels were calculated for each relationship. All graphical visualizations were generated using R software [61].

Results

Irrigation water pH significantly affected Cd concentrations in all plant tissues and soil-available Cd (Table S1; Fig. 1). Root Cd concentration was the highest among tissues and ranged from 24.50 to 64.83 mg kg⁻¹ DW, with the highest value at pH 5 and the lowest at pH 9. Stem, leaf, and panicle Cd concentrations ranged from 10.67 to 17.67, 4.50–10.83, and 2.50–4.67 mg kg⁻¹ DW, respectively. Leaf and panicle Cd concentrations peaked at pH 6, whereas their lowest values were recorded at pH 8. Soil available Cd varied from 2.60 to 3.61 mg kg⁻¹, with the maximum value at pH 7. Overall, Cd concentration followed the order root > stem > leaf > panicle across irrigation water pH treatments.

Fig. 1.

Effects of irrigation water pH on soil available Cd concentration and cadmium concentration in roots, stems, leaves, and panicles of sorghum grown on cadmium-contaminated soil. Different lowercase letters above boxplots indicate significant differences among irrigation water pH (P < 0.05). Boxes represent the interquartile range, the central horizontal lines indicate the medians, and the whiskers represent the data spread. Plant tissue Cd concentrations are expressed as mg kg⁻¹ DW, whereas soil available Cd is expressed as mg kg⁻¹ soil

Morphological traits were strongly influenced by the pH of irrigation water, including plant height, stem diameter, panicle length, shoot dry weight, and root dry weight (Table S1; Fig. 2). Panicle length exhibited the most significant response to treatment (F = 321.11, P < 0.001).

Fig. 2.

Effects of irrigation water pH on morphological traits of sorghum grown on cadmium-contaminated soil. Different lowercase letters above boxplots indicate significant differences among irrigation water pH treatments (P < 0.05). Boxes represent the interquartile range, central horizontal lines indicate medians, and whiskers represent data dispersion

Plant height ranged from 96.77 cm to 118.00 cm across pH treatments. The longest plants were observed at pH 7, followed by pH 6 and pH 8, while the shortest plant height was recorded at pH 9. Stem diameter varied from 16.03 to 17.90 mm, with the smallest diameter recorded at pH 5 and larger values at pH 6–9.

Panicle length varied significantly among treatments, ranging from 5.57 to 13.67 cm. The longest panicles were recorded at pH 7, which was not significantly different from pH 6, whereas the smallest panicles were observed at pH 5. Shoot dry weight ranged from 63.37 to 71.37 g, with the highest at pH 6 and the lowest at pH 9. Root dry weight ranged from 6.27 to 8.70 g, with the peak value at pH 8, followed by pH 9, while the minimum root dry weight was reported at pH 5.

The pH of irrigation water significantly influenced all analyzed soil chemical parameters, including soil pH, electrical conductivity (EC), lime content, organic matter, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) (Table S1; Fig. 3). The soil pH ranged from 6.26 to 7.05, with the lowest and highest values recorded at pH 5 and 9, respectively. Conversely, EC decreased from 830.7 µS cm⁻¹ at pH 5 to 133.0 µS cm⁻¹ at pH 8, followed by a marginal rise to 160.2 µS cm⁻¹ at pH 9.

Fig. 3.

Effects of irrigation water pH on soil chemical properties in cadmium-contaminated soil cultivated with sorghum. Different lowercase letters above boxplots indicate significant differences among irrigation water pH treatments (P < 0.05). Boxes represent the interquartile range, central horizontal lines indicate medians, and whiskers represent data dispersion

The lime content of the soil ranged from 1.01% to 1.47%, with the highest value at pH 6 and the lowest at pH 9. Soil organic matter ranged from 0.39% to 0.45%, peaking at pH 8. Soil P increased from 21.7 mg kg⁻¹ at pH 5 to 38.1 mg kg⁻¹ at pH 8, then dropping to 34.3 mg kg⁻¹ at pH 9.

Soil K levels varied from 351.7 to 418.7 mg kg⁻¹, with the highest and the lowest values recorded at pH 6 and 9, respectively. Soil Ca varied between 3225.0 and 3475.0 mg kg⁻¹, with the peak value noted for pH 7. Soil Mg concentration varied from 480.7 to 613.0 mg kg⁻¹, with the highest and the lowest values recorded for pH 6 and 9, respectively.

Forage quality traits were strongly influenced by the pH of irrigation water, while the reaction differed across plant parts and traits (Table S1; Fig. 4). Stem ADF increased from 31.50% at pH 5 to 38.50% at pH 8–9, while panicle ADF varied from 37.10% at pH 5 to 41.00% at pH 7. Conversely, leaf ADF had a limited range, fluctuating between 34.40% and 35.60% across pH treatments.

Fig. 4.

Effects of irrigation water pH on forage quality traits of sorghum grown on cadmium-contaminated soil. Different lowercase letters above boxplots indicate significant differences among irrigation water pH treatments (P < 0.05). Boxes represent the interquartile range, central horizontal lines indicate medians, and whiskers represent data dispersion

Stem NDF increased from 59.60% at pH 5 to 69.40% at pH 8, whereas panicle NDF varied from 83.10% to 86.30%, peaking at pH 8. Leaf NDF ranged from 68.40% to 71.10%, with the maximum value at pH 5 and the minimum at pH 8. The crude protein concentration varied across irrigation water treatments. Stem crude protein ranged from 8.20% to 10.30%, with the highest and the lowest values recorded under pH 6 and pH 8, respectively. Leaf crude protein varied between 6.70% and 8.20%, with the lowest percentage observed at pH 7 and the highest at pH 9. Panicle crude protein ranged from 7.40% to 9.20%, with the peak at pH 5 and the minimum at pH 7.

Cadmium translocation and bioconcentration indicators indicated significant fluctuations across different treatments (Table S1; Fig. 5). The stem/root TF varied from 0.246 to 0.498, peaking at pH 9 and reaching the lowest level at pH 7. The leaf/root TF varied between 0.108 and 0.232, with the highest value noted at pH 9 and the lowest observed at pH 8. The panicle/root TF varied from 0.060 to 0.123, with the highest value observed at pH 9 and the lowest at pH 8.

Fig. 5.

Effects of irrigation water pH on cadmium translocation factors (TF) and bioconcentration factors (BCF) of sorghum grown under cadmium-contaminated soil. Different lowercase letters above boxplots indicate significant differences among irrigation water pH treatments (P < 0.05). Boxes represent the interquartile range, central horizontal lines indicate medians, and whiskers represent data dispersion

Root BCF varied between 9.44 and 24.99 across different irrigation water pH treatments. The highest root BCF was observed at pH 5, followed by pH 6, 8, 7, and 9. The higher numerical values among the TF indices were recorded for stem/root TF, followed by leaf/root TF and panicle/root TF.

Linear regression analysis showed significant relationships between irrigation water pH and selected Cd, soil, and forage quality traits (Fig. 6). Root Cd concentration decreased as irrigation water pH increased, indicating a strong negative relationship (R² = 0.92, P < 0.001). Root BCF also decreased significantly with increasing irrigation water pH (R² = 0.79, P < 0.001).

Fig. 6.

Linear relationships between irrigation water pH and selected Cd, soil, and forage quality traits in sorghum grown under Cd-contaminated soil conditions. Panels show stem ADF, root Cd concentration, stem crude protein, stem NDF, root BCF, soil EC, and soil pH. Shaded areas indicate 95% confidence intervals around the fitted regression lines. P values and coefficients of determination are shown within each panel

Among soil properties, soil pH increased linearly with irrigation water pH (R² = 0.90, P < 0.001), whereas soil EC decreased significantly as irrigation water pH increased (R² = 0.85, P < 0.001). For forage quality traits, stem ADF and stem NDF increased with irrigation water pH, with strong positive relationships (R² = 0.89 and 0.83, respectively; P < 0.001). In contrast, stem crude protein decreased as irrigation water pH increased (R² = 0.77, P < 0.001).

Overall, the strongest regression was observed for root Cd concentration (R² = 0.92), followed by soil pH (R² = 0.90), stem ADF (R² = 0.89), soil EC (R² = 0.85), stem NDF (R² = 0.83), root BCF (R² = 0.79), and stem crude protein (R² = 0.77).

Pearson correlation analysis was conducted on Cd concentration in different organs, soil chemical properties, growth traits, forage quality, TF, and BCF variables (Fig. 7). Cd concentrations in plant tissues exhibited a positive correlation with one another. Root Cd concentrations exhibited strong positive associations with panicle Cd (r = 0.97), leaf Cd (r = 0.88), and stem Cd (r = 0.72). Leaf Cd exhibited a robust correlation with panicle Cd (r = 0.90), but stem Cd showed a positive correlation with both leaf Cd (r = 0.63) and panicle Cd (r = 0.62).

Fig. 7.

Pearson correlation heatmap illustrating relationships among cadmium accumulation, morphological, soil chemical, forage quality, translocation, and bioconcentration traits of sorghum grown under different irrigation water pH levels in cadmium-contaminated soil. Red indicates positive correlations, whereas blue indicates negative correlations. Color intensity corresponds to the strength of the correlation coefficient, with values ranging from −1 to +1. Correlation coefficients are displayed within each cell. Traits were hierarchically arranged according to similarity in correlation structure

Soil pH exhibited negative correlations with Cd concentration in roots, stems, leaves, and panicles, with correlation values between r = − 0.83 and r = − 0.91. Conversely, soil EC exhibited positive associations with Cd concentration in plant tissues, with r values ranging from 0.66 to 0.91. Root BCF had a positive correlation with Cd concentrations in roots (r = 0.84), leaves (r = 0.77), panicles (r = 0.76), and stems (r = 0.62); however, it showed a negative correlation with soil pH (r = − 0.61).

Plant height exhibited a robust correlation with stem diameter (r = 0.91) and panicle length (r = 0.76) among growth variables. Panicle length was substantially correlated with shoot dry weight (r = 0.87). Root dry weight had negative relationships with cadmium contents in plant tissues, ranging from r = − 0.56 to − 0.68, and demonstrated a positive association with soil pH (r = 0.81).

The stem ADF and stem NDF exhibited a high positive correlation (r = 0.97). Stem ADF and stem NDF exhibited a positive correlation with soil pH (r = 0.94 and 0.93, respectively) and a negative correlation with Cd concentration in plant tissues. The crude protein content in the stem had a positive correlation with Cd concentrations in the roots (r = 0.88), leaves (r = 0.84), panicles (r = 0.94), and stems (r = 0.75), while demonstrating a negative correlation with stem ADF (r = − 0.78) and stem NDF (r = − 0.84).

The TF indicators exhibited positive relationships with the respective tissue Cd concentrations. The stem/root TF had a positive correlation with stem Cd (r = 0.70) and leaf Cd (r = 0.71). However, the leaf/root TF and panicle/root TF showed a significant correlation with one another (r = 0.99).

Principal component analysis revealed that the first six principal components explained 95.76% of the total variation in the measured traits, indicating a strong multivariate structure (Table 2). PC1 accounted for 47.60% of the total variation and represented the dominant response axis associated with changes in Cd concentration, soil chemical properties, and forage quality characteristics across irrigation water pH treatments. PC2 explained an additional 22.40% of the total variation, increasing the cumulative explained variance to 70.00%, and primarily reflected variation related to plant growth, nutrient dynamics, and Cd translocation responses.

Table 2.

Principal component analysis (PCA) eigenvalues and explained variance for cadmium accumulation, morphological, soil chemical, forage quality, translocation, and bioconcentration traits of sorghum grown under different irrigation water pH levels on cadmium-contaminated soil

PC	Eigenvalue	Variance (%)	Cumulative variance (%)
PC1	17.30	47.60	47.60
PC2	7.43	22.40	70.00
PC3	4.60	13.13	83.13
PC4	1.77	5.06	88.19
PC5	1.64	4.68	92.87
Pc6	1.01	2.89	95.76

The first three PCs explained 83.13% of the total variation, demonstrating that most treatment-related responses were concentrated within a limited number of multivariate axes. Subsequent components contributed comparatively smaller proportions of the total variation, with PC4, PC5, and PC6 accounting for 5.06%, 4.68%, and 2.89%, respectively. Overall, the high cumulative variance explained by the first few principal components indicates strong differentiation among irrigation water pH treatments and confirms that irrigation water pH substantially regulated coordinated responses among Cd dynamics, soil chemistry, forage quality, and plant growth traits in sorghum cultivated under cadmium stress conditions.

The PCA biplot revealed clear separation among irrigation water pH treatments based on combined plant, soil, and Cd-related responses (Fig. 8). Acidic treatments (pH 5–6) were strongly associated with higher Cd concentration, BCF, soil EC, and crude protein traits, indicating enhanced Cd uptake and bioavailability under acidic conditions. In contrast, alkaline treatments (pH 8–9) were associated with higher soil pH, ADF, NDF, and Cd translocation factor, reflecting reduced Cd uptake but increased accumulation of structural fibers under alkaline conditions. The pH 7 treatment occupied an intermediate position and was positively associated with plant height, shoot dry weight, soil lime, and soil Mg, indicating more favorable growth conditions. Overall, the PCA confirmed that irrigation water pH strongly regulated coordinated responses among Cd dynamics, soil chemistry, forage quality, and plant growth traits in sorghum under Cd stress.

Fig. 8.

Principal component analysis (PCA) biplot showing the multivariate relationships among cadmium accumulation, translocation, bioconcentration, morphological, soil chemical, and forage quality traits of sorghum grown under different irrigation water pH levels in cadmium-contaminated soil. Arrows indicate the direction and magnitude of trait contributions to the principal components. Traits oriented in similar directions are positively associated, whereas traits positioned in opposite directions indicate negative relationships

The integrated heatmap revealed clear clustering patterns among irrigation water pH and measured traits (Fig. 9). Acidic irrigation treatments (pH 5–6) were associated with higher Cd concentration, BCF, soil EC, and crude protein traits, indicating enhanced Cd bioavailability and plant uptake under acidic conditions. In contrast, alkaline treatments (pH 8–9) were associated with higher soil pH, ADF, NDF, and Cd translocation factor, reflecting reduced Cd concentration but increased accumulation of structural fibers under alkaline conditions. The pH 7 treatment exhibited an intermediate response pattern and was generally associated with improved growth-related traits.

Fig. 9.

Heatmap illustrating the integrated responses of cadmium accumulation, translocation, bioconcentration, morphological, soil chemical, and forage quality traits of sorghum grown under different irrigation water pH levels in cadmium-contaminated soil. Trait values were standardized using Z-score normalization prior to visualization. Red indicates relatively higher standardized values, whereas blue indicates relatively lower standardized values. Both treatments and traits were hierarchically clustered according to similarity patterns

Discussion

The results demonstrated that irrigation water pH strongly influenced DTPA-extractable Cd, Cd uptake and accumulation, forage quality, and soil chemical dynamics in sorghum. Increasing irrigation water pH reduced Cd concentrations in roots, stems, leaves, and panicles, confirming that rhizosphere chemistry strongly influenced Cd mobility. The higher tissue Cd concentrations under acidic irrigation were consistent with greater Cd mobility and availability for root uptake, whereas the lower concentrations under alkaline irrigation were consistent with increased Cd adsorption or precipitation; however, these mechanisms were not directly measured. Recent studies have similarly reported that elevated pH decreases Cd mobility and suppresses Cd uptake in crops by reducing metal solubility and altering metal speciation in the rhizosphere [1, 32, 62–64].

Significantly higher Cd concentration in roots than in aboveground tissues indicates that sorghum retained a large proportion of absorbed Cd within belowground organs. Root sequestration through cell wall immobilization, vacuolar compartmentalization, and chelation by phytochelatins and metallothioneins limits Cd translocation [65, 66]. Environmental stress conditions frequently impair biomass accumulation and plant development by disrupting physiological and nutritional homeostasis [67]. Immobilization of Cd within cell walls, vacuolar compartmentalization, and chelation by phytochelatins and metallothioneins restrict Cd translocation toward metabolically active tissues [68]. The strong positive correlations among root, stem, and leaf Cd concentration further indicate coordinated Cd uptake and internal redistribution. Similar root-dominant Cd accumulation patterns have recently been reported in sorghum and other crops under heavy metal stress [69, 70].

Although Cd uptake declined under alkaline irrigation conditions, Cd translocation factors increased at higher pH levels. This suggests that reduced total Cd accumulation was accompanied by proportionally greater redistribution of absorbed Cd from roots to aerial tissues. This pattern indicates that reduced total Cd uptake under alkaline irrigation was accompanied by a change in the proportional partitioning of absorbed Cd between roots and aboveground tissues. However, the present study did not assess subcellular Cd sequestration, transporter activity, or gene expression; therefore, the mechanisms underlying the observed increase in translocation factors cannot be determined from the current data and require further physiological and molecular investigation [31, 71, 72].

Morphological traits indicated that moderately acidic to neutral irrigation water (pH 6–7) was more favorable for sorghum growth than highly acidic or alkaline water. Higher Cd toxicity and ionic imbalance likely impaired root growth and nutrient acquisition under strongly acidic conditions, while highly alkaline conditions restricted nutrient availability. The observed growth responses suggest that the tested sorghum cultivar retained some capacity to maintain growth across the irrigation pH gradient. Cd-responsive molecular pathways in sorghum, including the SbWRKY54–SbHKT2b regulatory cascade, have been shown to enhance Cd tolerance; however, such mechanisms were not measured in the present study [73]. Similar reductions in growth under extreme rhizosphere pH conditions have recently been reported in millet and other cereal crops exposed to abiotic stress [74, 75].

The decrease in soil EC with increasing pH suggests reduced ionic mobility under alkaline conditions. Conversely, acidic irrigation produced substantially higher EC values, likely due to increased ion solubility and the addition of acidifying compounds. Changes in soil chemical conditions substantially affect nutrient availability, ionic balance, and overall soil fertility status [76]. Similar pH-dependent nutrient dynamics have been reported in calcareous and semi-arid soils, where changes in rhizosphere chemistry strongly influence nutrient mobility and plant uptake [34, 77].

One key finding is a clear trade-off between reduced Cd accumulation and forage nutritional quality under alkaline irrigation. Increasing pH increased ADF and NDF while reducing CP in stem and panicle tissues. This trade-off is particularly important because strategies aimed solely at reducing Cd may unintentionally compromise forage quality. The forage quality implications for livestock nutrition and feed management are directly relevant to ruminant production systems [78]. Previous studies have similarly shown that abiotic stress conditions and altered nutrient dynamics can significantly modify structural fiber deposition, lignification, and protein metabolism in forage crops [1].

The PCA and hierarchical clustering clearly separated acidic and alkaline irrigation treatments, indicating coordinated changes among Cd accumulation, soil chemistry, forage quality, and morphological traits. Acidic treatments were strongly associated with Cd accumulation variables, soil EC, and crude protein traits, whereas alkaline treatments clustered with soil pH, ADF, NDF, and Cd translocation variables. These findings suggest that irrigation water pH regulates not only Cd mobility but also broader physiological and biochemical processes affecting nutrient allocation, carbohydrate metabolism, and tissue structural composition.

Modifying the pH of irrigation water may provide a rapid, low-cost management strategy to partially regulate rhizosphere chemistry and reduce Cd accumulation in forage biomass. However, the accompanying decline in forage nutritional quality under alkaline irrigation stresses the need to balance heavy-metal mitigation with feed-quality preservation. Wastewater irrigation practices that alter soil-plant metal dynamics in ways analogous to the present study further support the relevance of irrigation water chemistry as a management lever [79]. Therefore, moderately acidic to neutral irrigation conditions may represent the most practical compromise between minimizing Cd toxicity, maintaining plant productivity, and preserving forage nutritional quality under contaminated conditions.

The study was conducted under controlled pot conditions, which may not fully reflect complex field interactions. Only one cultivar and one Cd level were evaluated, and the underlying molecular mechanisms remain unexplored. Furthermore, the absence of a Cd-free control group limited the ability to distinguish direct pH effects on plant physiology from Cd–pH interactions. Additionally, the soil buffering capacity—influenced by texture and organic matter—may have limited the magnitude of pH change achievable through irrigation water adjustment, particularly under field conditions [80]. Future studies should include both non-Cd controls, more Cd doses, and field-scale trials across diverse soil types.

Limitations

The experiment was performed under pot conditions, which may not accurately reflect the intricate physicochemical and biological interactions found in long-term field environments. Only one sorghum cultivar and one Cd dose were evaluated. The study focused on phenotypic and chemical responses, excluding investigation of the underlying physiological and molecular processes. Extended irrigation with modified pH may affect soil buffering capacity and microbial activity in manners not addressed herein. The experiment also lacked a control treatment free from Cd contamination. Thus, the effects of irrigation water pH on plant growth and forage quality may indicate both Cd–pH interactions and direct pH influences on plant physiology. Additionally, the use of HCl and NaOH to adjust irrigation water pH may have altered soil ionic composition through the addition of Cl⁻ and Na⁺, potentially confounding the isolated effect of pH and further emphasizing the need for field validation. Further studies should use uncontaminated controls to clarify their contributions. Future research must include multi-season field experiments, molecular analysis, and evaluations of economic viability.

Conclusion

The pH of irrigation water significantly affected Cd behavior, soil chemical characteristics, plant development, and forage quality in sorghum cultivated in Cd-contaminated environments. Higher irrigation water pH decreased Cd concentrations in roots, stems, leaves, and panicles, with the lowest tissue Cd levels usually observed under alkaline irrigation conditions. Nonetheless, alkaline conditions were associated with increased fiber fractions and reduced crude protein content, suggesting a trade-off between reduced Cd transport to plant tissues and forage nutritional quality. Under the present experimental conditions, irrigation water within the mildly acidic to near-neutral range (pH 6–7) represented the most favorable overall compromise among plant growth, forage quality, and Cd accumulation. This range should not be interpreted as universally optimal because individual tissue responses differed across pH treatments, and some aboveground Cd concentrations remained relatively high, particularly at pH 6. Multivariate analysis revealed that Cd concentration, soil chemistry, plant morphology, and forage quality traits were intricately interconnected throughout the irrigation pH gradient. These results illustrate the significance of managing the pH of irrigation water as a practical means of mitigating heavy-metal risks in forage production systems. Field-scale and long-term studies are essential to confirm the agronomic viability, environmental sustainability, and economic feasibility of adjusting the pH of irrigation water across various soil and climatic conditions.

Authors’ contributions

All authors were actively involved in conceptualization, funding acquisition, investigation, methodology, validation, visualization, resources, software, writing – original draft, writing-review, and editing.

Funding

This research was financially supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under the 2209A project numbered “1919B012106993”.

Data availability

Data will be available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Consent for publication

All authors consent for publication.

Competing interests

The authors declare no competing interests.