1 Introduction

Bean (Phaseolus vulgaris L.) is a member of the Fabaceae family. It is one of the most widely consumed legumes worldwide due to its substantial contents of sugars, proteins, fats, vitamins, dietary fibers, and nutrients (Uebersax et al. 2023). The edible part of beans is their seeds, which can be consumed dry or offered canned as industrial crops or green as vegetables. In concern of bean cultivation and production on a global scale, 27.5 million metric tons of beans was produced in 2020 from an estimated 34.8 million hectares (Uebersax et al. 2023). According to the most up-to-date estimate, bean production has grown since 1990 by approximately 60%, while harvested area has expanded by 36% over the same span (FAO 2022). The increased production has been attributed to improve bean breeding schemes and agronomic practices.

After seed harvest, bean straw is frequently utilized as fodder since it is affordable and beneficial for feeding cattle. Legume straws have relatively high nutritive value and protein and carbohydrate contents as compared to wheat or barley straw (Katoch 2022). In addition, bean straw can be employed as an organic fertilizer to improve the physicochemical characteristics of the soil. The addition of organic crop residues to the soil has been reported to enhance the soil porosity, water-holding capacity, mineral contents, microbial community, and soil enzyme activity (Akhtar et al. 2019).

Crop performance and human, animal, and ecosystem health are seriously threatened when soil is enriched with heavy metals (e.g., Cd, Co, Ni, Pb, Hg, As, and Cu). Although these detrimental metals are naturally occurring in the environment in trace attentiveness, human activities like mining, urban, and industrial emissions, in addition to increased agricultural and manufacturing activities, are typically what cause their levels to rise in soil, water, and atmosphere (Palansooriya et al. 2020). Since it is becoming more prevalent in agricultural fields, cadmium (Cd) has emerged as a serious heavy metal contaminant, reducing crop performance and posing a potential health hazard due to its ease of incorporation into the food chain (Lu et al. 2019). Industrial and mining industries, as well as fertilizers and pesticides used in agricultural systems, are the predominant human-caused sources of Cd in the ecosystem (Manzoor et al. 2019). Even at low doses, the non-essential metal Cd, which has no specific biological function, is harmful to plants. Consequently, Cd contamination has a deleterious impact on plants at the structural, physiological, molecular, and metabolic aspects (El Rasafi et al. 2020). Furthermore, Cd disrupts the architecture of the chloroplast, impairs photosynthetic activity, and elevates ROS (reactive oxygen species) (Cuypers et al. 2023). As a result, the imposed ROS accumulation promotes nucleotide damage, enzyme inhibition, lipid peroxidation, and protein degradation, which eventually impairs crop growth and yield (Moradi et al. 2019).

Cobalt (Co) is a precious transition metal that primarily exists in nature as oxides, sulfides, and arsenides (Long et al. 2020). Since Co has a low biodegradability and a prolonged half-life (Jiang et al. 2022), its avoidance in natural ecosystems like water, soil, and rocks is not possible. Plants absorb cobalt in a variety of ways, most frequently as di- and trivalent cations (Kosiorek and Wyszkowski 2020). As a micronutrient, Co contributes significantly to plant growth and development by boosting water use efficiency and reducing transpiration rate (Elshamly and Nassar 2023). Other positive impacts of Co include higher drought tolerance and a delay in senescence mediated by suppression of ethylene production (Fatma et al. 2022). Typically, uptake and distribution involve active transport across cell membranes; thus, its highest concentrations build up in the roots, followed by stems, and the smallest concentrations in the leaves (Hu et al. 2021). Cobalt toxicity is species-dependent and closely related to soil acidity. The most frequent symptoms resulting from the high accumulation of cobalt in plants are reduced plant growth and the emergence of necrosis as well as disorders in the uptake of nutrients (Kosiorek and Wyszkowski 2020). Ali et al. (2023) reported that Co toxicity is associated with oxidative stress, photosynthetic impairment, and iron deficiency. Lowered plant growth, biomass, and chlorophyll content were reported to be due to the results of Co toxicity, which negatively affects antioxidant status, photosynthetic pigments, and nutritional homeostasis (Lwalaba et al. 2017). According to Ton et al. (2021), Co-induced oxidative damage is associated with increased lipid peroxidation, ROS production, and DNA fragmentation, contributing to cell death.

Subsequent investigations have focused on the administration of plant extracts to improve vegetative growth, crop yield, stress tolerance, and pathogen resistance since these extracts contain a variety of essential nutrients, growth regulators, osmomodulators, vitamins, secondary metabolites, and organic acids, which are found in varying amounts and forms in different plant parts (Byan 2020; Kasim et al. 2016; Sobhy et al. 2022). Palm pollen extract (PPE) has been identified as a viable source of various phytochemicals including proteins, sugars, amino acids, lipids, sterols, triterpenes, hormones, glycosides, vitamins, nutrients, fibers, enzymes, and many cofactors (Sebii et al. 2019). As a result, PPE can be employed after extraction as a bioactive promoter for plants to increase their resistance to stressful conditions (Taha et al. 2020). According to the study of Taha et al. (2020), foliar application of palm pollen grain aqueous extract considerably reduced water stress in basal plants by improving water use efficiency, photosynthetic rate, antioxidant properties, membrane stabilization, and osmolyte levels. However, there is little or no available data regarding the potential of PPE to boost the tolerance of crop plants to heavy metal stress. To the best of the authors’ knowledge, there are no studies undertaken to date to assess the effectiveness of PPE as a foliar treatment to alleviate cadmium and cobalt toxicity in crop plants. We are interested to determine whether this extract contains any specific phytochemicals that could reduce metal-induced toxicity in bean plants and how exactly these phytochemicals reduce metal toxicity, whether through metal-chelation, interfering with metal uptake, stimulating a particular defense system, or through another related mechanism. In light of this, this study aimed to assess the efficacy of PPE in alleviating the toxicity of cadmium and cobalt on bean (Phaseolus vulgaris L.) plants in terms of growth criteria, photosynthetic pigments, antioxidant capacity, nutritional status, and yield performance, as well as concentrating on the key components of this extract that could exert positive effects on the metal-stressed plants.

2 Materials and Methods

2.1 Collection of Palm Pollen Grains and Preparation of Their Extract

After reaching full maturity, pollen grains were harvested from the Egyptian date palm (Phoenix dactylifera L.), growing in Tanta, Egypt. The procedures described by Kasim et al. (2016) with some modifications were followed to prepare the methanolic-aqueous palm pollen extract (PPE). A weight of 50 g pollen grain powder was well mixed with 600 ml distilled water and 400 ml of 95% methanol then incubated in dark for 24 h on an orbital shaker set at 120 rpm. After that, the extract was once filtered using non-absorbent cotton and twice using Whatman No. 1 filter paper, evaporated till dryness, redissolved in 1000 ml distilled water with the aid of methanol, and then kept at 4 °C until use.

2.2 Identifying the Active Components of PPE

A Clarus 580/560S (PerkinElmer, Inc., Waltham, MA, USA) gas chromatograph/mass spectrometer (GC/MS) was employed to determine the chemical constitution of PPE. The column used in this investigation was an Elite-5MS (30 m × 0.25 mm × 0.25 μm film thickness) column, and the oven temperature was first held at 80 °C for 7 min before being increased by 10 °C min−1 to 140 °C withhold for 1 min, followed by an increase to 200 °C withhold for 1 min by a rate of 10 °C min−1, and finally increased to 280 °C withhold for 10 min by a rate of 5 °C min−1. Temperatures were maintained in the transfer and input lines at 250 °C. As a carrier gas, helium was employed at a constant flow rate of 1 m min−1. Using the Autosampler AS3000 and GC in split mode (1:20), a sample of 1 μl was automatically injected after a solvent delay of 5 min. At an ionization energy of 70 eV, EI mass spectra were obtained over the range of m/z 40–650 in full scan mode. The temperature of the ionization chamber was fixed at 200 °C. By comparing their retention times and mass spectra to those of the WILEY 09, replib, and NIST 11 mass spectral databases, all acquired components of the examined extract were recognized. The phytochemical constituents of the methanolic-aqueous PPE are summarized in Table 1.

Table 1 Phytochemical analysis of methanolic-aqueous palm pollen extract (PPE) as determined by GC-MS
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2.3 Experimental Setup and Treatments

A pot experiment was undertaken and repeated twice in the period from the 15th of February to the 22nd of May 2022 at the greenhouse of the Faculty of Science, Tanta University, Egypt (30° 80′ N, 30° 99′ E) to assess the effectiveness of methanolic-aqueous palm pollen extract (PPE) as a foliar application in reducing the deleterious implications of cadmium (Cd) and cobalt (Co) as an individual or combined stress treatments on the growth, biochemical, and yield attributes of beans (Phaseolus vulgaris L. cv. Giza 6). Bean seeds were acquired from the Agricultural Research Center, Egypt. The seeds were surface sterilized for 5 min with a 0.1% solution of HgCl2 with continuous agitation, carefully rinsed five times with distilled water, and then allowed to air-dry. Thereafter, seeds were sown in plastic pots (30 × 35 cm, diameter × depth) containing 8 kg of clay-sandy soil (2:1 w/w) and watered with tap water. The soil utilized in this experiment had the following physio-chemical characteristics: clay-sandy texture, pH of 7.28, EC of 1.43 dS/m, and a bulk density of 1.28 g/cm3. The mineral content analysis revealed that this soil contained a total of 2.03 N, 0.74 P, 5.29 K, 3.02 Mg, 2.18 Na, and 4.37 Cl kg-1 soil.

Following the complete establishment, a week after seeds sowing, the seedlings were thinned into six uniform seedlings for each pot. Following that, the pots underwent the following treatments: control tap water, 50 mM CdCl2, 50 mM CoCl2, or a combination of 50 mM CdCl2 and CoCl2. Each pot received 1000 ml of metal treatments topically applied once to the soil surface. The treated pots were then divided into two separate groups, with one receiving distilled water and the other receiving an aqueous palm pollen extract (PPE) as a foliar application (10 ml PPE pot-1). The foliar application was made on the upper surface of the leaves using an atomizer in the morning after sunrise. The spraying solutions were administered every 4 days (three times during the experiment’s course). The pots were arranged in a completely random layout and allowed to grow in the greenhouse under natural conditions (12:12 day/night, 29/13°C ± 2, and 58 ± 10% relative humidity), with each treatment replicated three times in the two experiments. Pots were watered with 1000 ml of tap water every 2 days until the end of the experiment. Three seedlings from each pot were harvested at a 21-day-old age to evaluate their growth performance, biochemical responses, and mineral composition. The remaining seedlings, on the other hand, were permitted to continue growing until the end of the growing season; then, the yield criteria were assessed.

2.4 Evaluation of Growth Parameters

The harvested seedlings were washed repeatedly under tap water, rinsed in distilled water, separated into shoots and roots, and had their lengths (cm) and fresh weights measured (g). They were then placed in an oven set at 60 °C until they attained a constant weight, at which point their dry weights (g) were determined, and the water content (%) was subsequently calculated. A BenQ 500B desktop scanner (BenQ Inc., Taipei, Taiwan) was used to measure the leaf area in the second fully expanded leaf of three different plants. The leaf area was then calculated (cm2) using the Scion imaging software (Saad-Allah et al. 2022).

2.5 Quantification of Leaf Pigments

The total photosynthetic pigments were extracted from 0.1 g of leaf tissues using 85% cold acetone following the protocol provided by Metzner et al. (1965). The extract was centrifuged for 5 min at 5000 rpm, and the absorbance at 663, 644, and 452 nm was recorded. The pigment fractions, Chl a, Chl b, and carotenoids, were reported as mg g-1 FM from the following equations:

$$\textrm{Chl}\ \textrm{a}=10.3\times \textrm{E}663-0.918\times \textrm{E}644$$
$$\textrm{Chl}\ \textrm{b}=19.7\times \textrm{E}644-3.87\times \textrm{E}663$$
$$\textrm{Carotenoids}=4.2\ \textrm{E}452-\left[0.0264\times \textrm{Chl}\ \textrm{a}+00.426\times \textrm{Chl}\ \textrm{b}\right]$$

2.6 Quantification of Osmoregulatory Compounds

Soluble proteins, free amino acids, and free proline as osmoregulatory molecules were quantified in the bean’s finely ground dry leaves. Following the protocol of Bradford (1976), total soluble proteins were extracted in 80% aqueous ethanol and quantified (as mg g-1 DM) using the Coomassie Brilliant blue G-250 reagent and BSA (bovine serum albumin) protein as a standard. The amount of free amino acids in the ethanolic extract was measured (as mg g-1 DM) using the approach established by Lee and Takahashi (1966). The ninhydrin reagent was combined with the extract or the standard glycine and boiled for 10 min, and the absorbance at 570 nm was recorded. Free proline in the powdered leaf tissues was extracted using 3% aqueous sulfosalicylic acid. The extract or standard proline was combined with the acid ninhydrin reagent and acetic acid for 1 h at 100 °C. The absorbance was measured at 520 nm, and proline content was determined (as μg g-1 DM) based on the calibration curve by proline (Bates et al. 1973).

2.7 Stress Biomarkers

Electrolyte leakage (EL) of bean leaves was measured according to Sairam et al. (2005). Fresh leaves were cut into equal-sized discs, and 1 g of these discs was gently agitated in 25 ml of deionized water using an orbital shaker. The EC1 and EC2 were measured at 1 and 24 h, respectively, following which the EL was calculated and represented as a percentage according to the following formula: \(\textrm{EL}=\left(\frac{\textrm{EC}1}{\textrm{EC}2}\right)\times 100\), where EC1 is the electrical conductivity after 1 h and EC2 is the electrical conductivity after 24 h.

Using the procedures outlined by Velikova et al. (2000), H2O2 concentration was determined. Leaf tissues were extracted using 0.1% TCA and centrifuged for 15 min at 10,000 rpm. The extracts were then combined with a 10 mM phosphate buffer solution of pH 7.0 and 1 M KI, and the absorbance at 390 nm was recorded. Using the extinction coefficient (028 M−1cm−1), the concentration of H2O2 was calculated and reported as μmol g-1 FM. Malondialdehyde (MDA), a byproduct of unsaturated fatty acids peroxidation, was quantified to determine the level of lipid peroxidation using the method of Heath and Packer (1968). Plant leaves were extracted in 5% TCA and centrifuged at 6000 rpm for 15 min. The supernatant was combined with 0.67% thiobarbituric acid and incubated in a boiling water bath for 20 min. The absorbance was recorded at 532 and 600 nm, and the MDA level was determined using the molar coefficient (155 mM−1 cm−1) and expressed as nmol g-1 FM.

2.8 Antioxidant Enzyme Activity

Fresh leaves of beans were pulverized in liquid nitrogen, extracted in a solution of 50 mM phosphate buffer (pH 7.0), EDTA, and polyvinylpyrrolidone (PVP), and then, the mixture was centrifuged for 25 min at 8000 rpm in a cooling centrifuge. Six antioxidant enzymes were quantified in the supernatant.

The reduction in H2O2 absorption in a mixture of 50 mM phosphate buffer (pH 7.0) and 20 mM H2O2 at 240 nm was used to evaluate the activity of catalase (CAT) according to the method of Havir and McHale (1987). Likewise, the reduction in H2O2 absorption at 290 nm in a reaction medium comprising 0.5 mM ascorbate, 0.2 mM EDTA, 5 mM phosphate buffer (pH 7.0), and 0.25 mM H2O2 was employed to evaluate the activity of ascorbate peroxidase (APX). Utilizing 2.8 mM−1 cm−1 as the extinction coefficient, the activity of APX was assessed (Nakano and Asada 1981). The assay of peroxidase (POD) was based on the increased light absorption at 470 nm as a result of POD converting guaiacol into tetraguaiacol in a solution of 100 mM phosphate buffer (pH 5.8), 7.2 mM guaiacol, and 11.8 mM H2O2. By applying an extinction coefficient of 26.6 mM−1 cm−1, the activity of POD was determined (Kato and Shimizu 1987).

To measure the activity of superoxide dismutase (SOD), formazan production from the photochemical reduction of nitroblue tetrazolium (NBT) in the presence of SOD was monitored at 560 nm (Beyer and Fridovich 1987). The enzyme extract was incorporated into a 50 mM phosphate buffer solution (pH 7.8) containing 9.9 mM L-methionine, 0.057 mM NBT, 0.025% Triton X-100, and 0.0044% riboflavin; then, the mixture was illuminated for 15 min. Following the measurement of absorbance, the activity of SOD was assessed by applying 21.1 mM−1 cm−1 as the extinction coefficient. By monitoring the decrease in NADPH absorbance brought on by its oxidation in the presence of oxidized glutathione (GSSG), the activity of glutathione reductase (GR) was assessed. The enzyme extract was combined with a 20 mM phosphate buffer (pH 7.5) solution containing 0.5 mM NADPH, 2 mM EDTA, and 0.5 mM GSSG. The absorbance at 340 nm was recorded, and the extinction coefficient 6.2 mM−1 cm−1 was employed to calculate GR activity (Halliwell and Foyer 1978). The activity of glutathione S-transferase (GST) was determined by monitoring the change in the absorbance brought on by the development of a conjugate between reduced glutathione (GSH) and 1-chloro-2,4-dinitrobenzene (CDNB). The enzyme extract was added to a 100 mM phosphate buffer solution (pH 6.5) containing 30 mM GSH and 0.75 mM CDNB. The absorbance at 340 nm was measured, and the extinction coefficient of 9.6 mM−1 cm−1 was used to calculate the activity of GST (Habig et al. 1974).

2.9 Non-enzymatic Antioxidants

The amount of total polyphenols in the ethanolic extract of powdered leaf tissues was estimated according to the procedures of Jindal and Singh (1975). Ethanolic extracts were mixed with Folin-Ciocalteu’s reagent and 20% Na2CO3; then, the absorbance at 650 nm was measured after 1 h of dark incubation. Based on a calibration curve constructed by gallic acid, polyphenol concentrations (mg g-1 DM) were determined. The total flavonoid amount in the ethanolic leaf extracts was determined by coupling an aliquot of the leaf extract with 10% AlCl3, 1 M potassium acetate, and 95% ethanol. After 30 min, the mixture’s absorbance was recorded at 417 nm against the reference flavonoid quercetin, and the amount of total flavonoids was given as mg g-1 DM (Chang et al. 2002).

2.10 Elemental Analysis

The wet digestion of dry powdered root and leaf samples was carried out using a combination of 70% HNO3 and 30% H2O2 (4:2 v/v). Using an inductively coupled plasma-optical spectrophotometer (Polyscan 61 E, Thermo Jarrell-Ash Corp., Franklin, MA, USA), the concentrations of K, Cd, and Co in the digested samples were determined. However, phosphorus (P) was measured calorimetrically using molybdenum blue reagent and monopotassium phosphate as a standard, whereas nitrogen (N) was measured using Rochelle reagent and ammonium chloride as a standard according to Allen et al. (1974).

2.11 Yield Characteristics

Plants in each pot were collected individually at harvest time. Each plant was counted for the number of pods, and the pods were then detached from the plant and left to dry in the open air. Each of the treatment’s seeds was extracted from their pods, and the number of seeds per pod, the weight of 100 seeds (g), and the filling ratio (%) were all measured according to Bondok et al. (2022).

2.12 Statistical Analysis

The results of the two experiments were combined and reported as mean ± SD (standard deviation). The acquired experimental findings were statistically examined using the two-way analysis of variance (ANOVA) to determine the impact of the metal stress and PPE foliar application, as well as their interactions on the measured variables. Additionally, a one-way ANOVA was conducted to separate the means and assess the level of significance between various treatments using LSD at the 0.05 level as a post hoc test. The entire statistical analysis was performed using the CoStat program (v 6.311, CoHort Software).

3 Results

3.1 Phytochemical Constituents of the Methanolic-Aqueous PPE

The results of GC/MS analysis of the methanolic-aqueous palm pollen extract (PPE) revealed that it included a total of 40 different compounds (Table 1 and Supplementary Fig. 1). According to the results, the extract contained a heterogeneous mixture of many different compounds, most of which were only present in trace concentrations. The versatile abundant components found in PPE included n-hexadecanoic acid (8.24%), 1-octadecene (7.16%), 1,2-benzenedicarboxylic acid, diisooctyl ester (5.53%), eicosane, 2-cyclohexyl- (3.18%), dibutyl phthalate (2.09%), and sulfurous acid, octadecyl 2-propyl ester (1.97%). In addition, there are other phytochemicals present in intermediate quantities.

3.2 Two-Way ANOVA Results of the Quantified Parameters

Table 2 displays the two-way completely randomized analysis of variance (ANOVA) influences of Cd or Co stress separately or in combination, foliar application of PPE, and their combined interaction on the evaluated parameters of bean (Phaseolus vulgaris L.). The findings showed that all the investigated parameters were highly significantly (P < 0.01) impacted by the stress treatments used, except for the shoot water content, which was significantly (P < 0.05) impacted. However, foliar application of PPE resulted in highly significant effects on all the evaluated variables, except for carotenoids, which had significant effects, and flavonoid content, which experienced non-significant effects (P > 0.05). Furthermore, there were highly significant interactions between metal stress and PPE foliar application, except for the significant effects on Chl b and soluble protein contents and the non-significant effects on root length, shoot water content, phenolic compounds, and the number of seeds per pod.

Table 2 Results of a two-way ANOVA analysis demonstrating the effects of metal stress, PPE, and their combined treatments on the tested variables of bean
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3.3 Growth Parameters

Figure 1 depicts how metal-stressed (Cd or Co single and combination) bean seedlings responded to the foliar application of the methanolic-aqueous PPE in terms of their growth characteristics (root and shoot lengths, water content, and leaf area). The results indicated that Cd stress substantially reduced root length, shoot height, root water content, shoot water content, and leaf area. In comparison to the unstressed control, the percentages of decrease for the aforementioned parameters were 15.31, 10.44, 14.16, 3.94, and 47.14%, respectively. Additionally, Co treatment caused decreases in root length, shoot height, root water content, shoot water content, and leaf area by 29.58, 22.62, 21.56, 2.47, and 55.34%, respectively. Moreover, treatment with a combination of Cd and Co markedly decreased these growth parameters with percentages of 27.56, 20.01, 27.54, 3.73, and 45.57% for root length, shoot height, root water content, shoot water content, and leaf area, respectively.

Fig. 1
figure 1

The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the growth parameters of bean. Different letters provide significantly different values at 5% level

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Contrastingly, spraying bean seedlings with PPE elicited meaningful increases in root length, shoot height, root water content, shoot water content, and leaf area, as they were 9.18, 14.79, 13.29, 0.61, and 15.06%, respectively, over the unstressed control values. Additionally, when PPE was combined with the Cd and Co treatments, the growth characteristics of bean seedlings were noticeably improved as compared to the individual stress treatments, while the values were still lower than those of the normal control. According to the obtained data, beans grown in Cd- or Co-contaminated soils can receive PPE as a foliar spray to lessen the negative consequences on their growth.

3.4 Photosynthetic Pigments

The effect of foliar application of methanolic-aqueous PPE on the concentration of photosynthetic pigments in the leaf tissues of bean seedlings under individual or combined Cd or Co stress is depicted in Fig. 2. The results demonstrated that Cd, Co, and their combined treatment significantly reduced Chl a and Chl b as compared to the control, with reduction ratios of 31.98 and 23.92% by Cd, 23.85 and 44.31% by Co, and 23.12 and 48.04% by the combined treatment. Contrarily, when exposed to Cd and Co stress, either separately or together, carotenoids demonstrated a significant increment when compared to the control, with an increasing percentage of 14.74% by Cd, 19.94% by Co, and 18.99% by the combined administration.

Fig. 2
figure 2

The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the leaf pigments of bean. Different letters provide significantly different values at 5% level

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PPE spraying, on the other hand, caused 6.09 and 5.10% increases in Chl a and Chl b, respectively, when compared to the control. Nonetheless, spraying PPE had no discernible impact on carotenoids (5.20% decrease) when compared to the unsprayed control. In contrast, when metal-stressed bean seedlings were treated with PPE as a foliar spray, their chlorophyll fractions significantly increased, but carotenoids showed a relative decline in their content in comparison to the stressed-untreated seedlings. The most noteworthy improvement in Chl a was brought about by applying PPE to seedlings that had received a combination of the two metals, whereas in Chl b, it was brought about by using it on seedlings that had been exposed to Cd stress. The lowering effect on carotenoids due to the foliar application of PPE, however, was rather consistent, regardless of the metal stress treatment.

3.5 Osmoregulatory Molecules

Figure 3 provides an overview of the concentrations of the osmoregulatory molecules (soluble proteins, free amino acids, and free proline) in bean seedlings exposed to metal stress brought on by Cd and Co single or mixed, PPE foliar treatment, and their combined interactions. Comparing the Cd, Co, and combined treatment (Cd+Co) to the control, the total soluble proteins (TSP) exhibited declines of 28.03, 25.05, and 21.18%, respectively. Free amino acids (FAA) also displayed a pattern resembling that of TSP, with decreases resulting from Cd, Co, and their combined treatment of 3.51, 14.32, and 45.24%, respectively. The administration of Cd, Co, and their combined treatment, on the other hand, increased the content of free proline (FP) in bean seedlings by 33.67, 25.99, and 45.79%, respectively, over the control value.

Fig. 3
figure 3

The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the osmoregulatory molecules of bean. Different letters provide significantly different values at 5% level

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However, compared to the unsprayed control, spraying PPE on bean seedlings increased TSP and FAA by 21.64 and 7.72%, respectively, but decreased FP content by 20.52%. Owing to the combined interactions of metal and PPE treatments, the results demonstrated that stressed seedlings treated with PPE as a foliar application showed an increment in TSP and FAA levels, as contrasted to the stress treatments alone. In some cases, the increases were on par with or even exceeded the control values. The combined metal and PPE treatments showed significant decreases in the content of FP compared to the level of standalone stressor treatments, even though their concentrations were still higher than that of the untreated control and PPE single treatments.

3.6 Stress Biomarkers

As shown in Fig. 4, the irrigation of bean seedlings with Cd, Co, or a combination of Cd and Co was extremely stressful in terms of stress biomarkers including electrolyte leakage (EL), hydrogen peroxide (H2O2), and malondialdehyde (MDA) as a product of lipid peroxidation. In comparison to the control, the levels of EL were significantly higher after receiving the Cd, Co, and Cd+Co treatments by percentages of 40.82, 48.04, and 71.35%, respectively. In addition, the treatment with Cd, Co, and Cd+Co resulted in increases in H2O2 concentration in bean seedlings of 18.25, 28.37, and 44.33%, respectively, as compared to the control level. Remarkably, the metal stress had a damaging effect on cellular membranes, causing a massive buildup of MDA. When compared to the control level, the percentages of MDA increases brought on by the administration of Cd, Co, and Cd+Co were 174.50, 250.96, and 324.04%, respectively.

Fig. 4
figure 4

The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the stress biomarkers of bean. Different letters provide significantly different values at 5% level

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When compared to the control, the foliar spray of PPE had no discernible impact on the level of stress biomarkers. However, when PPE and the metal-stress treatments were applied simultaneously, the consequences of metal toxicity resulting in the evaluated stress indices were attenuated. The most detectable augmenting effect of PPE was noticed with EL and MDA levels. The level of these parameters exhibited a substantial decrease as a result of PPE administration to the metal-stressed plants. The EL level of stressed plants nevertheless reached levels that were comparable to the control treatment.

3.7 Antioxidant Enzymes

Activities of the antioxidant enzymes catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POD), superoxide dismutase (SOD), glutathione reductase (GR), and glutathione S-transferase (GST) in bean seedlings were significantly affected by metal stress and PPE foliar application treatments (Fig. 5). The administered metal treatments boosted the activities of CAT, SOD, and GST while diminishing those of APX, POD, and GR. Treatments with Cd, Co, and Cd+Co raised CAT activity by 37.21, 61.73, and 58.82%; SOD activity by 86.71, 93.82, and 121.52%; and GST activity by 272.00, 228.00, and 257.40%, respectively, in comparison to the control. However, following Cd, Co, and Cd+Co treatments, APX activity exhibited 34.84, 42.42, and 46.36% decreases; POD activity showed 9.65, 63.70, and 36.50% decreases; and GR activity showed 64.07, 42.48, and 54.06% decreases, respectively, in comparison to the control.

Fig. 5
figure 5

The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the antioxidant enzyme activity of bean. Different letters provide significantly different values at 5% level

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The data showed that when bean seedlings were sprayed with PPE, the activities of APX, POD, SOD, GR, and GST were significantly increased (by 7.58, 114.53, 33.54, 23.31, and 92.00%, respectively), whereas CAT activity was decreased (by 5.51%) when compared to the control. After the foliar application of PPE, the metal-induced increases in CAT, SOD, and GST activity were lessened compared to the activities of metal stress single treatments, while their level was still somewhat greater than those of the control. Oppositely, the metal-induced declines in APX, POD, and GR activity, particularly POD activity, were boosted when compared to the activities of metal stress single treatments, while their level was still somewhat lower than those of the control.

3.8 Antioxidant Molecules

The change in the level of the antioxidant molecules represented in total phenols and flavonoids in bean seedlings as affected by Cd or Co single and coupled treatments, PPE foliar spray, and their interactions is represented in Fig. 6. The data showed that when phenolic content was compared to the control, it was increased upon treatment with Cd, Co, or their combination (Cd+Co) by percentages of 12.53, 13.48, and 22.25%, respectively. However, when exposed to Cd, Co, or their combination (Cd+Co), the content of flavonoids significantly decreased by percentages of 25.78, 28.90, and 11.72%, respectively, as compared to the control level.

Fig. 6
figure 6

The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the antioxidant molecules of bean. Different letters provide significantly different values at 5% level

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Moreover, foliar application of PPE increased phenolic content by 17.34% while decreasing flavonoid content by 6.25% in comparison to the control. The integrative interactions between foliar PPE and metal treatments led to a considerable increase in the levels of phenols and flavonoids in bean seedlings as compared to the solitary treatment of metal stressors.

3.9 Mineral Content

The effects of Cd or Co single and paired treatments, PPE foliar spray, and their interactions on the levels of mineral ions in the shoot and root of bean seedlings are reported in Table 3. As demonstrated by the results, nitrogen (N), phosphorus (P), and potassium (K) contents in both the roots and shoots of bean seedlings were significantly reduced by either a single Cd or Co treatment or by the administration of both metals together. As a result of the combined metal stress (Cd+Co), the levels of P in the root and shoot and K level in the root were significantly reduced, whereas Cd administration resulted in the least richness in shoot N and K levels, while Co administration produced the least abundance in root N levels. In conformity with the utilization of clear soil in the experiment, neither Cd nor Co was detected in the shoot or the root of bean seedlings. But because they were applied onto the soil, the soil became contaminated, making it feasible for plant roots and shoots to uptake them. Consequently, it was found that bean shoots accumulate higher Cd and Co concentrations than roots.

Table 3 The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the shoot and root ion content of bean
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As opposed to the control as well as other treatments, foliar PPE application boosted the nutritional status of bean seedlings, as evidenced by the greatest concentrations of N, P, and K in the seedlings’ shoots and roots. The decline of the detected macronutrients (N, P, and K) was also somewhat mitigated by the integration of PPE foliar application and metal stress treatments, resulting in a state of relative nutritional homeostasis, particularly for the N of shoots in the case of the integrative interaction between Cd and PPE. Additionally, the administration of PPE in conjunction with metal stress treatments reduced Cd and Co accumulation in the shoots and roots of bean seedlings.

3.10 Yield Parameters

The repercussions of Cd or Co single and coupled treatments, PPE foliar application, and their interactions on the yield components of beans as characterized by the number of pods per plant, number of seeds per pod, pod filling ratio, and 100 seeds’ mass are reported in Fig. 7. As shown from the data, metal treatments significantly decreased bean yield, particularly when the two metals were applied as a mixture. When Cd and Co were administered together, the number of pods per plant, the number of seeds per pod, the pod filling ratio, and the mass of 100 seeds were decreased by 57.80, 33.38, 29.31, and 34.43%, respectively.

Fig. 7
figure 7

The effects of Cd or Co single and coupled treatments, PPE foliar spray, and their interactions on the yield attribute of bean. Different letters provide significantly different values at 5% level

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The yield characteristics of beans were slightly improved following the foliar administration of PPE than the control treatment. The number of pods per plant, the number of seeds per pod, the pod filling ratio, and the mass of 100 seeds each increased by 40.00, 2.46, 0.85, and 5.19%, respectively, as a result of this treatment. Aside from that, the co-application of metal treatments and PPE significantly improved the yield characteristics of the stressed plants, except for the number of seeds per pod, which was non-significantly affected.

4 Discussion

In addition to endangering plant development and productivity, rhizospheric microorganisms, soil structure, and even animal and human health, heavy metals pose a serious threat to the environment. One of the most frequently occurring metals in soil is cadmium (Cd), which tends to accumulate in vegetative organs and adversely affects vital physiological activities. Additionally, cobalt (Co) is a micronutrient that has beneficial effects on plant metabolism, but toxic doses of Co hinder active transport, impede plant growth, and cause a variety of physiological dysfunctions in plants. The target of the current investigation was to determine whether the foliar application of the methanolic-aqueous palm pollen extract (PPE) could lessen the adverse effects of the two well-known heavy metals, Cd and Co, applied either individually or in combination, on beans (Phaseolus vulgaris L.) in terms of growth, physiological activities, and yield attributes.

The surrounding environment and its components have a significant impact on the growth and development of the plants. In both individual and combined treatments, plants exposed to Cd and Co showed reduced growth in terms of root length, shoot height, water content, and leaf area, with the Co effect being more intense than the Cd effect. Cd has been reported as a non-active redox metal; however, exposure to excessive concentrations of Cd limits root and shoot growth by increasing oxidative stress; disrupting photosynthetic performance, carbohydrate metabolism, and nutrient availability; and lowering carbon assimilation and disorder water balance, all of which impair plant growth (Qin et al. 2020; Spormann et al. 2021). Notwithstanding findings that cobalt (Co) is essential for the normal growth, metabolism, and formation of root nodules in legumes (Akeel and Jahan 2020), in addition to its function as a coenzyme (Adolfo et al. 2016), it is one of the toxic metals that is readily assimilated into the food chain. The described reduction in growth caused by Co treatment has been associated with its promotion of discoloration and necrosis as well as suppression of root development, which impaired nutrient transport and water absorption (Akeel and Jahan 2020). In addition, Co impairs the enzymes necessary for chlorophyll biosynthesis and substitutes Mg in rubisco, leading to reduced uptake of CO2 (Ozfidan-Konakci et al. 2020). When Cd and Co were administered combined, this may have prompted further iteration of ROS, which might have increased cellular damage and impeded normal growth. Additionally, both metal ions may accumulate in plant tissues, causing lower water content, disruption of physiological activities, and interfering with nutrient homeostasis.

Due to the high sensitivity of the leaf pigments to metal ions, this sensitivity may either positively contribute to enhanced biosynthesis or adversely interfere with it and promote degradation. Our current investigation demonstrated that bean plants exposed to Cd and Co treatments had significantly lower levels of Chl a and Chl b despite possessing higher levels of carotenoids. According to Zhang et al. (2020), Cd causes a reduction in the amount of chlorophyll in leaves by suppressing the expression of significant enzymes involved in the chlorophyll biosynthetic pathway. Furthermore, Yang et al. (2018) concluded that the buildup of Cd within plant cells was correlated to the decline in chlorophyll levels because it interferes with the biosynthesis of aminolaevulinic acid or interacts with thiol groups of the enzymes that participate in chlorophyll synthesis. In addition, the reported loss in chlorophyll as a result of Co treatment may be brought on by deterring the incorporation of Fe into protoporphyrin molecules, interference with the proteins involved in chlorophyll biosynthesis, and the reduction in the uptake of essential minerals like potassium and nitrogen (Mohamed and Hassan 2019). Overall, it has been established that the accumulation of heavy metals in plant tissues results in redox disequilibrium, distortion of organelle membranes, the substitution of several activators of metal-containing enzymes, and subsequently the decrease in pigments involved in photosynthesis (Kumar and Prasad 2018). On the other hand, the increased carotenoids in bean plants after Cd and Co application in this study may be related to the function of carotenoids in preventing ROS-induced photooxidation of chlorophyll, stabilizing organelle membranes, shielding biomolecules from damage, and maintaining enzyme activity. The observed increase in carotenoid content reported in this study was observed in rice (Roychoudhury et al. 2012), lettuce (Dias et al. 2013), and duckweed (Piotrowska et al. 2010). Moreover, some plant species have been identified to accumulate carotenoids after receiving Co like mung bean (Reshma et al. 2014) and common beans (Zeid 2001).

It is generally known that soluble protein concentration in living organisms is a direct response to a variety of biological and environmental stressors and is a key factor in both reversible and irreversible modifications in metabolic activity. Our results showed that the soluble protein content predominantly showed a considerable reduction in plants treated with Cd, Co, or both. According to de Dios (2019), the side chains of amino acids can be degraded by metal stress-generated ROS, or they can interact with the aldehydic intermediates of lipid peroxidation and impair protein stability. As a result, the current study offers substantial proof that beans experienced oxidative stress as a result of metal stress via a decrease in protein content. Metal stress treatments may be responsible for the decreased soluble protein content due to the decreased protein biosynthesis as well as accelerated protein degradation brought on by proteolytic enzymes (Saleh et al. 2020). The decreased amino acids following metal stress treatments reported in our study were previously described in spinach and mustard (Břendová et al. 2016) and Chinese cabbage (Khan et al. 2020a). The decreased concentration of free amino acids (FAA) suggests that it was likely used as a defensive strategy against metal toxicity. Due to their functions as metal chelators, signaling molecules, and antioxidants, FAA have been suggested to play a crucial role during metal stress (Khan et al. 2020b). Nonetheless, our results demonstrated that plants subjected to both single and combined treatments of Cd and Co experienced higher proline levels. The rising trend of proline content after exposure to Cd and Co implies that metal stress triggered proline synthesis to fend off the imposed osmotic stress. According to Zouari et al. (2016), proline is a defensive molecule that contributes to stabilizing biomolecules, enzymes, and membranes; hence, plants directly respond to stress by promoting proline biosynthesis and slowing proline catabolism. Furthermore, Szabados and Savouré (2010) pointed out that proline operates as an osmoprotectant, a metal-chelating agent, fixes harmed chlorophyll, regulates cytosolic pH, shields against protein degradation, and sustains protein synthesis.

Higher levels of oxidative stress can be determined as an indicator of metal toxicity by assessing stress biomarkers. The elevated levels of the stress biomarkers assessed in this investigation (EL, H2O2, and MDA) confirmed a reasonable degradation brought on by Cd and Co, individually or in combination, on bean cellular membranes. H2O2 overproduction, lipid peroxidation, and membrane leakage are basic signs of prolonged oxidative stress in response to heavy metal treatment. Our findings concur with those of Chattha et al. (2021), who linked the upsurge in EL to diminished membrane fluidity and cellular degradation driven by the increase in lipid peroxidation and H2O2 buildup. Accordingly, to confront oxidative damage whenever confronted with metal toxicity, plants have developed enzymatic and non-antioxidant protection systems.

The buildup of ROS is indeed identified to trigger several antioxidant enzymes, each of which has a specific function in minimizing oxidative stress. In our study, we determined that Cd and Co treatments, both separately and in combination, stimulated the activities of CAT, SOD, and GST in bean plants. These results are in accordance with those of Xu et al. (2022) and Lu et al. (2019) in cadmium-stressed seashore paspalum and tartary buckwheat, respectively. Also, the induction of those enzymes has been reported in cobalt-stressed rapeseed (Jahani et al. 2020) and maize (Ozfidan-Konakci et al. 2022; Selim et al. 2022). The induction of CAT, SOD, and GST is a pivotal defensive mechanism for minimizing oxidative damage in plants undergoing metal stress. As a result, the activity of those enzymes seems to be essential to the elimination of oxygen radicals. According to the reports of Siritantikorn et al. (2007), GST is valuable in the process of eliminating ROS and heavy metals from plant tissues as well as restoring the redox balance. However, the increased activity of SOD and CAT supports their involvement in the elimination of oxygen radicals like O2•− and H2O2.

Contrarily, the results of our investigation showed that bean APX, POD, and GR activity decreased in response to Cd and Co treatments, either alone or in combination. These decreases may indicate a deterioration in the active oxygen removal capability. The accumulation of ROS could be the cause of the decline by disturbing the equilibrium between the detoxification mechanism and the overwhelming generation of ROS (Sarmast et al. 2015). Arif et al. (2020) reported that the reduced activity of some antioxidative enzymes may result from the downregulation of enzyme synthesis or the modifications in the subunit architecture of these enzymes. As a result, it is possible to correlate the declined activities of APX, POD, and GR to the buffering effects of other antioxidant enzymes on the redox potential as well as to the detoxification of harmful ROS and maintenance of GSH/GSSG homeostasis in the metal-stressed beans. Overall, the decline in APX, POD, and GR activities was substantially compensated by an increase in CAR, SOD, and GST activities.

The fundamental mechanism for restoring the oxidative balance and eradicating the damage caused by plant exposure to different stressors is the production of specific antioxidant molecules. The antioxidant molecules assessed in this work, including phenols and flavonoids, displayed a distinct trend, with phenols increasing and flavonoids declining as a result of Cd and Co treatments, either independently or combined. The key factor contributing to the elevation in phenols is the increased activity of phenylalanine ammonia-lyase (PAL), which is triggered by the oxidative stress imposed by metal stress (Ampofo and Ngadi (2021)). Although phenols have been recognized as not being directly included in cellular metabolism, they are a vital component of the non-enzymatic antioxidant system and promote the plant in overcoming both biotic and abiotic stressors (Asgari Lajayer et al. 2017). In addition, phenols are involved in the construction of cell walls, photoassimilation, respiration, allelopathic interactions, and the defense of plants against invading animals (Medda et al. 2020). Accordingly, by complexing the heavy metals and neutralizing ROS, these phenols defend the plant against metal stress sustaining normal growth under stressful conditions. As a class of complex polyphenols, flavonoids operate in plants to suppress lipid peroxidation, eliminate ROS, chelate metals, and protect against radiation damage (Baskar et al. 2018). Saad-Allah and Ragab (2020) asserted that the biosynthesis of flavonoids is suppressed in response to abiotic stress; thus, plants are forced to rely on alternative enzymatic and non-enzymatic antioxidants. The decline in flavonoids could be related to their involvement in the chelation of uptaken metal ions to restrict their dispersion throughout plant tissues.

The current study results demonstrated that the levels of the essential macronutrients (N, P, and K) were significantly reduced in the roots and shoots of beans after being subjected to Cd and Co stress treatments, whereas the concentration of the toxic metals was increased in the plant shoots and roots. Research works with similar consequences were documented in Cd-stressed wheat (Kaya et al. 2020), Indian mustard (Ahmad et al. 2015), and rice (Bhuyan et al. 2020). Likewise, similar results were recorded in Co-stressed wheat (Hagagy et al. 2023), barley (Lwalaba et al. 2020), and maize (Salam et al. 2022). Due to competition between heavy metals and essential minerals for uptake and assimilation by the plant, nutritional insufficiency is one of the major repercussions of metal stress. In addition, metal-induced oxidative stress damages cellular membranes, which can additionally result in decreased nutrient content, hampered uptake of nutrients, and disturbed ionic homeostasis (Bhuyan et al. 2020). As a result, excessive Cd and Co buildup in bean plants hinders the growth and yield of the plant by affecting root and shoot development, metabolic processes, and the absorption of water and minerals.

Our investigation showed that Cd and Co, either alone or in combination, significantly reduced the yield characteristics of bean plants. The decreased bean yield following exposure to Cd and Co treatments could be attributed to a number of factors, including impaired root and shoot growth, disruption of chlorophyll biosynthesis, elevated levels of oxidative stress, decreased enzyme activity, and interference of Cd and Co with ionic homeostasis. Similar investigations on sunflower (Wyszkowska et al. 2022) and rapeseed and maize (Boros-Lajszner et al. 2021) similarly corroborated the reduction in yield components reported by Cd and Co in our study.

The results of this study demonstrated that spraying stressed plants with the methanolic-aqueous palm pollen extract (PPE) could reduce the detrimental effects of Cd and Co on bean growth, biochemical processes, antioxidant properties, and yield characteristics. PPE has been reported as a potent source of naturally occurring proteins, minerals, lipids, carbohydrates, vitamins, sugars, amino acids, hormones, enzymes, cofactors, and antioxidant compounds that may promote plant growth and development (Sebii et al. 2019). Moreover, it was demonstrated by Sadeq et al. (2021) that PPE contains a variety of antioxidative molecules, including tannins, flavonoids, sterols, alkaloids, catechin, and gallic acid. In their study on water-stressed basil plants, Taha et al. (2020) reported that PPE is a feasible source of growth regulators, mineral nutrients, carbohydrates, amino acids, and vitamins that could alleviate drought stress. Consequently, the better growth traits and the higher water content of PPE-sprayed bean seedlings could be attributed to the increased intracellular osmotic pressure, which enhances nutrient and water absorption (Zhao et al. 2021).

Our findings demonstrated that the adverse consequences of Cd or Co anxiety or their combination (Cd+Co) on the reduction in Chl a and Chl b levels might be mitigated by treating beans with PPE as a foliar spray. This enhancement may be attributable to the high content of PPE of minerals like Ca, Fe, and Mg, which are vital for the manufacture of chlorophyll (Sofy et al. 2022). Additionally, Fadhil and Abdulkadhim (2020) demonstrated that lime leaves treated with PPE exhibited higher chlorophyll due to the minerals it contains, including Zn, Mn, Fe, K, Ca, Mg, and P as well as other substantial components including organic acids, vitamins, and proteins. Moreover, the phytohormones in PPE may support stressed plants in maintaining hormonal balance, delaying senescence, and boosting chlorophyll synthesis (Taha et al. 2020). The minor fall in antioxidant carotenoids mediated by PPE treatment may be due to the protective effects this extract has on both normal and stressed plants. Since PPE might minimize ROS accumulation throughout plant tissues, there is no longer need to shift its metabolism to produce antioxidants. Alternatively, the plant can direct its energy to develop more cells to sustain its growth.

Plants treated with PPE showed higher protein and amino acid contents, as compared to the unstressed and stressed plants. This could be explained by the stimulated nitrogen metabolism following PPE treatment due to its abundance in nitrogen and other minerals, which could activate the enzymes responsible for amino acid biosynthesis and protein assembly. Kocira et al. (2020) asserted that the use of biofertilizers, such as seaweed and PPE, boosts the absorption and translocation of nitrogen from roots to shoots, which is reflected in the improved protein and amino acid contents of leaves. As PPE contributes to boosting nitrogen absorption and enhancing amino acid biosynthesis, it results in higher de novo synthesis and lower protein degradation, which contribute to the increased nitrogenous compounds in PPE-treated plants (Hamouda et al. 2022). The decreasing proline level following PPE application found in this study, however, suggests that bean plants are switching from overproducing the antioxidant molecule proline to the biosynthesis of other structural components. In plants treated with biostimulants, Hamouda et al. (2022) demonstrated a correlation between the decrease in proline concentration and the increase in soluble proteins, suggesting that proline was incorporated into protein synthesis. Furthermore, the interplay between proline and the accumulated ROS suggests that PPE reduced the overproduction of ROS, and thus, no further proline accumulation is needed in PPE-treated plants.

Bean seedlings treated with PPE, whether stressed or not, displayed decreased levels of the stress biomarkers EL, H2O2, and MDA. PPE treatment could be regarded to offer protection to the proteins and phospholipids found in membranes to promote cellular activities by suppressing the formation of harmful ROS like H2O2 and OH and maintaining membrane fluidity by avoiding peroxidation. PPE has been reported as a source of many antioxidants like vitamin A, ascorbic acid, glutathione, and vitamin E (Taha et al. 2020). These antioxidants offer the plant protective impact by accelerating metabolic processes and sustaining plant health through improved turgidity and reduced membrane degradation.

Following PPE application onto bean plants, there was a noteworthy rise in some enzymatic antioxidants’ activity, including APX, SOD, GR, and POD enzymes, whether the plants were stressed or not. Consequently, by maintaining membrane integrity and redox balance in stressed plants, the application of PPE assisted in limiting the harm that these enzymes may experience. The observed decrease in CAT activity in bean seedlings treated with PPE is consistent with the findings of Daoud et al. (2017), who attributed this decline to PPE’s significant concentrations of flavonoids, phytosterols, and carotenoids, which are antioxidants with redox activities acting as reducing agents for H2O2 quenching. As a result, PPE prevented the denaturation or malfunctioning of antioxidant enzymes by sustaining their high expression to offset the overwhelming metal-induced ROS formation.

The obtained results demonstrated that PPE supplementation as a foliar spray markedly boosted the essential mineral ions of bean shoots and roots including N, P, and K while reducing the uptake of Cd and Co into the stressed plants. According to Taha et al. (2020), the mineral nutrients in PPE can reach the leaf tissue and keep the balance of minerals for optimal plant growth even when under stress. PPE also contains essential minerals, growth regulators, osmoprotectants, and antioxidants such as phenols and flavonoids that promote plants to uptake more water, improving their hydration status, metabolic responses, and yield performance.

The results of GC/MS analyses showed that PPE includes an assortment of phytochemicals that are advantageous for plant growth under both normal and stressed conditions such as pentanoic acid, heneicosanoic acid, diphenylamine, citronellol, benzenedicarboxylic acid, 1,3-propanediol, mannitol, hydroxylamine, and γ-linolenic acid. These compounds have been demonstrated to increase plant growth through the induction of the antioxidant system, stabilization of cellular membranes, enhancement of cell wall rigidity, improvement of root cell growth and division, establishment of osmoregulation, enhancement of chlorophyll content, improvement of nutritional balance, adjustment of gene expression, and relief of stress-related conditions (Ankati and Podile 2019; Hasan et al. 2019; Jayaraj et al. 2022; Mubeen et al. 2022; Sukačová et al. 2023). Furthermore, the findings of this study demonstrated that PPE contains high concentrations of n-hexadecanoic acid, 1,2-benzenedicarboxylic acid, and diisooctyl ester, which are well-documented for promoting plant physiological responses towards the equilibrium under stress conditions by inducing antioxidant defenses, improving enzymatic activities, sustaining the nutritional homeostasis, and avoiding membrane deterioration in a variety of stress situations, including heavy metals, salinity, water deficiency, and fungal invasion (Mubeen et al. 2022; Terletskaya et al. 2021; Xiong et al. 2020).

5 Conclusion

In this study, we have demonstrated that the foliar application of methanolic-aqueous palm pollen extract (PPE) positively amended the growth pattern, photosynthetic pigments, osmomodulators, oxidative stress indices, antioxidant potential, mineral ion content, and yield performance of Cd- and Co-stressed beans (Phaseolus vulgaris L.). Our results revealed that PPE elicited physiological and protective adaptations, potentially having promoted better and more efficacious interactions to the administration of Cd and Co in bean plants. In addition to the boosted antioxidant enzymatic activity of ascorbate peroxidase (APX), peroxidase (POD), superoxide dismutase (SOD), glutathione reductase (GR), and glutathione S-transferase (GST), the improved pigment levels; declined oxidative indications like electrolyte leakage (EL), malondialdehyde (MDA), and hydrogen peroxide (H2O2); and promoted buildup of non-enzymatic antioxidants (phenols and flavonoids), as well as osmoregulatory molecules (proteins, amino acids, and proline), might be the basic keys for the relief of metal toxicity (Cd or Co) in bean plants. Therefore, in this study, we defined the presence of specific significant phytochemicals in PPE which enable bean plants to endure the metal-induced toxicity. In the future, studies ought to focus primarily on phytoconstituents, hormonal, biochemical, and transcriptional domains to comprehend the underlying approaches through which PPE foliar treatment could retrieve Cd and Co toxicity in bean plants.