Influence of Lead (Pb) and Its Relationship with the pH of Water on the Growth of Creole Maize (Zea mays L.)

Abstract

Lead (Pb) as a pollutant is not biodegradable, tends to accumulate in different organisms, and can affect, for example, the performance of maize crops. However, only a few studies have reported on the effect of lead (Pb) and the relationship with the potential of hydrogen (pH) of water on crop performance. Thus, this study aimed at determining the influence of Pb and its relationship with the pH of water on the growth of Creole maize (Zea mays L.). In order to achieve this, a double bottom vessel system, isolated from the soil, was used to expose the plants to an aqueous Pb solution based on the equivalence of 1.5985 g of lead nitrate (Pb(NO₃)₂) per g of Pb. An experimental design of the two-factor response surface methodology (RSM) was applied. The Creole maize plants were exposed to four different concentrations of Pb in water [0 g L⁻¹ (P1: control); 0.33 g L⁻¹ (P2); 0.66 g L⁻¹ (P3); 1 g L⁻¹ (P4)], as well as to three different pH levels (5.5 (pH1); 6.5 (pH2); 7.5 (pH3)) in clay soil typical of the region. Subsequently, the relationship of these variables with maize growth was determined. The results showed a decrease in leaf growth, height, stem circumference, and root. However, it was also determined that these negative effects can be mitigated by controlling the pH of water in 7.5. Failure to control the combination of these two factors in the cultivation process generally results in an impact on the growth of the maize seedling. In addition, leaf discoloration was also observed in the leaves of maize plants from the concentration of 0.33 g L⁻¹ (P2), which suggests a nutritional anomaly that is toxic to the plant.

1. Introduction

Technological development and human activities in general produce waste that can affect organisms and microorganisms, which in turn can threaten human health by contaminating soils, air, rivers, and other bodies of water [1,2,3,4]. Much of this pollution is caused by heavy metals and metalloids. This is one of the most severe problems, as it compromises food safety and public health at global and local levels [5,6,7,8]. Heavy metals are chemical elements that have a relatively high density and are toxic or poisonous (even at low concentrations); therefore, they are commonly associated with contamination, potential toxicity, or ecotoxicity [9,10,11]. Untreated waste (whether industrial, sanitary, or from anthropogenic activities in general) contaminates water and soil with a high content of heavy metals such as nickel (Ni), cadmium (Cd), manganese (Mn), and lead (Pb), among others [12,13,14,15,16]. Heavy metals can be hazardous due to their non-biodegradable nature [17,18,19,20,21], their toxicity to certain crops, and their bioavailability [22,23], especially when found in soil and water used in agriculture.

One of the most toxic heavy metals is lead (Pb) (from the Latin plumbum) [20,24,25], the widespread use of which has caused great environmental pollution and health problems in many parts of the world [2,5,15,26,27,28,29,30]. Pb is a high-density element (11.35 g/cm³), greyish in color, naturally present in the earth’s crust [31,32]. Despite being a metal, it is a bad conductor of electricity and, in humid environments, it is covered with an oxide layer [32,33]. Its atomic number is 82, its melting point is 327.4 °C, and its boiling point is 1740 °C [34,35]. In its pure state, it is soft, ductile, and malleable. These characteristics make Pb susceptible and very suitable for industrial use [31,36].

Pb and its derivatives have the ability to bioaccumulate. For this reason, the concentration in soil, plants and animals increases along the food chain [15,37,38]. In plants, for example, it is stored mainly in the roots [39,40], minimizing its presence in reproductive structures. Phytotoxicity by Pb causes disorders in normal physiological activities, killing cells when organisms are exposed to high concentrations [41]. Symptoms can be divided into specific and non-specific symptoms. Specific symptoms show a decrease in the percentage of germination of seeds and in the germination index (GI). The GI represents the product of the relative germination of seeds by the relative growth of the radicle [42,43]. Nonspecific symptoms are related to rapid inhibition of radicle growth, reduction of leaf area, inhibition of seed germination, and plant growth retardation [44,45].

On the other hand, the quality of soil and water is given by a set of physical, chemical, and biological properties, as well as their interactions with the organisms that inhabit them [46]. In agriculture, for example, they directly influence crop yield and quality [47]. Therefore, it is also important to evaluate the concentration of hydrogen ions, represented by the potential of hydrogen (pH). pH is an index with which irrigation water is quickly evaluated [48]. This is a measure of water acidity [49]. The influence of the pH of irrigation water on different agricultural products has been studied by several authors. Regarding this, it has been shown that an extreme value produces a decrease in morphological parameters [50,51]. pH values above the normal range may cause nutrient imbalance or contain poison ions [48,52]. Different studies have focused on determining the effect of Pb and water pH on different crops, but independently; that is, without considering a relationship between both variables [21,53,54,55]. Thus, this study aimed not only at determining the influence of lead on the growth of maize, but also the relationship that exists with the pH of water. We are particularly interested in maize, as it is considered a primary product for food sovereignty in Mexico [56,57]. In addition, the State of Veracruz is one of the main producers at the national level, with a sown area of just over 566,000 hectares [58]. Maize represents 12.7% of the total agricultural production of the State of Veracruz and is considered the second most important crop [59]; however, its average yield is 2.2 tons ha⁻¹, which is considered low [58]. Thus, due to its economic importance and the impact that it has on the Mexican diet, this study seeks to determine the influence of pH in the presence of water contaminated with Pb in the cultivation of Creole maize. This is in order to check if the pH influences the performance of the crop, and where appropriate, control it to increase its yield.

2. Materials and Methods

2.1. Experimental Site and Plant Matter

The study was carried out in an area specially designated for the experiment. This took place in the Higher Technological Institute of Xalapa (ITSX), which is located in the south of the City of Xalapa, in the State of Veracruz, Mexico. The region is characterized by a humid temperate climate with a minimum temperature of about 2 °C to a maximum of 34 °C. The study began in June 2021 and ended in December 2021. Two complete replicas were executed from the sowing to the analysis of results. Data were collected from 216 seeds, 72 plants, and 288 leaves. The seeds used were maize of the Creole type (Zea mays L.) and were obtained in local commerce. The basic information regarding seeds is presented in

Table 1. Description of the seeds used in the study.

Parameter	Average	Standard Deviation	Minimum Value	Maximum Value
Diameter (mm)	1.48	0.27	1.0	1.9
Weight (g)	0.4730	0.08554	0.3547	0.6821

Calculated from the total population N = 216.

.

2.2. Experimental Design and Treatment of Maize Plants

In order to determine the effect of lead (Pb) and its relationship with pH, an experimental design based on the two-factor response surface methodology (RSM) was applied. The first factor was the concentration of Pb in water, measured in g L ⁻¹, with 4 concentration levels. The second factor was the potential of hydrogen pH with 3 levels. The design had a total of 12 experimental units. Groups of three pots were placed in which the seeds were grown. The pots were conical in shape (with a volume of 8.9 L or 2.3 gallons, with measures of 17 cm regarding major diameter, 12 cm of minor diameter, and a height of 13.2 cm). Subsequently, they were placed in containers in the form of a rectangular prism (with a volume of 60.6 L or 16 gallons, with the following measures; 32 cm wide, 60 cm long, and 32 cm high). Each of these rectangular containers was defined as an experimental unit. The experimental unit was designed with a double vessel or double bottom in order to remain isolated from the ground. Figure 1 shows the experimental unit scheme that was also used as an automated drip irrigation unit and included a centrifugal pump and hose.

According to the design of the experiments, to determine the effect of lead (Pb) and its relationship with the pH in the crop, the crop was irrigated with an aqueous solution in which the two factors (Pb and pH) were controlled. Lead nitrate (Pb(NO₃)₂) was used to obtain this solution, so it was necessary to calculate an equivalence based on the atomic mass of both substances (as shown in

Table 2. Lead equivalence by lead nitrate.

Substance	Chemical Symbol	Atomic Mass (g/mol)	Pb/Pb Ratio (NO3)2
Lead nitrate	Pb(NO3)2	331.2
Lead	Pb	207.2	1.5985

Data collected by the authors [33,60].

). Based on the equivalence of 1.5985 g of (Pb(NO₃)₂) per g of Pb [33,60], the necessary amount of nitrate was diluted according to the indicated lead concentration. The mixtures were made by adding the necessary quantity for the 20 L of water contained in each container in the form of a rectangular prism (see

Table 3. Solutions per liter.

Concentrations	Pb/L (g L −1)	Pb(NO3)2/L (g)	Pb(NO3)2/20 L (g)
P1 (Control)	0.000	0.000	0.000
P2	0.333	0.532	10.646
P3	0.666	1.065	21.291
P4	1000	1.598	31.969

Data collected by the authors.

). Thus, the plants seeded in clay soil typical of the region of Xalapa, Veracruz, Mexico were exposed to water mixtures with four different Pb concentrations (0 g L⁻¹ Control; 0.33 g L⁻¹; 0.66 g L⁻¹; 1 g L⁻¹) and three different pH levels (5.5; 6.5; 7.5).

Optimal conditions were provided to the seedlings. For example, they were fertilized with nitrogen, potassium and phosphorus (NPK) in order to contribute to their strengthening and growth [61,62]. The seedlings were watered every 3 days with this aqueous fertilizer solution at a concentration of 20 g L⁻¹. In addition, as a control insecticide to prevent the presence of pests, 21.5% P P⁻¹ cypermethrin was applied every 15 days through an aqueous insecticide solution with a concentration of 5 mL per liter of water [63].

The aqueous solution was supplied by using an automated drip irrigation system for the twelve double bottom irrigation units, with a control circuit that ignited a centrifugal pump for 80 min every 48 h. In this way, a different concentration of Pb and a different pH level were supplied for each of the twelve irrigation units (see

Table 4. Experimental matrix.

Double Bottom Irrigation Unit	Lead Concentration (g L−1)	Potential of Hydrogen (pH)
U1	0	5.5
U2	0	6.5
U3	0	7.5
U4	0.33	5.5
U5	0.33	6.5
U6	0.33	7.5
U7	0.66	5.5
U8	0.66	6.5
U9	0.66	7.5
U10	1	5.5
U11	1	6.5
U12	1	7.5

Each double bottom irrigation unit has 3 pots, therefore N = 36. Data collected by the authors.

). The required amount of liquid was determined by considering the flow rate of the pump, the surface of the pots in which the seedlings were sown, and the recommended irrigation amount for the crop—between 500 and 700 mm of water [64]. The level of water volume of the solution was monitored every day to ensure the Pb concentration and pH level originally established by each treatment according to the experimental matrix (see Table 4). To maintain the pH values, potassium hydroxide (KOH) was added to increase it, and hydrochloric acid (HCL) to reduce it [64]. Table 4 shows the experimental matrix and Figure 2 an experimental crop photograph, in which we can see the twelve double bottom irrigation units, with a density of about 200,000 plants ha⁻¹.

Once the seeds germinated and the seedlings began to develop, the height, stem circumference, and leaves were measured in each plant. The radicle was measured at the end of the experiment, every Monday of each week. All of these measurements were made based on a field guide [65]. The photographs in Figure 3 and Figure 4 show examples of the 2 replicas of the experimental crop.

2.3. Statistical Analysis

The experimental design based on the response surface methodology (RSM) considered two factors: the concentration of lead (Pb) in water, measured in g L⁻¹, with four concentration levels; and, the potential of hydrogen (pH), with three levels (see Figure 5).

The coefficient of determination (R²) and the correlation coefficient were used to evaluate the relationships of the variables [66,67] by using the statistical software Minitab^® 17.1.0 for Windows 10 Pro [68].

3. Results

3.1. Leaf Length, Seedling Height, Stem and Root Circumference

Figure 6 shows the average results of the different pH combinations and Pb concentrations (in the seedling leaves, the height of the entire plant, and the stem circumference, which was measured at the base of the seedling) at 30 and 60 days after having been planted. In addition, the results of the main root length 60 days after having been sown are shown.

3.2. Association of the Results with a Fixed pH as a Function of the Pb Concentration

Table 5. Relationship of the maize leaf length, height of the entire plant, stem circumference, and length of the main root as a function of the lead concentration in the water CPb, at the three different pH levels at 60 days.

pH	Equation	R2	p-Value	Pearson r	p-Value
5.5	Blade Lenghth = 32.962 − 2.36 CPb	67.06	0.000	−0.819	0.181
6.5	Blade Lenghth = 35.30 − 3.652 CPb	99.80	0.000	−0.999	0.001
7.5	Blade Lenghth = 45.40 − 6.14 CPb	91.72	0.000	−0.958	0.042
5.5	Seedling height = 32.526 − 9.29 CPb	94.38	0.001	−0.971	0.029
6.5	Seedling Lenghth = 41.17 − 10.62 CPb	59.85	0.009	−0.774	0.226
7.5	Seedling Lenght = 46.49 − 14.35 CPb	90.69	0.002	−0.952	0.048
5.5	Stem circumference = 5.833 − 1.976 CPb	94.38	0.001	−0.918	0.082
6.5	Stem circumference = 7.659 − 3.686 CPb	90.90	0.004	−0.953	0.047
7.5	Stem circumference = 8.378 − 4.077 CPb	93.18	0.002	−0.965	0.035
5.5	Root = 16.397 − 1.048 CPb	88.63	0.000	−0.941	0.059
6.5	Root = 18.149 − 1.81 CPb	42.65	0.003	−0.653	0.347
7.5	Root = 20.202 − 1.66 CPb	56.95	0.001	−0.755	0.245

Data collected by the authors.

shows the equations of maize leaf length, full plant height, stem circumference, and main root length as a function of the lead concentration in water (CPb) with the three different levels of potential of hydrogen (pH) handled in the experiment at 60 days. In general, it can be seen that the coefficients of determination (R²) are high, as 42.65 is the lowest (corresponding to the root length data with a fixed pH of 6.5), and 99.8 is the highest (corresponding to the leaf length data with a fixed pH of 6.5), clearly indicating that there is a moderate to strong correlation for all cases. On the other hand, it is observed that all Pearson’s coefficients (r) are negative, as −0.653 is the highest (corresponding to the root data with a pH of 6.5), and −0.999 is the lowest (corresponding to the leaf length data with a pH of 6.5), indicating that there is a strong inverse correlation.

3.3. Association of Results with Fixed Pb Concentration as a Function of the pH

Table 6. Relationship of the maize leaf length, height of the entire plant, stem circumference, and length of the main root according to the pH of the water, at the four levels of CPb at 60 days.

CPb	Equation	R2	p-Value	Pearson r	p-Value
0	Blade Length = −2.8 + 6.20 pH	92.31	0.849	0.961	0.179
0.33	Blade Length = −0.9 + 5.4 pH	82.50	0.969	0.980	0.096
0.66	Blade Length = 5.4 + 4.61 pH	82.43	0.765	0.908	0.275
1	Blade Length = 4.4 + 4.48 pH	88.39	0.750	0.998	0.042
0	Seedling Length = −3.93 + 6.507 pH	99.23	0.485	0.996	0.056
0.33	Seedling Length = −5.0 + 6.58 pH	79.54	0.856	0.892	0.299
0.66	Seedling Length = −3.4 + 5.85 pH	92.31	0.818	0.961	0.179
1	Seedling Length = 1.10 + 3950 pH	96.43	0.862	0.982	0.121
0	Stem circumference = −0.33 + 1.200 pH	94.53	0.889	0.972	0.150
0.33	Stem circumference = −0.88 + 1.100 pH	93.56	0.722	0.967	0.163
0.66	Stem circumference = 1.608 + 0.4500 pH	99.59	0.075	0.998	0.041
1	Stem circumference = 2.808 + 0.2500 pH	98.68	0.043	0.993	0.073
0	Root = 6.958 + 1.750 pH	99.32	0.086	0.997	0.052
0.33	Root = 5.96 + 1.750 pH	85.47	0.424	0.924	0.249
0.66	Root = 3.208 + 2.250 pH	99.18	0.182	0.998	0.041
1	Root = 8.38 + 1.250 pH	89.29	0.208	0.945	0.212

Data collected by the authors.

shows the equations of maize leaf length, height of the entire plant, stem circumference, and main root length as a function of the pH with the four different lead concentration levels (CPb) handled in the experiment at 60 days. In general, it can be seen that the coefficients of determination (R²) are high: 79.54 is the lowest (corresponding to the plant length data with the fixed CPb of 0.33), and 99.59 is the highest (corresponding to the circumference length data with a fixed CPb of 0.66). This indicates that there is a significant to strong correlation. On the other hand, it is observed that all Pearson’s coefficients (r) are positive: 0.980 is the highest (corresponding to the leaf length data with a fixed CPb of 0.33), and 0.892 the lowest (corresponding to the data of the seedling height with a fixed CPb of 0.33), indicating that there is a strong direct correlation.

3.4. Association of Results as a Function of the Lead Concentration (CPb) and Potential of Hydrogen (pH)

Once the data was captured in the statistical software, multiple linear regression was carried out, relating the two independent input variables and the dependent output variable. As a result, four equations were obtained. They were all based on pH and CPb at 60 days of growth for the 24 double bottom units. Equation (1) shows the length of the leaves measured in centimeters. Equation (2) shows the total height of the plant in centimeters. Equation (3) shows the stem circumference in centimeters. Equation (4) shows the length of the root in centimeters.

Blade length (cm) = 3.65 + 5.267 pH − 4.048 CPb,

(1)

Seedling height (cm) = 3.01 + 5.705 pH − 11.45 CPb,

(2)

Stem circumference (cm) = 2.35 + 0.758 pH − 3.236 CPb,

(3)

Root (cm) = 6.77 + 1.767 pH − 1.519 CPb

(4)

CPb means the concentration of lead in the water in g L⁻¹ and pH is the potential of hydrogen of water. Figure 7 shows the graph of response surface obtained through the mathematical model by using the statistical software Minitab^® 17.1.0 for Windows 10 Pro.

3.5. Comparative Analysis of the Parameters Evaluated without Lead and with the Maximum Lead Concentration (CPb)

Table 7. Comparison of the results of all the parameters contrasting double bottom irrigation units without lead and with the maximum lead concentration (CPb) at 60 days.

	Parameter	Double Bottom Irrigation Unit		Difference (cm)	Decrease Percentage (%)
		U1	U10
pH 5.5	Blade length	32.3	30	2.3	7.2%
	Seedling height	32.2	22.4	9.8	30.5%
	Stem circumference	6.1	4.2	1.9	31.1%
	Root	16.5	15.5	1	6.1%
		U2	U11
pH 6.5	Blade length	35.4	31.7	3.7	10.5%
	Seedling height	37.7	27.7	10	26.7%
	Stem circumference	7.8	4.4	3.4	43.6%
	Root	18.5	16	2.5	13.5%
		U3	U12
pH 7.5	Blade length	44.7	39	5.7	12.8%
	Seedling height	45.2	30.3	14.9	33.0%
	Stem circumference	8.5	4.7	3.8	44.7%
	Root	20	18	2	10.0%

Data collected by the authors.

shows the comparison between the parameters of the double bottom irrigation units at the maximum concentration of lead established (CPb = 1) and those of the corresponding units without lead contamination (CPb = 0). The results of the comparison are expressed as differences (in cm) and percentages of decrease 60 days after having been planted. The differences and the percentages of affectation that occurred with respect to the values without contamination, at a pH of 7.5, are as follows: the leaf length was affected by 12.8%, the height of the seedling by 33%, the circumference of the stem was the most affected with 44.7%, and, finally, the main root by 10%.

Figure 8 plots the percentages of affectation by pH level, and Figure 9 shows the comparison in the growth of the factors with maximum concentration of lead and without contamination at pH 5.5 and pH 7.5.

In the experiment, a discoloration was also observed in the plants from CPb 0.33 g L⁻¹. The discoloration became more frequent in leaves with higher CPb, turning purple due to the influence of lead (see Figure 10). Because the control seedlings did not present this discoloration, a phosphorus deficiency or a viral disease caused by lead nitrate is suggested [65].

4. Discussion

4.1. Leaf Length, Seedling Height, Stem and Root Circumference

The results suggest that there is less growth regarding the leaf length as the concentration of CPb increases. This can be observed from 30 days of growth (see Figure 6) and becomes significant at 60 days. For example, if we analyze the average length of the leaves without contamination with a pH of 5.5, we find that is of 32.3 cm, and as the concentration of CPb in water increases, the average length of the leaves begins to decrease. This behavior occurs in the height of the plant and the circumference of the stem. This effect can also be observed when we use a pH of 6.5 and 7.5.

Regarding the main root, it was only possible to measure it at 60 days, because it involved separating the seedling from the substrate; however, the results showed a lower growth as CPb contamination increases.

These results are consistent with several studies that mention the negative effects on maize due to the presence of irrigation water contaminated with heavy metals [25,27,44,65,69].

4.2. Association of Results with a Fixed pH as a Function of the Pb Concentration

The leaf length is related to the pH and to the level of lead concentration in water. In order to study this relationship with the mathematical model, the pH was kept fixed. The results shown in Table 5 suggest that, regardless of the pH level, if the concentration of lead in the water (CPb) increases, the leaf length decreases. For example, in the case of the pH of 6.5 the equation obtained through the formula “Blade length = 35.30–3.652 CPb” presents the best coefficient of R² and the best adjustment of the data with an r= −0.999, in which the value −3.652 corresponds to the slope of the linear equation product of the correlation shown in Table 5.

Regarding the height of the seedling, the results show that regardless of the pH level in the irrigation water, if the lead concentration (CPb) increases, the height is negatively affected. In this case, the formula that presented the best R² coefficient is for the pH of 5.5 “Seedling Length = 32.526–9.29 CPb as can be seen in Table 5.

When analyzing the stem circumference in Table 5, there are some singularities that are worth mentioning. For example, in the case of the pH of 5.5 the equation “Stem circumference = 5.833–1.976 CPb” presents the best value of R², but the associated r value is the lowest. With a pH of 7.5, the equation “Stem circumference = 8.378–4.077 CPb” presents the best value of r = −0.965. Furthermore, the value of R² is very close to that of the equation with the pH of 5.5. This may be due to the fact that the variations in the growth of the stem circumference are small.

In the case of the main root in Table 5, we observe for pH 5.5 the equation “Root = 16.397–1.048 CPb” presents the best value of r and R². However, in the case of a pH of 6.5 the equation “Root = 18.149–1.81 CPb” presents the lowest values obtained from all equations of r and R².

4.3. Association of Results with a Fixed Pb Concentration as a Function of the pH

Studying the parameters and the relationship they have with pH and lead contamination in water (CPb), in the mathematical model, keeping the CPb variable fixed (see Table 6), the results suggest that regardless of the level of CPb, if the pH increases the leaf length increases. For example, in the case of the sample with CPb = 1, the equation obtained through the formula “Blade length = 4.4 + 4.48 pH” presents a high value of r = 0.998 and a positive value of 4.48, corresponding to the slope of the linear equation. This allows us to observe a positive tendency in the growth as the pH value increases.

Regarding the total length of the plant, a similar situation occurs. The results show that regardless of the level of CPb in the water, if the pH level increases, the height of the plant increases; however, unlike the length of the leaf, the greatest contribution of the pH in the height of the plant is given with CPb = 0.33 and the equation “Seedling length = −5.0 + 6.58 pH”.

With respect to the stem circumference, the same situation occurs, which means that regardless of the value of the lead concentration in water (CPb), if the pH value increases, the stem increases. In addition, the equations for concentrations of CPb = 0.66 and CPb = 1 show the best values of R² and r.

Finally, when we analyzed the length of the main root, the results show the same coefficient of pH for the case of the equations with CPb = 0 and CPb = 0.33. This indicates that the variation in root growth between these two points is barely perceptible.

4.4. Association of the Results as a Function of the Lead Concentration (CPb) and Potential of Hydrogen (pH)

On the other hand, when analyzing the two variables of this study—that is, the concentration of lead in water (CPb) and the pH level shown in Table 7, as well as the coefficients of equations 1, 2, 3 and 4—it was observed that in the case of the variable CPb the sign is negative, which shows a downward tendency, whereas in the case of the variable pH the signs are positive, which shows an upward tendency.

In the case of the leaf length (Equation (1)), the coefficient accompanying the pH is the second highest as a value of 5.267, and the coefficient accompanying the CPb is the second lowest.

With regard to seedling height, in Equation (2) the coefficients accompanying both pH and CPb are the most significant of the four equations.

Analyzing Equation (3) corresponding to the stem circumference, the results obtained from the coefficients show that it is the least affected with respect to the pH level and one of the least affected parameters with respect to the CPb.

For the case of the root, Equation (4) shows that based on the coefficients it is one of the least affected parameters with respect to the pH level and the least significant with respect to the CPb.

4.5. Comparative Analysis of the Parameters Evaluated without Lead and with the Maximum Lead Concentration (CPb)

When examining the growth tendencies over time with the data available and plotted in Figure 9, if we analyze graph (b) U10 with CPb = 1 and pH 5.5 and graph (d) U12 with CPb = 1 and pH 7.5 vertically, we can appreciate the effect on the growth of all the parameters due to the maximum concentration of lead and the minimum and maximum level of pH used (the exact values can be seen in Table 7). Considering that in both cases we have the maximum concentration of Pb, it is evident that for the case of pH 7.5 the seedling presents a significant increase in the growth of the leaf length and the height of the plant.

In the case of the root and the stem circumference, it was observed that the difference in growth that occurs with CPb = 1, pH 5.5 and CPb = 1, pH 7.5 is small; however, when contrasting these values with graphs (a) U1 with CPb = 0 and pH5.5 and (c) U3 with CPb = 0 and pH 7.5, it was observed that there is also an improvement in the growth of the root with pH 7.5, but of less significance.

5. Conclusions

The comparison of the evaluation results of each of the parameters in this study, taking into account the lead concentrations P1 (0 g L⁻¹) and P4 (1 g L⁻¹), that is, the aqueous solution without lead contamination (control), and the maximum lead concentration in the water at the different pH levels, shows the affectation that maize suffers in its growth when irrigation water contaminated with lead is used.

According to the findings of this study, in the presence of Pb contamination in irrigation water, an adequate pH control can mitigate the negative effects produced by this heavy metal on the growth of plants. Thus, proper monitoring and control of pH in irrigation water is critical in the face of suspected Pb contamination.

It is worth mentioning that more studies are needed in relation to the combined effects of Pb and pH, as well as other heavy metals that have an impact on agriculture as there are very few studies in the scientific literature.

In addition to the negative effects on maize growth presented in this study, a foliar discoloration with a purple tone at high Pb concentrations was also observed, which suggests a nutritional anomaly due to phosphorus deficiency or plant toxicology.