Sulfur application alleviates chromium stress in maize and wheat

2.1 Materials and setup and procedure

Experiments were set up in four replicates in Wagner type pots of 5 kg of soil with the physical and chemical properties presented in Table 1.

Initially, the soil had an acidic pH, before liming of the soil was done, a medium level of phosphorus and low levels of potassium and magnesium. The amount of overall S and S-SO₄ in both soils was determined and classified as low fertility soil. The content of microelements was low for iron, medium for copper, and manganese, but high for zinc. The Cr content was determined to be within the naturally occurring range.

The studied agricultural plants were spring wheat (Tybalt variety) and maize (Mosso variety). The length of the vegetation period for the species of plants tested in the pot experiments was 115 days for wheat and 99 days for maize. Wheat was collected at full maturity stage, and maize was collected at the full bloom stage (BBCH 67).

Calcium was added to the soil (liming) before sowing. Calcium carbonate was applied at a dose calculated for 1 Hh (5 g CaCO₃). The experimental design included eight groups used to study the interaction of applied elemental sulfur fertilization and Cr (Table 2).

Elemental S and Cr were applied before sowing the seeds. Elemental S was ground into an average grain size of less than 0.1 mm to increase the rate of S oxidation in the soil [11]. Cr(vi) was applied to the soil in the form of an aqueous solution of potassium dichromate K₂Cr₂O₇. A potassium chloride solution was used to compensate for the amount of K in pots when no potassium dichromate was added.

For both plants, the same dose of nitrogen was applied (1.6 g per pot; NH₄NO₃ in an aqueous solution). Half of the dose was applied before sowing and a half during the top-dressing stage (spring wheat BBCH 30 and maize BBCH 19).

The size of the dose for the remaining macroelements depended on the soil properties. Doses were added to the 5 kg of soil per pot: 0.6 g phosphorus, 1.5 g potassium, and 0.3 g of magnesium. The fertilization of microelements was applied in standard quantities for pot experiments in compounds that did not contain S. Macro- and microelements were applied before sowing (in an aqueous solution or solids) and mixed into the entire amount of soil in the pot. Soil moisture was maintained throughout the entire vegetation period of the cultivated plants at 60% field capacity.

2.2 Methods for chemical analysis

Before and after the vegetation experiments, representative soil and plant samples were collected for agricultural and chemical analysis. After taking preparations of the soil material, the following was determined: the soil pH of 1 mol dm⁻³ KCl with the potentiometric method, the overall S content (S total) with the Butters–Chenery method [12], the content of S sulfates(vi) (S-SO₄) with the Bardsley and Lancaster method [13], and the soluble form of Cr in soil were determined in 1 mol dm⁻³ HCl by the Rinkis method [14].

In-plant material collected during the research, the following was determined: the overall level of nitrogen (N organic) with the Kjeldahl method, the overall level of S (S total) with the Butters–Chenery method. To determine other elements, the plant material was dry mineralized, then the ash was taken up with nitric acid and determined in solutions: phosphorus with the vanadic-molybdate method, potassium and calcium with flame photometry, magnesium with atomic absorption spectrophotometry (ASA), and Cr and microelements with the ASA method.

2.3 Statistical methods

The yield sizes and the results of the chemical analysis were subjected to a one-way variance analysis. An evaluation was made of the relevance of mean differences with the Tukey post hoc test with a significance level of p = 0.05. The statistical program Statistica v. 13 and R [15] were used to develop the test results.

1. Ethical approval: The conducted research is not related to either human or animal use.

3.1 Plant growth and yield

All the examined doses of Cr caused a marked decrease in both the fresh and dry weight of maize and applying S significantly increased these parameters (Figure 1).

Figure 1

Yield of maize.

Under the highest Cr dose, maize growth parameters were more than 20% worse than in the control plants. Applying S significantly improved the maize biomass, and the accumulation of fresh and dry mass was 36% and 26% higher, respectively, than in plants not treated with S.

Wheat was more tolerant of Cr than maize, and both lower Cr doses caused a statistically significant increase in the weight of straw (Figure 2).

Figure 2

Yield of spring wheat.

Cr used in the highest dose decreased the yield of straw and grain, and the grain yield was 20% lower than in the control plants. Applying S enhanced the grain yield by nearly 12% compared to plants treated with 52 mg Cr. In the remaining cases, S fertilization also caused statistically significant increases, and the yield of both grain and straw was higher than in the control plants.

3.2 Mineral nutrition

4.1 The influence of Cr

In the present study, we found that relatively low doses of Cr reduced crop productivity and changed the mineral status and applying elemental S to the soil may alleviate these negative influences. The studied data on crop growth showed that maize was more sensitive to Cr than wheat, and with increasing levels of Cr, the dry mass of maize decreased. There are many reports that plants exposed Cr have inhibited growth [21,22,23,24]. However, there are also reports indicating that Cr applied at low doses can have hormetic effects, namely low amounts of a contaminant have positive effects on an organism [25]. Our results showed that the lowest Cr dose (26 mg kg⁻¹ soil) stimulated the growth of wheat shoots.

The mechanisms that build plant tolerance to heavy metals are connected with processes that reduce the uptake and transport of metals and with detoxification inside the cells [26]. The toxicity of Cr to plants depends on the concentration of the metal, its oxidation state, soil type, and the plant itself.

Interestingly, the shoots of both cereals grown under the highest dose of Cr contained 13% more nitrogen than the control plants. This may indicate that the N uptake and transport within the plant were not disrupted by Cr, and it is possible that both cereals activate a similar defense process in response to Cr. Many compounds containing nitrogen play an important role in the cellular detoxification of heavy metals. In the present study, we did not examine these metabolites, but the increased N levels confirm that statement [27,28,29,30].

On the other hand, S concentrations in the shoots of the studied cereals varied when exposed to Cr. Cr treatment did not change the S concentrations in maize, but significantly lowered it in wheat. However, even the lower level of S in wheat straw was approximately three times higher than in maize. The greater tolerance of wheat to Cr may be associated with the high S concentration in this plant. Generally, plants differ greatly in their uptake and requirements for S, and S concentrations in plant tissues can vary over wide ranges. Small grains and maize usually contain the least amounts [31]. S is necessary for the proper growth and development of plants, however, like nitrogen, it also plays regulatory and defense functions in a plant’s response to environmental stress and participates in the biosynthesis of some defense metabolites. A S-containing amino acid, cysteine, plays a central role in the detoxification of heavy metals. Cysteine is a primary metabolite for the synthesis of glutathione, phytochelatins, and metallothioneins. A higher S concentration in wheat straw likely allows this plant to accumulate considerably more Cr than maize without visible growth inhibition. Zayed and Terry [32] state that a critical Cr concentration in most plants is between 1 and 10 mg kg⁻¹ dry mass. Our results showed that the determined Cr content in the wheat straw and maize shoots fell within this range.

The nutritional state connected with the remaining macronutrients (P, K, Ca, Mg) was different in wheat and maize. Maize contained significantly more of all of these elements than wheat, and in some cases, the exposure of plants to Cr led to a higher concentration of K, Mg, and Ca but a decreased P content. Cr can interfere with the uptake and transport of some essential ions, such as iron, sulfate, and phosphate. There is evidence that carriers involved in their uptake are also engaged in the uptake of Cr by plants [33,34]. Our results showed that Cr did not affect phosphorus uptake negatively in wheat, although it did disrupt this process in maize.

The used doses of Cr were relatively low, and the observed disturbances in the levels of microelements were not critical to plant growth and development. The increase in Zn and Mn concentrations in maize may be considered as a positive effect because these metals are important for cell protection against reactive oxygen species generated under heavy metal stress. Zn, Mn, and Cu are essential components of superoxide dismutase that is a key element in cellular antioxidative mechanisms. The studied wheat straw contained considerably more Zn, Mn, and also Cu than maize, and this may have been due to a greater tolerance of wheat to Cr. We found that grain quality was negatively affected by the highest Cr dose; the Cr content was 18% higher than in the control plants, and the S content was 18% lower. However, as expected, the Cr content in the grain was a few times lower than in the straw. In the aerial parts of plants, the leaves usually contain more Cr than other parts like the seeds. In general, Cr concentrations in the shoots of plants are very low and large amounts of the metal remain in the plant roots [35,36].

Concentrations of the microelements Mn, Fe, and Zn increased by 42%, 10%, and 11%, respectively, when compared with the control plants. Our results showed that Cr simultaneously affected the content of many minerals, which could lead to a mineral imbalance in plants. Several studies have demonstrated that Cr could induce changes in the mineral nutrition of plants [37,38,39,40]. However, it is not possible to make a clear generalization, because ultimately a plant’s response is dependent on many factors, such as the Cr concentration, metal speciation, the availability of other nutrients, the presence of some soil additives and the plant genotype.

4.2 Cr and supplementary S fertilization

Adequate mineral fertilization is very important under all stress conditions. As mentioned earlier, under heavy metal stress S is particularly important because of its role in the cellular detoxification of metals. The presented results showed that applying elemental S improved the mineral-nutrient status in plants and ultimately plant growth and grain quality. Under the highest Cr dose, the S supply improved both the dry weight of wheat straw and grain yield when compared with plants grown with Cr alone. As expected, applying S increased the S concentration in both cereals and significantly lowered the Cr concentration.

Wheat simultaneously exposed to Cr and S accumulated significantly less Cr and considerably more S than plants grown without this macroelement. Sulfate originating from elemental S may inhibit the penetration of Cr into the cells and the translocation within the plant. The absorption of Cr involves the use of S transporters and carriers; hence, a sulfate deficiency promotes Cr uptake [1]. Likewise, maize reacted very positively to the presence of elemental S in the soil. Thus, S fertilization provides an effective strategy for reducing the adverse effect of Cr, and the applying S may be particularly recommended in sites contaminated with this metal.

From an agricultural point of view, the total amount of minerals accumulated by plants cultivated in particular pots is very important. Obviously, this parameter depends on the total plant biomass. The total biomass production of maize was approximately four-fold greater than wheat; hence, the levels of the accumulated minerals were several times higher than in wheat. The greatest differences were observed in the case of phosphorus, potassium, calcium, and magnesium, and the smallest, only 20%, for zinc. Wheat was characterized by a high concentration of S in the examined parts of plants and this resulted in a higher total uptake of this element when compared with maize. In our opinion, this property is responsible for greater wheat tolerance to Cr than maize.

Changes in the soil pH of fertilized soil can be impacted by the dose of S or the form applied. Elemental S oxidation is a biological process that depends on many factors [41]. The rate of this process depends on the size, composition, and activity of the microbial population [42]. An increase in the share of sulfates(vi) in the total S in the soil after applying elemental S to the soil may indicate a high oxidation rate of elemental S [9,43]. In soils to which only Cr(vi) was added, a significant increase in soil pH in relation to the control group was found [40].