3. Results
The environment and its interaction with the cultivar significantly affected DM production in the straw (
Table 3). Dry matter production in the straw fluctuated between 7.0 and 14.7 Mg·ha
−1 (
Table 4). The highest value in all the cultivars was attained in La Serena (
p < 0.05) then Chillán and Los Tilos; there were non-significant differences between them (
p > 0.05) and values were 14.7, 9.3, and 7.0 Mg·ha
−1, respectively (
Table 4). Comparing DM production in the straw between cultivars, and as a mean of the environments and Cd rates, higher production was observed in Syngenta with 10.9 Mg·ha
−1, followed by Pioneer and Dekalb, with values of 9.0 and 9.7 Mg·ha
−1, respectively, and no differences between them (
p > 0.05) (
Table 4).
Dry matter production in the roots was also affected by the environment, cultivar, and environment × cultivar interaction (
Table 3). When comparing environments, the highest DM production in roots was found in La Serena (mean 2.0 Mg·ha
−1), significantly higher than in Chillán (mean 1.3 Mg·ha
−1) (
p < 0.05), which, in turn, was significantly higher than in Los Tilos (mean 0.8 Mg·ha
−1) (
p < 0.05). The comparison between cultivars indicated that the highest DM production in roots was obtained in Syngenta (mean 1.5 Mg·ha
−1), significantly higher (
p < 0.05) than in Dekalb and Pioneer (mean 1.2 and 1.1 Mg·ha
−1, respectively), and with non-significant differences between them (
p > 0.05) (
Table 4).
Cadmium uptake in the grain was affected by the environment, Cd rate, environment × Cd rate interaction, and environment × cultivar interaction (
Table 3). When contrasting environments (
Table 5), the highest Cd uptake in the grain was obtained in La Serena with a mean value of 0.45 g ha
−1, which was significantly higher (
p < 0.05) than in Chillán (0.12 g·ha
−1) and Los Tilos (0.08 g·ha
−1), and there were no differences between them (
p > 0.05). The contrast between Cd rates (
Table 5) indicated that the highest Cd uptake was attained with 2 mg·kg
−1 CdCl
2 (0.25 g·ha
−1), which was significantly higher (
p < 0.05) than for 1 mg·kg
−1 CdCl
2 (0.19 g·ha
−1); the latter was significantly higher (
p < 0.05) than the control (0.06 g·ha
−1) (
Table 5). The contrast between cultivars (
Table 5) indicated that the highest Cd uptake in the grain was reached in Pioneer (0.19 g·ha
−1), then in Syngenta (0.16 g·ha
−1) and Dekalb (0.14 g·ha
−1) with non-significant differences between them (
p > 0.05) (
Table 5). The environment × Cd rate and the environment × cultivar interactions exhibited greater variability associated with the environment and revealed high values obtained in La Serena (data not shown).
Cadmium uptake in the straw was only affected by the Cd rate (
Table 5). When contrasting the CdCl
2 rates (
Table 5), the highest Cd uptake in the straw was attained in Los Tilos with a mean of 23.9 g·ha
−1 (
p < 0.05), while La Serena and Chillán had 18.8 and 18.5 g·ha
−1, respectively, and with a non-significant difference between them (
p > 0.05).
As for Cd uptake in the roots, there was an effect of the environment and Cd rate with an interaction between both sources of variation and between environment × cultivar (
Table 5). The contrast between environments (
Table 5) indicated that the highest Cd uptake in the roots was obtained in La Serena with a mean of 4.5 g·ha
−1, which was significantly higher than in Chillán and Los Tilos where uptakes were 0.6 and 0.2 g·ha
−1, respectively; there was non-significant difference between them (
p > 0.05). When contrasting the Cd rate (
Table 5), higher Cd uptake in the roots was observed when using 2 and 1 mg·kg
−1 CdCl
2; however, they were both similar (
p > 0.05) and values were 2.1 and 1.5 g·ha
−1, respectively. They significantly surpassed the control (
p < 0.05), which had an uptake of 0.6 g·ha
−1.
Whole plant Cd uptake was only affected by the Cd rate (
Table 3;
Figure 1). The highest whole plant Cd uptake was attained with 2 mg·kg
−1 CdCl
2 (34.4 g·ha
−1), which was significantly higher (
p < 0.05) than with 1 mg·kg
−1 CdCl
2 (17.7 g·ha
−1); the latter was significantly higher than in the control without Cd (2.9 g·ha
−1) (
p < 0.05) (
Figure 1b). Although the cultivars did not show any differences in whole plant Cd extraction (
Table 3,
Figure 1c), these values were 21.5, 17.1, and 16.4 g·ha
−1 in Dekalb, Pioneer, and Syngenta, respectively. Cadmium distribution in the maize plant (
Table 3;
Figure 1) revealed that the environment was affected by the distribution to the grain, straw, and roots. There were no interactions between the sources of variation, with the exception of Cd distribution in the grain, which was affected by the environment × cultivar interaction (
Table 3).
The highest percentage of Cd distribution to the grain was attained in La Serena (3.0%) (
p < 0.05), exhibiting significant differences between Chillán (0.8%) and Los Tilos (0.7%); however, there were non-significant differences between them (
p > 0.05) (
Figure 1a). The values for Cd distribution to the grain were 1.2%, 0.9%, and 0.9% for Cd rates 2, 1, and 0 mg·kg
−1 CdCl
2, respectively, with non-significant differences between them (
p > 0.05) (
Figure 1b). When comparing cultivars (
Figure 1c), there was a higher Cd distribution to the grain in Pioneer (1.4%), while Syngenta and Dekalb obtained 0.9% and 0.8%, respectively, with non-significant differences between the three cultivars (
p > 0.05).
Comparing environments for Cd distribution in the straw indicated that Los Tilos and Chillán did not show any significant differences one from the other (
p > 0.05) and that their values were significantly higher than in La Serena (
p < 0.05). The values for Cd distribution to the straw were 97.3%, 94.9%, and 72.5% in Los Tilos, Chillán, and La Serena, respectively (
Figure 1a). The Cd rate and its effect on Cd distribution to the straw differed from that described for Cd distribution to the grain (
Figure 1a,b); non-significant differences were detected (
p > 0.05) with values of 90.8%, 90.2%, and 88.3% for rates of 2, 1, and 0 mg·kg
−1 CdCl
2, respectively (
Figure 1b). Non-significant differences were observed between the Syngenta (90.4%), Pioneer (90.0%), and Dekalb (88.9%) cultivars (
Figure 1c) for Cd distribution to the straw.
Cadmium distribution to the roots when comparing environments (
Figure 1a) revealed a higher value in La Serena (24.8%) (
p < 0.05) than in the other environments. In turn, there were non-significant differences between Chillán and Los Tilos (
p > 0.05) with values of 4.3% and 2.0%, respectively. As for Cd rates, values for distribution to the roots were 10.5%, 8.9%, and 8.3% for rates of 0, 1, and 2 mg·kg
−1 CdCl
2, respectively (
Figure 1b); only the control was significantly higher than the other treatments (
p < 0.05) with no differences between rates of 1 and 2 mg·kg
−1 CdCl
2 (
p > 0.05). The comparison of cultivars (
Figure 1c) indicated values for distribution to the roots of 10.3%, 8.7%, and 8.6% in Dekalb, Syngenta, and Pioneer, respectively (
Figure 1c), with non-significant differences between them (
p > 0.05). In addition, when comparing Cd distribution between the three maize plant structures, the straw usually concentrated the highest accumulation of the metal (
p < 0.05), independently of the environment, Cd rate, and evaluated cultivar (
Figure 1). The percentage accumulation of Cd in the grain was lower than in the root (
p < 0.05) for the different Cd rates and in most of the environments and evaluated cultivars (
Figure 1).
Table 6 displays the translocation factors (TF) used to evaluate plant capacity to translocate Cd from the roots to the aerial part of the plant (straw + grain), which are the mean values (
Table 6;
n = 27) of the different: (a) environments; (b) Cd rates; and (c) maize cultivars. It is noted that there are significant differences in TF between environments (
p < 0.05), TF in Los Tilos is 20 times higher than in Chillan and 2.6 times higher than in La Serena with values of 11.82, 4.63, and 0.57, respectively, for the three environments. Significant differences for TF between the different rates (
p < 0.05) (
Table 6) were observed for the 0, 1, and 2 mg·kg
−1 CdCl
2 treatments with values of 0.73, 1.96, and 2.84, respectively. The variations of TF, in accordance with the different Syngenta, Pioneer, and Dekalb cultivars (
Table 6), did not indicate any significant differences between treatments (
p > 0.05) with values of 2.31, 2.23, and 2.21, respectively.
The BCF, or enrichment factor, is used to quantify plant capacity to accumulate Cd in the root with respect to Cd concentration in the soil.
Table 6 indicates that BCF shows significant differences (
p < 0.05) between environments and highlights the fact that La Serena has a greater capacity to accumulate Cd in the plant root than Chillán or Los Tilos, values are 1.2, 0.7, and 0.2, respectively. When comparing Cd rates, BCF showed non-significant differences among 0, 1, and 2 mg·kg
−1 CdCl
2 treatments (
p > 0.05), and values were 0.66, 0.82, and 0.75, respectively. The same trend was observed when evaluating BCF for the different cultivars (
Table 6) where non-significant differences were detected (
p > 0.05) between Syngenta, Pioneer, and Dekalb with values of 0.68, 0.78, and 0.82, respectively.
On the other hand, BAF allows quantifying the capacity to accumulate Cd in the aerial part of the plant (straw + grain), with respect to Cd concentration in the soil.
Table 6 specifies significant differences between environments for BAF (
p < 0.05); a higher accumulation rate is observed in the Chillán environment, which is followed by Los Tilos (
p < 0.05) and a lower bioaccumulation in La Serena (
p < 0.05) with values of 2.54, 1.71, and 0.67, respectively. The same trend was observed when studying BAF in accordance with the Cd rates (
Table 6); there were significant differences between 0, 1, and 2 mg·kg
−1 CdCl
2 (
p < 0.05) and values were 1.7, 1.33, and 0.48, respectively. It should be noted that, when evaluating BAF for the different cultivars (
Table 6), there were non-significant differences between them (
p > 0.05) and the values were 1.35, 1.44, and 1.47 for Syngenta, Pioneer, and Dekalb, respectively.
Root TI (
Table 6) exhibited non-significant differences between the environments in La Serena, Los Tilos, and Chillán and TI values were 0.15, 0.10, and 0.16, respectively (
Table 6) (
p > 0.05). Differences were neither observed for the different CdCl
2 rates, and values were 0.17, 0.17, and 0.17 for 0, 1, and 2 g·kg
−1 CdCl
2, respectively (
Table 6) (
p > 0.05), nor for the three cultivars with values of 0.20, 0.14, and 0.16 in Syngenta, Pioneer, and Dekalb, respectively (
Table 6) (
p > 0.05). However, when evaluating TI in the grain, there were significant differences for both the environments and Cd rates (
p < 0.05). When observing the environments, Los Tilos had the lowest value (
p < 0.05) and was surpassed by Chillán, whereas La Serena had the highest value (
p < 0.05). Values were 0.71, 1.29, and 1.87 in Los Tilos, Chillán, and La Serena, respectively (
Table 6). Non-significant differences were detected between TI at the different Cd rates (
p > 0.05) and values were 1.33, 1.24, and 1.30 for 0, 1, and 2 mg·kg
−1 CdCl
2, respectively (
Table 6). The same trend was observed for TI in the grain for the different cultivars, non-significant differences were observed (
p > 0.05), and values were 1.32, 1.31, and 1.24 for Syngenta, Pioneer, and Dekalb, respectively (
Table 6). Finally, TI of the straw exhibited the same trend as in the grain (
Table 6) with significant differences only between environments (
p < 0.05) where values were 1.78, 1.13, and 0.85 for La Serena, Chillán, and Los Tilos, respectively (
Table 6). For the different CdCl
2 rates (0, 1, and 2 mg·kg
−1), TI values in the straw were 1.26, 1.25, and 1.25, respectively, and with non-significant differences (
p > 0.05) (
Table 6). The same statistical effect was obtained when comparing the different cultivars (
p > 0.05), which had values of 1.33, 1.15, and 1.28 for Syngenta, Pioneer, and Dekalb, respectively (
Table 6).
4. Discussion
The production of DM in the grain, straw, and root was not affected by the degree of anthropogenic contamination of the soil with Cd (1 and 2 mg·kg
−1 CdCl
2) or the different evaluated cultivars (
Table 4); this is corroborated by TI (TI > 1) (
Table 6) and concurs with Zhang et al. [
34]. Other authors point out that soils contaminated with Cd exhibit decreased crop development that can fluctuate between 55% and 80% [
35]; however, soil Cd contamination levels in that study were higher than those generated in the present study. Given that our results were similar to those obtained by other authors under similar Cd concentration conditions applied to the soil [
13,
36], it can be suggested that Cd concentrations achieved in the soil by applying CdCl
2 are below the danger threshold of 3.5 mg·kg
−1 [
13]. However, the environment significantly affected DM production in Los Tilos (
Table 4) for DM in the straw, grain, and root (
Table 4). The lack of heat accumulation during the development stage and higher than optimal soil pH could be limiting factors for DM production, which concurs with other authors who point out a significant correlation between DM production and degree-day accumulation [
36]. At the same time, the environment × cultivar interaction was affected by DM production, which corroborates what was previously mentioned and the fact that the response to the environmental factor is differentiating in accordance with the thermal and soil requirements of each cultivar [
13,
37].
Although initial total soil Cd was higher in La Serena compared with Los Tilos and Chillán (
Table 1), this order was not maintained at the end of crop development (
Table 2); this was probably generated by crop Cd absorption (
Table 1 and
Table 5) [
29]. For total plant Cd extraction, non-significant differences were observed between environments (
Table 5), which concur with Putwattana et al. [
21]. However, mean total Cd extraction between different environments, Cd rates, and maize cultivars (22.4, 22.4, and 22.4, respectively) (
Table 5) is higher than the mean among cultivars with phytoextraction potential reported by Slycken et al. [
38] (18.5 g Cd·ha
−1). These extraction levels demonstrate that the maize crop could be used for Cd phytoextraction [
20,
38] given that Cd extractions up to 42 g·ha
−1 were reported in our experiment. On the other hand, Cd distribution in the different plant parts was affected by the environment with higher Cd absorption in the grain in La Serena (
Table 5); this was 200% more than in Los Tilos, which, in turn, was 50% higher than in Chillán. This is mainly because DM production was higher in La Serena, which was pointed out by Trejo et al. [
29]. These results concur with those obtained by Sarwar et al. [
39], who also stated that high soil Zn concentrations, such as those found in the present study (
Table 1), form Zn-phytochelatin complexes in the cell cytoplasm to substitute the Cd-phytochelatin complexes. This leaves Cd free inside the root cells and results in increased Cd translocation in the different plant parts, which was observed in the present study. However, low Cd extraction in the maize grain (
Table 5) showed values within the range cited by several authors [
13,
40] with a mean Cd value of 0.18 g·ha
−1 (mean of 27 values that considered three environments, three Cd rates, and three cultivars). This is an indicator of normal translocation for this species. Wahsha et al. [
41] point out that Cd concentration in the maize grain was lower than the detection limit of 0.002 mg·kg
−1, leading to low extraction levels for the maize grain, such as that observed in the present study. This could be due to several factors, for example, absorption and translocation limitations generated in the root [
13], which suggests that maize plants have more efficient defense mechanisms than other crops to regulate Cd toxicity, including the accumulation of this metal at the root level [
22]. Secondly, the available soil Zn concentration acts as an antagonist for plant Cd absorption [
37]. Finally, the low Cd concentration could be attributable to high agronomic efficiency of nutrient use (kg of DM produced per kg of applied nutrient) obtained in the present experiment (
Table 5), and which usually implies a Cd dilution effect [
24].
It can also be observed in
Table 5 that, for the environment, Cd rate, and cultivar, total Cd extraction in the straw is always significantly higher than in the roots or grain. These results concur with those reported by Liang et al. [
42] and Stritsis et al. [
43], who point out that Cd concentrations in maize straw are higher than in the rest of the plant. The same trend was observed in other crops, such as lettuce (
Lactuca sativa) [
44] and bread wheat (
Triticum aestivum) straw [
1]. Zhang et al. [
45] explain these results by indicating that high S and Zn concentrations in the soil profile, also observed in the present study (
Table 1), facilitate Cd translocation from the root to the aerial part of the plant. As previously mentioned, this higher Cd concentration in the straw indicates a higher translocation rate of this metal to the aerial part. Liang et al. [
42] reported that it could be influenced by a decrease in sap flow that generates an increase of Cd concentration in the xylem when the crop is exposed to high Cd concentrations. It should be noted that the main factor for the higher translocation rate to the aerial part of the plant in the Los Tilos environment could be the high Ca levels in the soil profile associated with basic pH levels (
Table 1). Sarwar et al. [
39] pointed out that this generates increased Cd availability because it was substituted by Ca in the soil particles, thus increasing absorption by the plant and its translocation. Given the fact that high Cd extractions in the maize plant straw expose animals to this PTE and ultimately contaminate the food chain, which contributes in evaluating Cd levels in these maize cultivars destined for animal feed. Cadmium absorption by the maize plant is associated with the abovementioned factors and is benefited by high organic matter (OM) levels [
12]; these levels were observed in the different evaluated environments in the present study and reached values close to 63 g·kg
−1 in Chillán.
As expected, the Cd concentration in the aerial part of the plant was proportional to the Cd concentration in the soil (
Figure 1) where significant differences were noted in Cd extraction by the plant between the control and the 1 and 2 mg·kg
−1 rates with more than 50% and 100% (
Table 5), respectively. This concurs with the literature [
46,
47], which states that when the CdCl
2 rate applied to the soil is higher, absorption rates of this PTE are higher. The same trend is observed when evaluating extraction levels in the same plant parts (grain, straw, and root) (
Table 5). With respect to the studied maize cultivars, no effect on plant total Cd extraction was recorded and no differences existed between cultivars in the same plant part (
Table 5); these results concur with Slycken et al. [
38], who conducted a study with maize and pointed out that the different evaluated cultivars did not show any significant differences in Cd absorption.
Plant capacity to absorb soil contaminants can be expressed as BCF, which indicates the relationship between the metal content in the plant tissue (root) and the soil [
30,
32,
48]. Most of the BCF results in the present study fluctuated between 0.2 and 0.82, which concurs with results obtained by Usman and Mohamed [
31], who pointed out that BCF values fluctuated between 0.38 and 0.9. In the different environment treatments, Cd rates, and cultivars, only the environment in La Serena reached values above the mentioned range (1.2) (
Table 6). Different authors mention that BCF values >1 are high; this indicates that maize in the La Serena environment, independently of the cultivars and the degree of anthropogenic soil contamination, behaves as a plant with high Cd bioaccumulation efficiency at the root level [
48]. On the other hand, low root BCF generally found in the de Los Tilos and Chillán environments can be explained by the maize plant’s capacity to prevent Cd absorption and probably for the low soil Cd bioavailability in accordance with previously-analyzed factors [
22,
37]. Furthermore, the different evaluated maize cultivars can be classified as excluding Cd because all the BCF values are <1 [
31,
48].
However, phytoextraction capacity has been expressed as TF and is defined as the relationship between the Cd concentration in the aerial part and the Cd concentration in the roots [
36]. Results for TF obtained in the present study (
Table 6) do not coincide with TF values recorded by other authors, who reveal that TF for different fertilization treatments, rotations, and degrees of Cd contamination were <0.5 [
21,
36]. Mean TF in the present study (3.26) is higher than the results reported by Liu et al. [
36], which could be mainly influenced by soil pH in that study (pH between 8.31 and 9.06) and generate decreased Cd availability to the soil solution; this differs from the pH in the present study, which fluctuated between 5.74 and 8.25 (
Table 1). The observed TF values in the present study (TF > 1) (
Table 6) concur with results mentioned by other authors, who indicate TF values of 2.6 [
33]. These results could classify these cultivars as plants with high Cd translocation efficiency from the roots to the aerial part [
32]. The TF values obtained in Los Tilos (TF = 11.82) and BAF > 1 (
Table 6) classify this environment as having the highest capacity to contaminate the food chain [
7]. Therefore, more studies need to be conducted regarding PTE absorption and translocation to the aerial part of the crop in this environment, mainly cultivars used as a food sources for animals destined for human consumption. According to values reported by Usman et al. [
32], translocation factors for cultivars, Cd rates, and environments in the present study exceeded the efficient plant classification limit of Cd translocation from the root to the aerial part, with the exception of the La Serena environment and the 0 mg·kg
−1 Cd treatment rate where TF < 1. The three maize cultivars (Syngenta, Pioneer, and Dekalb) exhibited high risks of contaminating the food chain with Cd because of the high TF value (TF > 2) [
32]. Based on these results, new maize cultivars should be made available or production management strategies be evaluated for this crop to reduce translocation rates of this metal to the aerial part of the plant, as well as more research to decrease both Cd absorption and accumulation in different maize plant tissues destined for animal consumption.