Maize is an economic crop that is also a candidate for use in phytoremediation in low-to-moderately Cd-contaminated soils, because the plant can accumulate high concentration of Cd in parts that are nonedible to humans while accumulating only a low concentration of Cd in the fruit. Maize cultivars CT38 and HZ were planted in field soils contaminated with Cd and nitrilotriacetic acid (NTA) was used to enhance the phytoextractive effect of the maize. Different organs of the plant were analyzed to identify the Cd sinks in the maize. A distinction was made between leaf sheath tissue and leaf lamina tissue. Cd concentrations decreased in the tissues in the following order: sheath > root > lamina > stem > fruit. The addition of NTA increased the amount of Cd absorbed but left the relative distribution of the metal among the plant organs essentially unchanged. The Cd in the fruit of maize was below the Chinese government’s permitted concentration in coarse cereals. Therefore, this study shows that it is possible to conduct maize phytoremediation of Cd-contaminated soil while, at the same time, harvesting a crop, for subsequent consumption.
Heavy metal contamination of agricultural soils is a worldwide problem [
Maize is a familiar agricultural crop that is widely adapted to regions of China and can be cultivated easily. It has greater dry-mass than many heavy metal hyperaccumulative plants, such as Thlaspi caerulescens and Arabidopsis halleri. The roots and straws of maize can accumulate many kinds of heavy metals, including Cd, from contaminated soil. Fortunately, the seeds and fruits from maize generally accumulate metals at lower concentrations than leaves or roots [
One of the species used in the present study was sweet maize CT38, which was screened from many maize cultivars as an optimal cultivar for heavy metal by pot test [
In order to enhance the efficiency of phytoextraction, amendments are widely used to increase the root uptake of metals through metal solubilization, and substantially increasing the speed of transfer of metals to shoots [
Researchers have found that maize can take up Cd from contaminated soil and that different organs in the plant accumulate Cd at different concentrations [
Sheaths (inside the ellipse) and laminas (outside the ellipse) of maize leaves.
The aim of this study was to investigate the uptake and distribution of Cd in sweet maize enhanced by NTA under the field condition. The objectives were (1) to compare the potential of Cd phytoextraction by two sweet maize in field, (2) to confirm the distribution of Cd in different tissues, and (3) to investigate the impact of NTA on Cd uptake and distribution.
The experiment was conducted on a Cd-contaminated field in Hengli village, belonging to Renhe town, which is near the Guangzhou Baiyun airport in Guangdong, China. The region has a mild and warm climate (22.8°C mean temperature) with an annual average rainfall of 1982.7 mm. The soil type is a latosolic red soil with a loamy and silty texture typical of the region. The Cd contamination originated primarily from the disposal of city waste, including batteries, 30 years ago, a practice originally designed to increase the fertility of the reclaimed soil. While the fertility of the reclaimed soil was improved, Cd contamination was introduced into the soil at the same time. The concentrations of Cd in the ploughed topsoil were approximately 1.4 mg kg−1. Selected soil characteristics and heavy metal contents were shown in Table
Selected physical and chemical characteristics of soil in the field.
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Total Cd (mg·kg−1) |
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Extractable Cd (mg·kg−1) |
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The test area consisted of eight
Test design and treatments code.
Maize genotypes | No NTA | NTA | ||
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Before pollination | After pollination | |||
25 mmol NTA/plant | 100 mmol NTA/plant | 25 mmol NTA/plant | ||
CT38 | C0 | C1 | C2 | C3 |
HZ | H0 | H1 | H2 | H3 |
The sweet maize was harvested 75 days after the sowing date. Three maturing maize plants from each treatment group were chosen at random for analysis. Roots were excavated and washed to remove adhering soil. The shoots were immediately divided into stems, lamina, sheath, and fruits. The samples were packed into plastic bags and immediately transported to the laboratory where they were washed carefully with distilled water to remove any soil, cut into pieces, and then oven-dried for 1 hour at 105°C and for an additional 24 hours at 70°C. All dried materials were ground to 0.5 mm size using a centrifugal mill. Subsamples (1 g) were microwave-digested in 10 mL of HNO3 (65%) and 5 mL of H2O2 (30%). The digests were diluted to 50 mL with high-purity water and filtered. The filtrate was analyzed for Cd by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500A) [
The Cd concentrations from the different treatments were analyzed by ANOVA and a post hoc Bonferroni test, using Excel (version 2007), Origin (version 8.0), and SPSS (version 17) software. Data were assessed for accuracy and precision using a quality control system that included reagent blanks and triplicate samples. The precision and bias of the chemical analysis was less than 10%. Duncan’s multiple range test at
All of the plants, irrespective of genotype or treatment, appeared to have normal growth. The dry-mass biomasses of the different organs in the plants are shown in Table
The biomass weight of different maize organs grown with different treatments (dry mass weight, g). Root dry weight is only collected in the plough layer soil, the stem weight includes the above ground stem and the underground stem. All values are mean ± SD (
Organ | C0 | C1 | C2 | C3 | H0 | H1 | H2 | H3 |
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Stem |
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Lamina |
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Sheath |
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Seeds |
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The biomass ratio of different maize organs with different treatments.
Organ | C0 | C1 | C2 | C3 | H0 | H1 | H2 | H3 |
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Root | 6.3% | 6.5% | 6.3% | 6.4% | 6.2% | 6.4% | 6.2% | 6.4% |
Stem | 58.8% | 58.6% | 58.8% | 59.2% | 58.7% | 58.2% | 58.9% | 58.8% |
Lamina | 15.5% | 15.5% | 15.5% | 15.3% | 15.5% | 15.7% | 15.4% | 15.3% |
Sheath | 7.7% | 7.8% | 7.8% | 7.6% | 7.9% | 8.1% | 7.7% | 7.7% |
Seeds | 11.8% | 11.7% | 11.7% | 11.4% | 11.7% | 11.7% | 11.7% | 11.8% |
The overall evaluation of field scale phytoremediation depends on the total Cd accumulation per plant and total maize biomass in the field. The different maize treatments can result in differences in the uptake of Cd from the soil. The total Cd accumulation per plant under different treatments are shown in Figure
The total Cd accumulation per plant under different treatments. All values are mean ± SD (
According to Figure
Probably, pollination can make maize CT38 have more hormones which can adjust the combination and transportation of dry mass [
As a widely planted coarse cereal crop, maize is important to mankind. The Cd concentration in seeds is of significant concern in food safety because the maize fruit is used for food and oil for human consumption and for forage for livestock.
From Figure
Cd levels in maize HZ dry seeds grown under with different NTA treatments. The broken line is the maximum permissible concentration of Cd in Chinese coarse cereals including the maize seeds (0.1 mg/kg, dry weight) (GB2762-2005) [
In particular, even though CT38 maize with postpollination NTA treatment concentrates Cd significantly more than HZ maize, the CT38 grain remains at a level that is safe for human consumption. Furthermore, maize can be easily planted in a variety of agricultural situations, which leads to lower planting costs and greater economic benefit than other hyper accumulator plants such as Solanum nigrum L and Sedum alfredii. In China, arable land is limited, so that low and even some moderately contaminated lands remain in production because of the great need for food to feed 1.3 billion people. Therefore, the CT38 maize genotype with NTA postpollination application is a strong candidate for phytoextraction of Cd from contaminated soil since the land can continue production even while it is being cleansed.
The distribution of Cd in sweet maize can provide insight on how maize accumulates the metal (see Figure
The Cd distribution fractions in different treatments. The leaf fraction is equal to the sum of the sheath and the lamina fractions.
The percentage of Cd accumulated in the different organs of maize HZ is similar to that of maize CT38. For prepollination application, increasing amounts of NTA lead to increased relative Cd accumulation in the leaves (lamina plus sheath) (cf.
Figure
Figure
The concentrations of accumulated Cd in different parts of the plant grown under different treatments are listed in Table
Cd concentrations in different parts of different treatments (mg/kg). All values are mean ± SD (
Organ | C0 | C1 | C2 | C3 | H0 | H1 | H2 | H3 |
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Root |
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Stem |
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Fruit |
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Lamina |
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Sheath |
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Leaf* |
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The highest Cd concentration in both genotypes occurred in the sheath. The Cd concentration decreased in the following sequence: sheath > root > lamina > stem > fruit (Table
The data in Table
In contrast with no NTA adding treatment, NTA adding in 25 mmol each maize before pollination makes the Cd concentration become higher in root and lamina, but not significantly changing in sheath. However, after pollination, NTA adding in 25 mmol each maize can make the sheath organ accumulate Cd in higher concentration. After pollination, the Maize is in reproductive growth period and mainly increases the fruit organ. Almost all nutrition is transported to the edible parts of the Maize; the Cd can also be transferred from soil to above ground organ following with the nutrition elements, such as Zn and Fe. Seeds and fruits generally accumulate metals at lower concentrations than leaves or roots [
From Table
From Table
From Table
Maize is a strong candidate crop for us in phytoremediation of low- and medium-grade Cd contaminated farmland in China. Compared with maize HZ, the maize CT38 had more significantly greater accumulating capability and more dry mass. The Cd concentration in the various parts decreased in the following order: sheath > root > lamina > stem > fruit, whether NTA was used for fortifying the effect of phytoremediation or not. Both maize genotypes produced fruit with Cd levels that were under the maximally permitted amount by the Chinese standards (GB2762-2005), so the two maize genotypes could be used to treat Cd contamination in contaminated farmland while still producing a consumable crop.
This work was funded by the Project Sponsored by the Guangzhou environmental protection agency (Project no. 2061760), the Program for New Century Excellent Talents in University (no. NCET-12-0199), and the Fundamental Research Funds for the Central Universities (no. 2013ZZ063). The authors thank the China National Analytical Center in Guangzhou for the ICP-MS determination. The authors also thank Donald Barnes in the South China University of Technology for the language editing in the writing work.
The authors declare that there is no conflict of interests.