arsenic toxicity and accumulation in radish as affected by arsenic chemical speciation

J. ENVIRON. SCI. HEALTH, B34(4), 661-679 (1999)

ARSENIC TOXICITY AND ACCUMULATION IN RADISH ASAFFECTED BY ARSENIC CHEMICAL SPECIATION

Key Words: Arsenic absorption, arsenic adsorption, arsenic speciation, Rhapanussativus, food contamination

A.A. Carbonell-Barrachina*, F. Burló, E. López and F. Martínez-Sánchez

Departamento de Tecnología Agro-Alimentaria, División Tecnología deAlimentos, Universidad Miguel Hernández, Carretera de Beniel, km 3.2,

03312 Orihuela, Alicante, España/Spain

ABSTRACT

Arsenic (As) uptake by Rhapanus sativus L. (radish), cv. Nueva Orleans,

growing in soil-less culture conditions was studied in relation to the chemical form

and concentration of As. A 4 × 3 factorial experiment was conducted with

treatments consisting of four As chemical forms [As(III), As(V), MMAA, DMAA]

and three As concentrations (1.0, 2.0, and 5.0 mg As L-1). None of the As

treatments were clearly phytotoxic to this radish cultivar. Arsenic phytoavailability

was primarily determined by the As chemical form present in the nutrient solution

and followed the trend DMAA As(V) As(III) << MMAA. Root and shoot As

* Corresponding author, e-mail: [email protected]

661

Copyright © 1999 by Marcel Dekker, Inc. www.dekker.com

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662 CARBONELL-BARRACHINA ET AL.

concentrations significantly increased with increasing As application rates.

Monomethyl arsonic acid treatments caused the highest As accumulation in both

roots and shoots, and this organic arsenical showed a higher uptake rate than the

other As compounds. Inner root As concentrations were, in general, within the

normal range for As contents in food crops but root skin As levels were close or

above the maximum threshold set for As content in edible fruit, crops and

vegetables. The statement that toxicity limits plant As uptake to safe levels was not

confirmed in our study. If radish plants are exposed to a large pulse of As, as

growth on contaminated nutrient solutions, they may accumulate residues which

are unacceptable for animal and human consumption without exhibiting symptoms

of phytotoxicity.

INTRODUCTION

Arsenic (As) has achieved great notoriety because of the toxic properties of

a number of its compounds. Fortunately there are great differences in the toxicity

of different compounds, and the species that are most commonly found in soils are

not the most toxic (O'Neill, 1995). Arsenic differs from many of the common

heavy metals in that the majority of the órgano As compounds are less toxic than

inorganic As compounds after foliar application (Sohrin et al., 1997). However,

residues from the use of organic-based As pesticides and herbicides are more toxic

than inorganic sources (Carbonell-Barrachina et al., 1998).

The major present-day uses of As compounds are as pesticides, wood

preservatives, and as growth promoters for poultry and pigs (O'Neill, 1995). The

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ARSENIC TOXICITY AND ACCUMULATION IN RADISH 663

toxicity of As to biological systems has made it a useful constituent of insecticides,

herbicides, fungicides, and desiccants (Marin, 1995). However, there has been

concern about the build-up of As residues in soils and lake sediments which has

occurred after the use of large quantities of inorganic As compounds which

phytotoxic effects can continue long after application has ceased (O'Neill, 1995).

In Spain, soils where inorganic arsenicals were widely applied are now frequently

used for vegetables growing (Carbonell-Barrachina et al., 1997), including radish.

The bioavailability and uptake of As is dependent on the source, form and

valency of the element (N.A.S., 1977), soil pH, redox and drainage conditions

(Marin et al., 1993), as well as the type and amount of organic matter present

(Mitchell and Barr, 1995). The uptake of As by many terrestrial plants is generally

low so that, even on relatively high As soils, plants do not usually contain

dangerous levels of As; therefore the uptake of As by animals and humans beings

from this source is also low (O'Neill, 1995). Arsenic uptake is also dependent on

plant species, seasonal effects and a number of physical and chemical factors

operating at the soil-root and root-shoot interfaces (Thornton, 1979). Unlike some

marine and fresh water organisms where very high concentrations were found

(similar levels to those in the sediments have been found in some macrophytes), the

levels in terrestrial plants remain well below the level in the soil and the statutory

limit for As in food destined for human consumption, 1 mg As kg"1 fresh weight,

f.w., (Mitchell and Barr, 1995). This suggests that certain plants act as a barrier

between man and toxic concentrations of As in the underlying geological material.

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In general roots contain higher As levels than stems, leaves or fruit (Lepp,

1981). Translocation of As to the shoots is stopped at the soil-root and root-shoot

interfaces because As is not an essential plant trace nutrient. Although these

interfaces play an important role in controlling the extent of element uptake by

plants, they can be overcome by environmental pressures (Mitchell and Barr,

1995).

The phytotoxic effects of As are indicative of a sudden decrease in water

mobility, as suggested by root plasmolysis and discoloration followed by necrosis

of leaf tips and margins (Machlis, 1941). The sensitivity of a plant to As appears to

be determined by the plant's ability to either not absorb or not translocate the As to

sensitive sites (Mitchell and Barr, 1995). Radish and turnip are amongst the less

sensitive plants to As toxicity (Wauchope, 1983). The degree of uptake and

concentration of As species absorbed from nutrient solutions by turnips plants was

arsenate > arsenite > monomethyl arsonic acid (MMAA) = dimethyl arsinic acid

(DMAA), with toxicity being proportional to the total concentration of As in the

nutrient solution (Guijarro, 1998).

Adherence of contaminated soil particles is a particular problem for root

vegetables, where inadequate washing can lead to the direct ingestion of As

compounds, even if levels in the vegetables themselves are low.

Arsenic can be toxic to radish plants and may accumulate in this plant

species, thereby entering the human food chain. A greenhouse experiment was

designed that allowed the study of As absorption and phytotoxicity to radish plants

in relation to its chemical form. The main objective of this study was to determine

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whether As could accumulate in the edible-part of radish plants in concentrations

potentially dangerous for human health. The effect of different chemical forms of

As (arsenite, arsenate, MMAA, and DMAA) and three As concentrations (1.0, 2.0,

and 5.0 mg As L"1) on radish plant growth and the distribution of the absorbed As

among root skin, inner root, and shoots is also reported here.

MATERIALS AND METHODS

Radish plants {Rhapanus sativus L.), cv. Nueva Orleans, were grown in

soil-less culture containing different chemical forms and concentrations of As. The

experiment was carried out under greenhouse conditions, using siliceous sand as

inert media for cultivation. The factorial treatments (4 x 3, As species x As

concentrations) were applied in three replicates of a complete randomized design.

The treatments consisted of four chemical forms of As (AsO2~, AsOt, MMAA,

and DMAA) with three As concentrations (1.0, 2.0, and 5.0 mg L'1). Controls with

no added As were also included. The chemical forms of As were added as their

sodium salts. These As levels were selected taking into account that in a previous

study, Carbonell-Barrachina et al. (1997), an arsenite application rate of 2 mg

As(III) L'1 was not phytotoxic to tomato plants, cv. Marmande. Tomato and radish

are both reported as tolerant plant species to As pollution (Wauchope, 1983).

The amount and form of As in solution were analyzed regularly using a

hydride generation atomic absorption technique (Masscheleyn et al., 1991) to

verify that the chemical form of the added As did not change over time. Arsenic

species were found to be stable with respect to oxidation/reduction and

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methylation/demethylation reactions for a period of 4 days. Thus, the nutrient

solution was replaced every 4 days in order to maintain the desired treatments.

Seeds were germinated in a commercial preparation of peat moss and

vermiculite. Fourteen days after germination, uniform seedlings were selected.

Organic residues were washed from the roots with distilled-deionized water and

seedlings were transferred to hydroponic pots containing 0.5 L of nutrient solution.

A single pot, representing a specific As form — As rate treatment, contained one

seedling. The basal nutrient solution (Feigin et al., 1987) contained (in mg L"1):

126 N; 46.5 P; 136.9 K; 31.6 Mg; 160.5 Ca; 2.0 Fe; 0.8 Mn; 0.3 Mo; 0.5 B; and

0.2 of Zn and Cu. The selection of the P level to be used in this study (46.5 mg P

L"1) was based in two factors: 1) this concentration can be considered as typical of

many fertilized soils where radish is grown (Junta de Extremadura, 1992), and, 2)

it would allow plants to live for a longer period of time than lower P

concentrations (Meharg and Macnair, 1991), though we were aware that a

competition between phosphate and As, mainly as arsenate, could happen and that

the results could be difficult to interpret.

Arsenic treatments started after 14 days of acclimatization. Plants were

grown for 31 days, and then harvested (plants were 59-day old after germination).

Plants were washed with tap water, P-free detergent, and rinsed several times with

distilled-deionized water. Roots and shoots were separated and fresh and dry (60-

70 °C for 72 h) matter productions were determined. Roots were divided in two

parts: root skin and inner root, to distinguish between the adsorption and

absorption processes. Dried samples were ground in a stainless-steel mill to obtain

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a homogeneous sample. Plant tissue samples were dry-ashed by applying the

methodology of dry mineralization developed by Ybáñez et al. (1992). Digested

samples were filtered and diluted with distilled-deionized water to 25 mL. Arsenic

in the extracts was determined by hydride generation atomic absorption

spectrometry (HGAAS) with a Perkin Elmer (PE) spectrometer Mod. 2100, with a

hydride generator (PE) MHS-10. Acid blanks were analyzed in order to assess

possible As contamination.

Statistical analyses were performed using the PROC ANOVA and PROC

GLM procedures available in SAS (SAS, 1987).

RESULTS AND DISCUSSION

Plant Growth

Radish plants growth (as represented by roots and shoots dry matter

production) was significantly affected by As treatments. Our results demonstrated

that the As chemical form was more important than the As level in solution in

determining the phytotoxic effect of As in this radish cultivar (Table 1). In fact, the

As concentration in the nutrient solution had not statistically significant effects on

plant growth. Arsenic form has been reported as the crucial factor determining As

phytotoxicity in several plants: rice (Marin et al., 1992), Spartina patens and

Spartina alternißora (Carbonell-Barrachina et al., 1998), and tomato (Guijarro,

1998).

Plants treated with arsenate had the highest total root dry matter

production followed by plants treated with MMAA and As(III), though differences

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TABLE 1

Effects of As Treatments on Dry Matter Production of Radish Plants

ControlAs(III)As(HI)As(m)As(V)As(V)As(V)MMAAMMAAMMAADMAADMAADMAA

Asíate

(mgL-1)

1.02.05.01.02.05.01.02.05.01.02.05.0

Source of variation

As formAs rateAs form x As rate

Source of variation

Arsenic formAs(ni)As(V)MMAADMAA

Arsenic rate1.0 mgL"1

2.0 mgL1

5.0 mg L*1

Root Skin

0.52±0.34f

0.16 ±0.020.28 ± 0.060.32 ±0.090.23 ±0.040.41 ±0.100.37 ±0.060.31 ±0.030.22 ± 0.020.17 ±0.060.22 ± 0.020.20 ± 0.030.13 ±0.03

Dry Matter Production

Inner Root Roots

(RPot1)

0.89 ±0.140.85 ±0.161.77 ±0.631.77 ±0.521.00 ±0.221.98 ±0.541.97 ±0.402.00 ±0.121.47 ±0.311.08 ±0.591.28 ±0.241.70 ±0.160.76 ±0.21

ANOVA F TEST

Root Skin

*t

NSNS

Inner Root

NSNSNS

1.41 ±0.481.01 ±0.182.05 ± 0.682.09 ±0.611.23 ±0.252.39 ±0.642.34 ±0.462.31 ±0.121.69 ±0.331.25 ± 0.651.50 ±0.261.90±0.130.89 ±0.24

Roots

NSNSNS

DUNCAN MULTIPLE RANGE TEST

Root Skin

0.25 ab¥

0.34 a0.23 b0.18 b

0.23 a0.28 a0.25 a

Inner Root

1.46 a1.65 a1.52 a1.25 a

1.28 a1.73 a1.39 a

Roots

1.71a1.99 a1.75 a1.43 a

1.51a2.01a1.64 a

Shoots

0.91 ±0.200.83 ±0.141.59 ±0.331.24 ±0.360.79 ±0.261.33 ±0.271.65 ±0.500.98 ± 0.010.99 ±0.320.73 ±0.291.04 ±0.231.34 ±0.121.70 ±0.39

Shoots

NSNSNS

Shoots

1.22 a1.26 a0.90 a1.36 a

0.91a1.31a1.33 a

t Standard error. JNS = non significant F ratio (p<ß.05), *, **, and •** significant atp < 0.05,0.01, and 0.001, respectively, ^"reatment means from the ANOVA test. Values followed by thesame letter, within the same source of variation, are not significantly different (p<0.05), DuncanMultiple Range Test.

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ARSENIC TOXICrrY AND ACCUMULATION IN RADISH 669

among treatments were not statistically significant; treatments with DMAA caused

similar root dry weights than those of controls. The influence of As concentration

on total root dry production was not significant.

Root skin and inner root dry weights followed a similar pattern, against As,

to that already described for total root dry weight, with DMAA being the most

phytotoxic As form to this plant species.

Arsenic has not been shown to be an essential plant nutrient, although it is

essential for animal metabolism (Lepp, 1981). Stimulation of growth by As

additions (mainly as arsenate) has been, however, reported to increase growth of

maize (Woolson et al. 1971), peas, wheat, potatoes (Jacobs et al., 1970), and rye,

soybean, and cotton (Cooper et al., 1932). It is possible that arsenate additions

may displace phosphate from the soil in certain situations with a resultant increase

in plant-P availability (Jacobs et al., 1970); thereby affecting growth. However, this

speculation cannot be responsible for the increased radish plants dry matter

production of our study because soil-free systems were used.

Marin et al. (1992) also reported an increase in the growth of rice in

hydroponic studies containing DMAA at rates of 0.05 and 0.2 mg As L"1.

Carbonell-Barrachina et al. (1995) reported an increase in tomato plant growth in

nutrient solution containing arsenite at a level of 2.0 mg As L"1 at the first stages of

development. More recently, Carbonell-Barrachina et al. (1998) observed that

applications of arsenate at rates of 0.2 and 0.8 mg L'1 (hydroponic culture)

significantly increased root, shoot and total dry matter production in Sp.

alterniflora and Sp. patens compared to control plants.

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The reason for the observed positive growth response is unclear, but may

be linked with phosphorus (P) nutrition. Phosphate and arsenate are taken into

plant roots by a common carrier; however, both high and low affinity

phosphate/arsenate plasma membrane carriers have a much higher affinity for

phosphate than arsenate (Meharg and Macnair, 1990). Conversely, Cox (1995),

who studied the effect of different arsenicals on the nutrition of canola plant,

postulated that since As can substitute for P in plant, but is unable to carry out P's

role in energy transfer, the plant reacts as if there is a P deficiency. Thus, as plant

As increases, the plant reacts by increasing P uptake. In this study, there was a

significant interaction between radish plant growth and plant-P, showing that the

growth of this cultivar of radish was dependent on plant-P status. In Figure 1,

radish root skin dry weight has been depicted versus P concentration in root skin

as an example of the "negative" relationship between P and plant growth, implying

that Cox's hypothesis was valid for this plant species.

In this particular experiment, roots- and shoots-P concentrations were

significantly influenced by both As chemical form and As concentration in the

nutrient solution (Table 2). The root skin of plants treated with MMAA presented

significant higher P levels than those of plants treated with inorganic arsenicals and

controls. On the other hand, the highest P concentrations were found in the inner

roots and shoots of plants treated with As(HI) and the lowest P accumulations

corresponded to the organic treatments. While the P concentration in root skin and

shoots decreased with increasing As levels in solution, the inner root P increased.

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ARSENIC TOXICUY AND ACCUMULATION IN RADISH 671

oQ .

S

ü>•535oc

00

ce«

0,5

0,4

0,3

0,2

0,1R =

u0

2- 0.7428; R = 0.5518 ***

2 4 6

Root Skin Phosphorus (g/kg)

• ^ ^

8 1

FIGURE 1Radish root skin dry weight as a function of root skin phosphorus concentration(*** significant at P < 0.001).

Tissue Arsenic Concentration

The total amount of As taken up by radish plants (roots + shoots) followed

the trend: DMAA < As(V) = As(III) « MMAA, with increasing As levels in the

nutrient solution resulting in a higher As uptake for all As species and

concentrations (data not shown; As uptake = As concentration x dry weight). The

highest residues of As are found in plant roots, with intermediate values in the

vegetative top growth, and edible seeds and fruits containing the lowest levels of

As (Lepp, 1981). Upon As absorption, radish plants accumulated As mainly in the

root system; 66.5% of all absorbed As remained in the root system and only 33.5%

reached the shoots.

Berry (1986) suggested three strategies of plant tolerance to metals:

avoidance (limited uptake by roots or limited transport to shoots), detoxification

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TABLE 2

Effects of As Treatments on P Concentration of Radish Plants

ControlAs(III)Asflll)AsflII)As(V)As(V)As(V)MMAAMMAAMMAADMAADMAADMAA

As rate

(mgL-')

1.02.05.01.02.05.01.02.05.01.02.05.0

Source of variation


Source of variation

Arsenic formAs(III)As(V)MMAADMAA

Arsenic rate1.0 mg I/1

2.0 mgL 1

5.0 mgl / 1

Root Skin

8.34±0.01f

5.38 ±0.458.90 ±0.186.11 ±0.035.40 ±0.015.26 ± 0.074.90 ± 0.2510.65 ± 0.266.84 ±0.228.21 ±0.308.52 ± 0.498.60 ± 0.23

Phosphorus

Inner Root

fe kg1)

6.48 ±0.137.38 ± 0.099.15 ±0.099.75 ±0.025.45 ±0.157.19 ±0.368.61 ±0.055.40 ±0.105.54 ±0.015.91 ±0.145.75 ±0.095.44 ±0.14

ANOVA F TEST

Root Skin

***t• * *

***

Inner Root

***

***• * *


Root Skin

6.80 c*5.19 d8.57 a7.94 b

7.49 a7.40 a6.48 b

Inner Root

8.76 a7.08 b5.61c5.52 c

5.99 c6.83 b7.41a

Shoots

6.82 ±0.0910.49 ±0.119.56 ±0.138.18 ±0.126.78 ± 0.066.83 ±0.225.53 ± 0.066.49 ± 0.095.45 ± 0.076.09 ± 0.065.48 ± 0.046.21 ±0.05

Shoots

* • *

* * •

• * *

Shoots

9.41a6.38 b6.01c6.06 c

7.31a7.01b6.58 c

Standard error. *NS = non significant F ratio (p<0.05), *, •*, and **• significant atp < 0.05,0.01, and 0.001, respectively. ^Treatment means from the ANOVA test. Values followed by thesame letter, within the same source of variation, are not significantly different (p<0.05), DuncanMultiple Range Test.

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(subcellular compartmentalization or by binding to cell walls), and biochemical

tolerance (specialized metabolic pathways and enzymatic adaptations).

When a toxic metal or metalloid has been absorbed by plants, the most

extended mechanism involved in plant tolerance is limiting the upward transport,

resulting in accumulation primarily in the root system (Meharg and Macnair,

1990). From the data on Table 3, it seems that the strategy developed by radish

plants to tolerate the different chemical forms of As was avoidance, limiting As

transport to shoots and increasing As accumulation in the root system. This,

however, does not explain how radish root tissue tolerates such high As

concentrations [up to 69.1 mg As kg'1 on a dry weight basis were found in the root

skin of plants growing in 5 mg MMAA L'1] without exhibiting visual symptoms of

toxicity. A possible explanation, not directly deduced from this study, could be that

As compartmentalization was so effective in radish roots that As impact on growth

and metabolism was minimal. Similar results and hypotheses were reported by

Carbonell-Barrachina et al. (1997) in a study on the influence of arsenite

concentration on As accumulation in tomato and bean plants. Arsenic

detoxification and compartmentalization in root cells are topics that will need

further research to verify their role in plant tolerance to As toxicity.

Besides this As compartmentalization hypothesis there is another factor

that contributed to the low toxicity of As to the radish root system. All the

arsenicals are strongly adsorbed to root surfaces from solution. This adsorption is

apparently limited only by availability. For this reason, observed As concentrations

in roots are very high in hydroponic experiments (Wauchope, 1983), including this

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674 CARBONELL-BARRACHIÑA ET AL.

TABLE 3Effects of As Treatments on As Concentration of Radish Plants, RAI (Root

Absorption Index), and ACR (Tissue As Concentration Ratio)

As rate

(mgL1)

Root Skin Inner Root

Arsenic

Shoots

(mg kg"1)

RAI ACR

ControlAs(ÜI)Asail)As(m)As(V)As(V)As(V)MMAA

AMMAAAMMA

A

ADMAADMAADMAA

1.02.05.01.02.05.01.0

2.0

5.0

1.02.05.0

0.67 ±0.05*24.65 ± 8.2240.94 ± 3.8918.81 ±4.659.46 ± 7.52

21.09 ±6.109.57 ± 0.85

10.90 ± 3.23

51.00 ±4.92

69.10 ±2.4910.36 ± 3.7810.34 ±3.45

0.51 ± 0.127.69 ± 4.048.30 ±0.1411.60 ±1.816.29 ± 5.327.01 ± 1.0815.03 ± 1.75

6.67 ± 1.99

20.86 ±0.35

38.54 ± 1.073.12 ±0.286.73 ±0.38

0.68 ±0.133.68 ± 1.697.81 ± 1.7211.17 ±0.623.67 ±0.923.03 ± 2.0913.78 ±3.87

3.11 ±1.05

11.38 ±1.01

15.42 ± 0.733.15 ±0.457.14 ± 1.52

7.69 ± 4.044.15 ±0.072.21 ±0.366.29 ±5.323.51 ±0.543.01 ±0.35

6.67 ± 1.99

10.43 ±0.18

7.71 ± 0.213.12 ±0.283.37 ±0.19

1.39 ±0.180.55 ±0.040.94 ± 0.221.06 ±0.172.72 ± 1.740.42 ±0.300.99 ± 0.34

0.53 ±0.15

0.55 ±0.05

0.40 ± 0.021.00 ±0.061.07 ±0.24

Source of variation


Source of variation

ANOVA F TEST

Root Skin

*•*!

***

In. Root

******• * *

Shoots

NS***

NS


Root Skin In. Root Shoots

RAI

•

NSNS

RAI

ACR

NSNSNS

ACR

Arsenic form

As(V)MMAADMAA

Arsenic rate1.0 mgL1

2.0 mgL1

5.0 mg L '

22.65 b¥

10.18 c41.60 a12.94 c

7.89 b26.14 a31.50 a

9.01b9.45 b

22.02 a8.56 b

5.94 c10.73 b20.11a

7.61a6.83 a9.97 a9.29 a

3.44 c7.34 b14.48 a

4.68 b4.27 b8.27 a3.22 b

5.94 a5.36 a4.02 a

0.85 a1.38 a0.49 a1.07 a

1.20 a0.78 a0.90 a

1 Standard error. *NS = non significant F ratio (p<0.05), *, **, and •** significant at p < 0.05,0.01, and 0.001, respectively. *Treatment means from the ANOVA test. Values followed by thesame letter, within the same source of variation, are not significantly difierent (p<0.05), DuncanMultiple Range Test.

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study. Our results clearly demonstrated that there is a significant difference

between As concentrations caused by adsorption (root skin) and those caused by

absorption (inner root), with the highest levels being due to adsorption onto the

root surfaces.

In general, As concentrations in roots significantly increased with

increasing As levels in the nutrient solution. The data on the root absorption index

(RAI = root As concentration / nutrient solution As level) seemed to indicate that

the chemical form of As present in the nutrient solution mainly determined the

phytoavailability of As to radish plants. Arsenic phytoavailability followed the

trend: DMAA <, As(V) <, As(III) « MMAA. The higher the As level in the

nutrient solution, the higher the As concentrations in roots and shoots.

The As addition rate had a significant effect on shoot As level; As

concentration in shoot tissue increased significantly with increasing As levels in the

nutrient solution. Tissue As levels found in shoots were lower than those found in

the root system, especially in the root skin. Shoot As concentrations were also

influenced by the As chemical form. Similarly to the As accumulation pattern in

root as affected by the As chemical form, MMAA caused the highest As

accumulations in shoots, though results were not statistically different.

The tissue As concentration ratio (ACR = shoot As concentration / inner

root As concentration) was not significantly affected by either As chemical form or

As application rate.

The data on tissue As concentration indicated that the As chemical form

present in the nutrient solution and the application rate not only determined the

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phytoavailability of As to radish plants but also determined the transport and

movement of As in the plant. Treatments with both organic arsenicals caused

higher As levels in shoots than those caused by inorganic compounds. This study,

therefore, has also shown that MMAA and DMAA, usually thought of as contact

herbicides (Wauchope, 1983), are also quite active via root entry and xylem

transport.

Arsenic levels in vegetables, grain, and other food crops at the consumer

level are low, and have decreased in last three decades (Jelinek and Corneliussen,

1977). The usual statement (e.g., Lepp, 1981) that toxicity limits plant As uptake

to safe levels was not, however, confirmed in our study. Under conditions of

exposure to threshold levels in the soil, the statement appears to be true. If,

however, crops are exposed to a large pulse of As, as growth on contaminated

nutrient solutions, they may accumulate residues which are unacceptable for human

consumption. These high As concentrations found in plant tissues of soil-less

culture studies are likely due to the fact that As concentrations in the nutrient

solutions are not high enough to cause plant death but are high enough to make

plants absorb As continuously leading to very high levels of pollutant.

The limit set for As content in ftuit, crops and vegetables is 1.0 mg kg'1

f.w. (Mitchell and Barr, 1995); considering an average water content of the edible

part of the radish plants in our experiment of 95%, this limit on a dry weight (d.w.)

basis is 20 mg kg"1. In our study, As concentration in the skin of radish roots were

in some plants close or even above this maximum threshold and ranged from 9.5 to

69.1 mg kg'1 (d.w.). These high As concentrations are, without a doubt, a result of

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all the arsenicals being firmly adhered to the root surfaces from solution

(Wauchope, 1983). Arsenic concentrations in the inner roots were, however, lower

than those of the root skins and ranged from 3.1 to 38.5 mg kg'1 (d.w.), but, in

some cases, they were still above this statutory limit. Therefore and taking into

account a possible incorporation of As in the food chain, soil residues from the use

of As-based pesticides and herbicides are potentially dangerous for the human

health due to their feasibility of accumulating high levels of As in the edible part of

radishes and even more if these radishes are consumed with peelings because of the

As adsorption to the root surface.

CONCLUSION

The statement that toxicity limits plant As uptake to safe levels was not

confirmed in our study. If radish plants are exposed to a large pulse of As, as

growth on contaminated nutrient solutions, they may accumulate residues which

are unacceptable for animal and human consumption without exhibiting symptoms

of phytotoxicity or growth reduction. Arsenic concentrations in the root skin were

statistically higher than those of the inner root likely due to arsenicals being firmly

adhered to the root surfaces, and, in general, both concentrations were clearly

above the maximum statutory limit for As in food destined for human

consumption.

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Received: November 9, 1998

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arsenic toxicity and accumulation in radish as affected by arsenic chemical speciation

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