Abstract: The aim of this study was to determine the level of heavy metals in the soil and in the plant parts (fruits, leaves and roots) of two cultivars of guava. The study was carried out at the Sungai Wangi Plantation in Sitiawan, Perak, Malaysia. Heavy metals in the soil were extracted using the sequential extraction method. Heavy metals in the soil and plants were determined using atomic absorption spectrometry. In general, it was found that the concentration of heavy metals in the soil was low and no Cu was detected in any fraction of the soil. Ni was detected in the RR fraction of all the four blocks studied and ranged from 2.71 to 4.52 mg kg-1. Cd was detected in all fractions of the four blocks except in the AR fraction of block 3. The concentration of Pb was considerably low in the soil of this plantation. Pb was not detected in the AR fraction of the four blocks. Mn was detected in all fractions. In the guava plants, Pb and Cu were not detected in all parts of the plants except that Cu (0.01 mg kg-1) was detected in the seeds from block 1. Of the heavy metals, only Fe was found in all plant parts from the four blocks. Similarly Zn was also found in all plant parts except those plants sampled from block 2. In conclusion, it can be stated that the concentration of heavy metals in both soil and guava plants from the Sungai Wangi Plantation at Sitiawan, Perak was considerably low.
INTRODUCTION
Agricultural activities have been identified as contributors in increasing heavy metal pollution through the application of various types of pesticides and fertilizers (McLaughlin et al., 2000). Faruk et al. (2006) mentioned that contamination of agricultural soils with heavy metals has always been considered a critical challenge in the scientific community. Heavy metals are generally present in agricultural soils at low levels. Due to their accumulative behavior and toxicity, however, they have a potential hazardous effect not only on crop plants but also on human health through the food chain (Das et al., 1997; Melamed et al., 2003).
One of the major problems concerning heavy metal pollution is that it persists indefinitely in soil. But at any point it can be reduced by transportation of the pollutant elsewhere for example to areas downslope or downwind by erosion, to surface or groundwater by leaching, to the atmosphere by volatilization, to animals or humans by removal in agricultural crops, or designated disposal areas by physical removal of soil or plants (phytoremediation) (McLaughlin et al., 2000). Heavy metals are identified as harmful to the environment because of direct potential toxicities to biota and indirect threats to human health from contamination of ground water and accumulation in food crops (Martinez and Motto, 2000).
Various studies on heavy metal pollution have been carried out in many agricultural areas throughout Malaysia. A study on bioaccumulation of heavy metals in selected vegetables such as brinjal (Solanum melongena), sweet potato (Ipomaea batatas) and spring onion (Allium cepa) in Sepang, Selangor and Cameron Highlands, Pahang, has indicated that heavy metal content in these vegetables was still low (Ismail et al., 2005). The concentration levels were found to be below the maximum limit permitted in the Malaysian Food Act (1983). Khairiah et al. (2006) also studied the heavy metal content in the Cameron Highlands (Pahang) and Sepang (Selangor) soil and found that most of the heavy metals in those areas were detected in the resistant fraction which is strongly bonded to silicate minerals and not available to plants grown at those particular areas.
Guava plants are grown in the Sungai Wangi Plantation, Sitiawan, Perak but no research was carried out to determine the level of heavy metals either in the plants or in the soil. This plantation supplies guava fruit for guava juice which is mainly for export. Therefore, the quality of the fruits produced should be of good quality. In the current study, the sequential extraction method was adopted to investigate the effects of the application of various types of fertilizers and pesticides to the soil. This study will be very useful in assessing the heavy metal pollution due to the agricultural activities besides other environmental factors that may also contribute to the pollution. The application of pesticides is very vital in the maintenance of this plantation. The pesticides are used in large quantities in order to control pests which could reduce the quality of the guava fruits. It is established that pesticides and fertilizers may contribute to the increasing level of heavy metals in soil environments. Therefore, the purpose of the study was to determine the level of heavy metals in guava plant parts (especially the fruits) and soil at the Sungai Wangi Plantation near Sitiawan, Perak.
MATERIALS AND METHODS
The Study Area
The study was carried out in the guava (Psidium guajava) farm at
the Sungai Wangi Plantation (4°13’S 100°42’U) at Sitiawan,
Perak, Malaysia, approximately 250 km from Kuala Lumpur. The growing of various
types of fruits was initiated in the plantation since Mac 1991 and guava planting
was introduced in 2002. Currently, there are two varieties of guava grown here,
namely the Semenyih and Sungkai cultivars. The Semenyih cultivar has pink pulp
whereas the Sungkai cultivar has white pulp.
Soil at this plantation consists mainly of the Sitiawan Series which contains sand at the upper portion of the soil and clay at the lower layer. The characteristics of the Sitiawan Series are shown in Table 1.
| Table 1: | The pH, organic carbon and grain size of the soil from the guava plantation at Sungai Wangi, Sitiawan, Perak |
Extraction of Heavy Metals from Plant Samples
Plant samples were collected from the four blocks of the guava plantation.
Two blocks consisted of the Semenyih cultivar (block 1 and 2) whereas the remaining
two blocks had the Sungkai cultivar (block 3, 4). In each block, three guava
plants were selected randomly for sampling.
Fruit, leaf and root samples were collected from each plant (3 plants per block). From each guava plant, three fruits were harvested. These samples were washed twice with distilled water and finally with distilled-deionized water. The samples were wiped with clean tissue paper and cut into small pieces with a stainless steel knife. The fruit was separated into skin, seed and fleshy pulp whilst for the leaves the young and old leaves were separated. Root parts were also treated the same way as the fruit and leaves. Before the digestion process, the fruits, leaves and roots were oven dried at 70°C until their weights were stable (AOAC, 1984) and then ground using a mortar and pestle. The wet digestion method was adopted to extract heavy metals from various parts of the guava plant (AOAC, 1984). One gram of each sample was weighed into a conical flask, then 10 mL of HNO3 was added followed by 3 mL of HClO4 (2:1) and the left for 2-3 h in a water bath. Ten milliliter of HCl was then added to dissolve inorganic salts and oxides. The digested samples were filtered through 0.45 μm pore size Milipore filter paper and made up to 50 mL with distilled water before the metal contents were determined through Atomic Absorption Spectrometry (AAS) Perkin Elmer (model 1100B).
Extraction of Heavy Metals from the Soil
Five plots from each block were identified for soil sampling. Soil samples
were collected from 0 to 30 cm depth and air-dried in the laboratory. The soil
was passed through 250 μm mesh. Prior to analysis, the soil samples were
ground using a mortar and pestle.
Heavy metal extraction in the soil was performed using the sequential extraction method of Badri (1984). The extraction was divided into four fractions namely easily leachable and ion exchange (ELFE), acid reducible (AR), organic oxidation (OO) and resistant (RR) fractions. The ELFE fraction normally extracted metals which were weakly bound to the clay surface area, secondary minerals and organic materials. Ten grams soil samples were put into kartell bottles followed by the addition of 50 mL 1.0 M NH4CH3OO (pH 7). The samples were then shaken for 1½ h and centrifuged at 3000 rpm for 30 min before being filtered through 0.45 μm millipore filter paper and made up to 50 mL with distilled water. Samples were washed with 50 mL distilled water, followed by further shaking and centrifugation as described earlier.
Then a total of 50 mL NH2OH.HCl (pH 2) was added to extract the metals from the acid reducible (RA) fraction using the procedure described above. Metals extracted from this fraction mainly came from those which were strongly bound to secondary minerals, Metals in the organic oxidation (OO) fraction were extracted by adding 15 mL of H2O2 to the sample placed in a water bath for 1-1½ h, followed by addition of 50 mL NH4CH3OO (pH 3.5). Metals in this fraction represented metals bound to organic matter. Samples were then digested using HNO3:HClO4 at 25:10 ratio in a sand bath at 100°C as in the RR extraction method. The digestion process was repeated until the samples turned whitish (Badri, 1984).
The determination of heavy metal concentration was carried out using AAS (atomic absorption spectrophotometry) Perkin Elmer (model 1100B).
RESULTS AND DISCUSSION
Heavy Metal Content in the Guava Fruits
Table 2 shows the average heavy metal content in the two
cultivars of guava fruit (Semenyih and Sungkai). Toxic metals such as Pb were
not detected in both the guava cultivars.
| Table 2: | Average heavy metal content in the guava fruit, leaves and roots (mg kg-1) |
| ND: Not detected. Values with different letters are not significant | |
This study showed that Fe, Mn and Zn were found in the guava plants except for Zn which was not detected in the Semenyih cultivar from block 2. Cu was only found in the seeds of the Semenyih cultivar from block 1. Cd was only detected in the leaves of both cultivars, while Ni was detected only in the fruits and seeds. Mn also was absent from the young and old leaves of the guava plants from block 1 which constituted the Semenyih cultivar (Table 2).
Metals detected in the guava plants were mainly plant nutrients namely Fe, Zn and Cu and these nutrients are required by the plants for optimal growth and various enzyme activities in the plant (Hopkins, 1999). This study also indicated that Zn and Mn were higher in the leaves than in the other plant parts studied. These results are in line with other findings, which showed that Zn acts as an activator of enzymes in synthesizing the hormone precursor the amino acid tryptophan (Hopkins, 1999; Marschner, 1986). Mn is required as a cofactor for a number of enzymes, particularly decarboxylase and dehydrogenase enzymes which play an important role in the respiratory carbon cycles (Hopkins, 1999). Interestingly, Fe was found in all the parts of the guava plants of both cultivars (Table 2). Fe is important for some enzyme activities, is a constituent in chlorophyll and is involved in electron transport in the photosystem (Hopkins, 1999).
The role of Ni in the plant is not very clear but it is needed in small quantities (Hopkins, 1999). It was reported that Ni is mostly found in the seeds of plants such as legumes and cereals, for instance barley (Hordeum vulgare) and that the lack of Ni may depress seedling vigour and cause chlorosis and necrotic lesions in the leaves (Brown et al., 1987; Dalton et al., 1988). A small amount of Cd was detected in the guava leaves. Cd is easily taken up and translocated to different parts of the plant (Andriano, 1986). As mentioned earlier, Pb was not detected in any part of the guava plant, thus confirming that guava plants from this plantation are free from Pb contamination. It should be mentioned that the Sungai Wangi Plantation is away from heavy traffic which are a known cause of Pb pollution.
Overall, the study indicated that no toxic metals such as Pb and Cd were found in the various parts of the guava plants studied with the exception of the leaves, contrary to the results of other studies. The metals detected were mostly nutrients which are required for plant growth. This was because application of pesticides and fertilizers at this plantation was at the recommended dosages, following good management practices. Furthermore, there were no other sources of pollutants found in the surrounding areas that could contribute to increase in the concentration of toxic metals.
Heavy Metals in the Soil
The guava plantation in Sungai Wangi constituted marine alluvial soils of
the Sitiawan Series. Most of the soil in the study area contained a fair percentage
of clay and silt (grain size <63 μm) except for the soil in block 2
(with the Semenyih cultivars) (Table 1). The soil in this
area was more clayey and silty in nature. Less than 2% organic matter was recorded
for all soil samples and the pH values ranged from 4.48 to 5.03 indicating acidic
soil.
Heavy metals in the soil samples occurred for a very wide range of concentrations. The highest value of total Fe was found in all the soil samples whereas Cu was not detected in all the soil fractions (Table 3). The total concentration of heavy metals in decreasing order were Fe>Pb>Zn=Mn>Ni>Cd>Cu. Generally, the studied soils contained lower heavy metal concentration compared to other agricultural soils such as those found in Sepang, Bangi and Cameron Highlands (Ismail et al., 2005). These findings suggest low heavy metal content in marine alluvial soils.
The level of Fe in the soil was associated with the RR fraction followed by the OO and AR fractions. Fe was not detected in the ELFE fraction of all the four blocks studied. The results of this study showed that relatively high amounts of Fe were found in all parts of the guava plants particularly the seeds. According to Merian (1991), the insoluble iron (mostly ferric hydroxide) is present in soils prior to plant uptake. Solubilization of iron takes place at the root-soil interaction due to the root reaction with soil and microbial activities. This study suggests that Fe uptake by the guava plants may occur in the AR and OO fractions.
| Table 3: | The average heavy metal content in the soil from the Sungai Wangi guava plantation, at Sitiawan, Perak (mg kg-1, n =5) |
| ND: Not detected. Values with different letters are not significant | |
Concentration of Pb in the ELFE and AR fractions was less than 0.1 mg kg-1. This showed a low availability of this metal to the guava plants because usually in this fraction it should be more available to plants (Andriano, 1986). The low concentration of Pb in the soil was confirmed by the undetectable Pb trace in all parts of the guava plants grown at the Sungai Wangi Plantation.
All the values for Mn at the ELFE fractions ranged from 0.87 to 1.16 mg kg-1 and this metal was found accumulated in the guava fruits and leaves (both young and old). The uptake of Mn in guava plants was due to the soil pH conditions (4.48-5.02). According to Andriano (1986), the uptake Mn by plants increased in slightly acidic soils compared to the strongly acidic or basic soil conditions.
The Zn in the ELFE fraction was in the range of 0.02-0.95 mg kg-1. As in the case of Mn, Zn accumulated in the guava fruits and leaves (young and old) except for plants in block 2. The slightly sandy and low percentage organic matter of this soil might have contributed to a better Zn uptake by plants because organic matter in soils could decrease the Zn bioavailability (Andriano, 1986).
Ni was not found in the ELFE, AR and OO fractions indicating the absence of Ni in exchangeable, reducing and organic bound forms. However, it should be noted that Ni was detected in the guava seeds and fruits but not in the leaves and roots. The concentration of Ni in the guava fruits might be due to bioaccumulation.
The Cd concentrations in the ELFE were low and in the range of 0.03 to 0.06 mg kg-1. The small amount of Cd detected in the leaves might reflect the low availability of Cd in the soils.
Cu was undetected in all the soil fractions and guava plants except in the seeds from the Semenyih cultivar in block 1. The concentration of Cu in the study area was far lower compared to that in other agricultural areas, namely Sepang, Bangi and Cameron Highlands (Ismail et al., 2005). This might be due to leaching at the soil surface (0-30 cm) from which the samples were collected. The leaching of Cu has been reported to increase in acidic and sandy soils with low organic matter content (Andriano, 1986; Merian, 1991). Thus, the absence of Cu in the guava plants is attributed to the unavailability of Cu in the soil.
In general it can be stated that the guava plants have a tendency to accumulate heavy metals which are also important plant nutrients. Statistically, no significant correlation was observed between the amount of heavy metals in the soil and in guava plants. This could be due to the ability of the guava plants to accumulate heavy metals. The uptake of these metals probably was via the transpiration system. On the other hand, these findings clarify that the heavy use of fertilizers and pesticides do not appear to cause an increase in the heavy metal content in the guava plant or the soil at this particular plantation.
CONCLUSION
This study indicated that most of the heavy metals found in the various parts of the guava plants studied were also plant nutrients, required for various processes for optimum plant growth. Toxic metals like Pb were undetected in the guava plants but low concentrations of Cd were detected in the seeds from block 1. The concentration of heavy metals in all fractions of the soil of the Sungai Wangi Plantation as seen by sequential extraction was considerably low.