ABSORPTION AND ACCUMULATION OF CD, PB,
CU, MN, AND ZN in Nepeta transcaucasica
Grosch. AS A FUNCTION OF DISTANCE FROM A POLLUTED ZONE
Valtcho Zheljazkov (Jeliazkov)*, Phil R. Warman (Department of Environmental Sciences, Nova
Scotia Agricultural College, PO Box 550, Truro, NS, Canada, B2N 5E3 Tel: (902)
893 7859, Fax: (902) 893 1404, e-mail: vjeliazkov@cadmin.nsac.ns.ca)
ABSTRACT
A two-year container experiment was conducted to study the absorption of Cd, Pb, Cu, Mn, and Zn in catmint (Nepeta transcaucasica Grosch.) cultivars Rozalia and Citronella. Soil was taken from the vicinities of a Pb and Zn smelter near Plovdiv, Bulgaria, at distances of 0.5, 3.0, 6.0 and 9.0 km (the latter was unpolluted and regarded as a control).
The accumulation of trace elements in plant parts was as follows: for Pb and Cu - roots > leaves > flowers > stems; for Cd and Zn - roots > leaves = flowers = stems; for Mn - leaves > roots > flowers > stems. In all essential oil samples, the concentration of Cd was less than the detection limit of the GFAA, and Pb, Cu, Mn and Zn content was extremely low.
High correlation was found between ‘total’ soil metal concentration of Cd, Pb, Cu, Mn and Zn and their concentration in the roots. Lower, but significant correlation was found between ‘total’ soil metal concentration and their concentrations in the leaves. The amount of heavy metal pollution of the soil, taken at 0.5 km from the smelter, reduced the yields of fresh herbage by 13-15 % relative to the control.
INTRODUCTION
In Bulgaria, there are thousands of hectares of contaminated agricultural soils around some of the biggest smelters, resulting in contamination of the crops grown in these regions (Sengalevitch, 1993; Angelova et al. 1999). The replacement of some crops in these areas with aromatic crops, grown for essential oils, might eliminate heavy metal contamination of the animal and human food chain (Zheljazkov and Nielsen, 1996a,b). Catmint (Nepeta sp.) is an aromatic plant traditionally grown in Bulgaria for its essential oil, which has antimicrobial and antioxidant activity.
Field experiments alone in the polluted areas do not give an estimate of the amount of metals taken up from the soils because of the continuous atmospheric pollution of the area. In order to estimate the uptake of heavy metals from the soils in the aboveground plant parts and the essential oil of catmint, and to evaluate its suitability for growing on metal polluted soils, container experiments were conducted using soils from a metal contaminated area.
METHODOLOGY
As a test material we used cultivars Rozalia and Citronella from Nepeta transcaucasica Grosch. family Lamiaceae (Labiatae). The soils were taken from region of Non-Ferrous Metals Combine (Lead and Zinc) around the city of Plovdiv, Bulgaria, at distances of 0.5 km, 3 km, 6 km and 9 km in one direction (south-east). The soil sampled from 9 km distance was considered unpolluted and was used as a control.
In each container of 10 kg of soil four equal size Nepeta seedlings were grown. Plants were harvested at full blossom, when the quality and quantity of the essential oil is the highest. The essential oil content in the fresh aboveground mass was determined via steam distillation of 50 g fresh aboveground material (leaves, stems and flowers) in a Klevenger type distillation apparatus.
The soil used in the experiment was a calcareous drained alluvial-meadow (or calcaric fluvisoils upon the FAO classification) with a pH 7.1-7.2. The humus horizon A has 25-28 cm, with organic carbon of 1.5-1.7 %, and clay content of 24 and 28 %. CaCO3 at the surface is around 2%, and reaches 6.9% in horizons B and C. The fertility of the soil was similar; the soil was fertilized with N, P, and K equivalent to 120 kg/ha (P and K and ½ of N were mixed with soils before filling the containers, the other half of N was applied and incorporated when the plants were 2 weeks old).
Soil, plant and oil samples were digested in concentrated HNO3, and analysed on AAS and GFAAS as indicated by (Zheljazkov and Nielsen, (1996a). Data was analysed using ANOVA in SAS.
RESULTS AND DISCUSSION
Plants grown in the soil taken at 0.5 km from the smelter reduced the yields of fresh herbage by 13-15 % relative to the control plants (Figure 1). Heavy metal concentration in soils taken at 3 and 6 km from the smelter did not cause significant yield reduction compared to the control. There were no significant differences in the overall productivity of the two cultivars tested. The essential oil content in the fresh plants did not show significant variation due to the different soils used. Thus, essential oil yields were influenced by fresh herbage yields only and not by essential oil content (data not shown).
Cadmium, Pb and Zn concentration in the soils at 0.5 km from the smelter was up to 40 times greater than their concentrations in the control soil (Table 1). Copper concentration in the soils taken at 0.5, 3 and 6 km from the smelter was high, around and above Cu critical soil concentration, while in the control soil it was within its normal background level. Manganese content in soils taken at 0.5, 3 and 6 km from the smelter was relatively high, but still way below its critical soil concentration.
Simple regression analysis revealed high correlations between root and soil ‘total’ content of Cd, Pb, Cu, Mn and Zn (Table 2). Similarly, high correlations were found between leaf and ‘total’ soil content of metals. However, except for Cu, there was no significant correlation between metal concentration in the soil and metal concentration in the essential oil. Both cultivars showed very similar accumulation of Cd, Pb, Cu, Mn, and Zn in the respective plant parts (roots, stems, leaves and flowers). Different plant parts accumulated unequal amounts of metals. As a whole, the concentration of the metals in the plant parts (Figure 2) was as follow: For Pb and Cu roots > leaves > flowers > stems; for Cd and Zn roots > leaves = flowers = stems; for Mn leaves > roots > flowers > stems.
Except for Cu,
there were no significant differences in metal concentration between the oils
from plants grown on different soils.
Cadmium in the essential oil of all treatments was below the detection
limit of the GFAA. The content of Pb,
Mn, Cu and Zn in the essential oil was below the maximum acceptable limits for
these elements in vegetable oils (CBSEP, 1989), so Nepeta essential oil
was not contaminated with heavy metals.
It is difficult to discriminate which of these metals was most responsible for the 13 -15% yield reduction. We assume there were interactions between these cations in soils, rhizosphere and in plants as well, since there are well documented interactions between Zn and Cu, Cd and Zn, Pb and Cd, Mn and Cu, as well as between these metal and some non-metal ions (Hasset et al. 1979; Kabata-Pendias and Pendias, 1992). Despite relatively high concentration of metals in the plants, we did not observe any toxicity symptoms, which confirm some of our other investigations (Zheljazkov and Nielsen, 1996 a,b).
It is generally
believed that Cu and Pb have little mobility within plants, so once they are
taken up, these metals tend to accumulate in the roots, while Cd, Mn and Zn are
easily transported up to the shoots (Kabata-Pendias and Pendias 1992). Our results support this understanding. Andrew and Thorne (1962) also found more Cu
was accumulated in the roots rather then in shoots of ten plant species. However, there are some reports where leaf
tissue concentration was equal or exceeded root Cu content in colza (Brassica napus) (Planquart et al.
1999). Using romaine lettuce, Sloan et
al. (1997) reported significant R2 coefficients for Cd, Pb, Cu and
Zn concentrations of aboveground lettuce tissue as a function of total metal
concentrations in the soils. We found
higher values for these coefficients, which may be due to differences in the
experimental conditions (soils, plants species). The sequence of trace element concentration we found in
different organs reflects specificity of the cultivars used, since other
authors reported a different sequence (Planquart et al. 1999), which may be due
to genetic differences between species.
REFERENCES
Andrew CD, Thorne PM, 1962, J. Agric. Res. 13: 821-835.
Angelova VR, Ivanov AS, Braikov DM., Ivanov KI, 1999. J. Sci. Food and Agriculture. 79: 713-721.
Collected
Bulgarian Standards on Environment Protection. 1989. Standardization. Sofia,
Bulgaria
Hassett
JE, Miller E, Koeppe DE, 1976, Environ. Pollut. 11: 297-301.
Kabata-Pendias
A, Pendias H, 1992, Trace Elements in
Soils and Plants CRC Press, Baton Rouge, USA.
Planquart P, Bonin G, Prone A, Massiani C, 1999, Sci. Total Environ. 241: 161-179.
Sengalevitch
G. 1993, Zemedelie 1(2): 18-33.
Sloan
JJ, Dowdy RH, Dolan MS, and Linden DR, 1997, J. Environ. Qual. 26: 966-974.
Zheljazkov
VD, Nielsen NE, 1996a, Plant & Soil, 1: 1-8.
Zheljazkov
VD, Nielsen NE, 1996b, J. Essential Oil Res. 8: 259-274.
Table 1. Average “total” metal concentration in the soils, depending on the distance from the source of pollution mg/kg
|
|
0.5 km |
3 km |
6 km |
9 km |
|
Cd |
21.6 |
3.1 |
3.0 |
0.6 |
|
Pb |
902.0 |
112.0 |
92.0 |
32.0 |
|
Cu |
101.0 |
90.0 |
91.0 |
29 |
|
Mn |
876.0 |
770.0 |
782.0 |
259.0 |
|
Zn |
772.0 |
88.0 |
71.0 |
15.0 |
Table 2. Regression models for Cd, Pb, Cu, Mn, and Zn concentrations in plant roots and leaves as related to their concentrations in the soils from the four locations.
___________________________________________________________
Heavy metals
Cd Pb Cu Mn Zn
_________________________________________________
Adjusted regression coefficient, R2
_______________________________________________________________________
roots/soil 0.94* 0.92* 0.93* 0.91* 0.98*
leaves/soil 0.63* 0.89* 0.74* 0.89* 0.91*
_______________________________________________________________________
Table 3. Metal concentration in the essential oil, mg/L
___________________________________________________
Heavy metals Distance from the source of pollution, km
0.5 3.0 6.0 9.0
___________________________________________________
Pb 0.056 0.045 0.041 0.041
Cu 1.013* 1.004 1.008 0.760
Mn 0.027 0.030 0.032 0.026
Zn 0.421 0.362 0.430 0.338
___________________________________________________