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Сельскохозяйственная биология, 2010, № 6, с. 94-99.

УДК 633.2:574.24:631.45

ESTIMATION OF COMPLEX PHYTOTOXICITY OF НЕАVY METALS AND DETERMINATION OF ADMISSIBLE CONCENTRATION FOR ZINC AND COPPER

E.A. Gladkov

In laboratory experiments by the use of the indices of germinating capacity, roots formation and growth in agricultural and some ornamental plants the author determined the phytotoxicity of Zn, Cu, Cd and Pb as anthropogenic ecological factors during their complex and individual action. It was shown, that complex action of two heavy metals at the unfavorable for plants doses may causes both the increase and the decrease of toxic effect. It is necessary to reconsider the maximum permissible concentration for some soil elements in consideration of toxic effect for plants, specifically for Zn this dose size must be reduced to 60 mg/kg, for Cu — to 30 mg/kg.

Key words: ecological factors, maximum concentration limit, heavy metals, salinity.

 

Contamination of man-made ecosystems usually involves the presence of several toxicants at contents often exceeding their maximum permissible concentrations (MPC). Fertilizers are one of sources of heavy metals (HM) in agroecosystems since they can contain lead, zinc, copper, cadmium and other HM (1), though, even a long-term use of fertilizers quite rarely leads to significant increase in contents of metals in soil. In urban ecosystems, the major pollutants are zinc, copper, cadmium and lead (2). For example, contamination with lead, zinc and cadmium in soil near the Domodedovo Airport has local nature and it is formed by the impact of traffic, the presence of solid waste landfills, sewage treatment plants and peat extraction areas (3). In most of agro-ecosystems and settlements, soils contain heavy metals, and even small amounts of them cause adverse effects on plants by inhibiting their growth and development, reducing yields and ornamental qualities (1, 4). Water-soluble and ion-exchangeable forms of HM can be easily absorbed by plants and then transferred in ecosystems along the food chain. In agricultural and medicinal plants, this process is the direct threat for human as a terminal link of these chains (4-8).

At the same time, MPC of toxicants in soil doesn’t completely imply their effects directly on plants. MPC are established upon the base of threshold levels and maximum ineffective concentrations: the first ones consider the reduce in indices of survival, fertility and growth up to 50% (mainly in long-term tests commensurate with duration of the object’s life cycle), the second ones – by no more than 25% (in chronic experiments) (9).

The processes of nutrients absorption in plants and HM income from soil are interconnected by both positive and negative correlation (10-12). Thus, the transport of trace elements was studied in potato during its life cycle when growing in soil contaminated with zinc, cadmium and lead; it has been revealed, that lead contributed to greater removal of zinc, cadmium and manganese, while cadmium stimulated accumulation of zinc in plants (by 6-20 g/ha) and zinc enhanced absorption of nickel (10).

The optimum zone and limits of resistance to a certain environmental factor (according to the law of interaction) in organisms may shift depending on strength and combination of simultaneously operating factors. This law is applicable to classical environmental impacts (pressure, temperature, etc.), but there’s no clear data about the existence of such regularities among anthropogenic factors – i.e., is it possible the enhancing or weakening of an adverse effect.

The purpose of this study was to assess phytotoxicity of environmental factors on forage grasses and plants of urban landscaping, as well as to establish relatively permissible concentrations (RPC) of zinc and copper in urban soils for the plants as the base for further establishing of MPC of these metals (total content).

Technique. The degree of phytotoxicity was assessed by germination of seeds of the plants - creeping bentgrass (Agrostis stolonifera L.), red fescue (Festuca rubra L.), swanriver daisy (Brachyscome iberidifolia Benth.), red cornflower (Centaurea cyanus L. ‘Carminea’), white clover (Trifolium repens L.), dwarf godetia (Godetia grandiflora Lindl. ‘Dwarf’), meadow-grass (Poa pratensis L.) and some others - on filter paper placed in Petri dishes, as well as by sowing in soil (pH 6,0-6,5, nitrogen, phosphorus and potassium contents – respectively, 150, 250 and 300 mg/l) containing toxicants. The control - plants grown in aqueous solution or in soil without toxicants. Heavy metal salts were brought into soil at watering. Phytotoxicity indices were determined: germination of seeds and growth rates of shoots and roots. Each variant was performed upon 100 seeds in replicates: 4-fold on a filter paper and 3-fold in soil tests.

A confidence interval was calculated using the Student’s t-criterion (reliability - 95%).

Results. Previously, the author has shown that actual MPC of zinc and copper in soil (in contrast to lead and cadmium) for plants differ from conventional levels (2, 13). Experimental establishment of RPC for zinc and copper by germination of flowering plants should consider at germination on a filter paper the indices of germination, growth and rooting, while at sowing in soil - germination, growth and total appearance of plants. The criteria for determining RPC were inhibition of majority of the indicators by no more than 25% and rooting of the most sensitive species without lethal effects.

 
Sensitivity to zinc (Table 1) was significantly variable by main indicators: strong inhibition of seed germination occurred only at values above MPC (in both Petri dishes and soil). Root system showed high susceptibility to zinc: 300 mg/l suppressed formation of roots in Swanriver daisy and red fescue, while other plants exhibited growth rates of roots 14-26% of control. Zinc content of 100 mg/l inhibited root growth by more than 50% in almost all species. The highest sensitivity to zinc was found in red cornflower: 80 mg/l during 4-6 days resulted in nearly lethal effects in plant roots (growth - 4% of control). The content of 40 mg/l didn’t inhibit shoot growth in tested species. In the variant with zinc content in soil 40 mg/kg, no effects on plants were observed and RPC amounted to 60 mg/kg. White clover didn’t show inhibition of growth, while creeping bentgrass and dwarf godetia demonstrated 25% reduce in growth by the 26th day of the experiment (Table 2). The plants cultivated in Petri dishes with zinc content of 60 mg/l demonstrated growth indices equal to more than 75% of control; after 4-6 days of such cultivation, root growth in different species reached 50-80% of control. The obtained results indicate that zinc content of 60 mg/kg corresponds to RPC (Table 1, 2), because it didn’t cause lethal effects on root growth and inhibited most of other indicators by no more than 25% (maximum ineffective concentrations).

2. Growth indices of shoots (relative to control, %) in tested plant species depending on contents of zinc and copper when growing in soil (Х±х)
Concentration, mg/l	Creeping bentgrass	Swanriver daisy	Dwarf godetia
Z n
40:
day 14	100	82±4,5	100
day 26	90±3,6	100	100
60:
day 14	100	75±5,1	100
day 26	75±4,8	78±6,0	80±5,1
100:
day 14	98±2,6	60±3,0	100
day 26	60±3,9	70±2,7	70±5,1
C u
20	90±5,4	80±4,7	96±5,8
30	80±3,2	75±3,9	86±2,4
40	72±4,6	67±2,9	74±3,4
50	68±2,9	62±3,2	64±6,0
75	65±4,2	58±3,6	60±4,5
100	60±2,6	52±4,4	58±3,9

According to the author’s data, RPC of copper in soil is 30 mg/kg. Plants germinated in Petri dishes at copper content 30 mg/l (creeping bentgrass, meadow-grass, red fescue, swanrifer daisy, red cornflower, white clover, red clover, dwarf godetia) exhibited shoot growth 73-84% of control . In contrast to zinc, copper provided strong inhibitory effect in soil just on the 14th day of the experiment. The dose of 30 mg copper per 1 kg soil was a maximum ineffective concentration: most of the plants had general appearance similar to control and growth inhibition did not exceed 25% in creeping bentgrass, swanriver daisy and dwarf godetia by the 14th day of the test (Table 2). In dwarf godetia, the toxicity of copper increased after 1 month while the shoots’ growth amounted to 75% of control. In white clover, growth inhibition wasn’t observed.

A simultaneous introduction of copper and zinc provided slight increase of copper effects (Table 3). The highest phytotoxicity of copper was observed on the 4th day of the experiment: 100 mg/l resulted in germination of 33% seeds compared with control, which value increased by 60% on the 10th day. In the variants with HM added in soil (mg/kg soil) Cu 30 + Zn 60; Cu + Zn 50 100; Cu 75 + Zn 150 and Cu 100+ Zn  150 growth indices amounted to, respectively, 72 ± 3,9; 65 ± 4,8; 61 ± 2,3 and 58 ± 4,5% of control – in creeping bentgrass, 65 ± 3,2; 58 ± 3,7; 56 ± 5,1 and 46 ± 5,1 % - in swanriver daisy. The similar trend was indicated in dwarf godetia and white clover.

Swanriver daisy grown in soil keeping both copper and zinc manifested weakened flowering: in the variant with HM doses (mg/kg soil) Cu 100 + Zn 200, number of buds was twice less than at only copper presence of 100 mg/kg without adding zinc. Probably, the increased toxicity of copper was the result of its enforced accumulation caused by zinc. Synergistic effect of zinc and copper may be also determined by location of these elements in neighboring groups of the periodic system. At the same time, it has been reported about both synergistic and antagonistic effects at the combined action of copper and zinc (for example, the antagonism between zinc, cadmium and copper has been shown in some invertebrates and plants) (14). Accumulation of zinc can reduce as well, but it less influences plants since zinc is less toxic than copper.

Studying the effect of cadmium and its combined action with zinc, it was noted that zinc content of 150 mg/l relieved toxic effects of cadmium in some species (especially in red cornflower) (Table 4, 5). This fact was also observed in red fescue, phacelia (Phacеlia tanacetifolia Benth.) and other plants (data not shown). Seeds of red clover and white clover were found to be less sensitive to cadmium than red cornflower.

Zinc and cadmium have fairly close chemical properties but distinct biological action (including toxicity). There are quite controversial literature data about their combined action – these are antagonistic and synergistic effects, as well as the absence of any effects. However, most of the authors tend to admit antagonistic effects, which has been shown in agricultural and medicinal plants (5, 10-12). Possibly, manifestation of a certain effect depends on proportion of cadmium and zinc contents in soil.

Significant raise of toxic effects at simultaneous presence of cadmium and lead in medium was observed in all studied plants, especially in swanriver daisy. Synergistic effect of these elements can be concerned with their location in neighboring groups of the periodic system.

Thus, the combined action of two heavy metals (as man-made environmental factors in agrobiocenoses) at adverse doses can lead to elevation or reduce of their toxic effects. It is necessary to revise current MPC of some substances in soil considering their phytotoxicity. Particularly, MPC of zinc should be reduced by 60 mg/kg, MPC of copper - by 30 mg/kg.

REFERENCES

1. Pronina N.B., Ekologicheskie stressy (Ecological Stresses), Moscow, 2000.
2. Gladkov E.A. and Gladkova O.V., Evaluation of Complex Impact of Ecological Factors on Urban Plants, Sb. nauch. tr. MGUIE,  2006, pp. 96-101.
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12. Rasteniya v ekstremal’nykh usloviyakh mineral’nogo pitaniya (Plants in Extreme Conditions of Mineral Nutrition), Shkol’nik M.Ya. and Alekseeva-Popova N.V., Eds., Leningrad, 1983.
13. Gladkov E.A. and Gladkova O.V., Assessment of Complex Phytotoxicity of Heavy Metals and Obtaining of Plants with the Complex Resistance, Biotekhnologiya, 2007, no. 1, pp. 81-86.
14. Stepanok V.V., The Effect of Complexes of Technogenic Elements on Chemical Composition of Agricultural Crops, Agrokhimiya, 2003, no. 1, pp. 50-61.

K.A. Timiryazev Institute of Plant Physiology, RAS, Moscow 127276, Russia , e-mail: gsc@ippras.ru Moscow State University of Environmental Engineering, Moscow 105066, Russia

Received June 1, 2010

All-Russian Institute for Selection and Seed-Breeding of Vegetables All-Russian Selection and Technological Institute of Horticulture and Breeding Nursery Institute of Agricultural Biotechnology Joint Stock Company "GosNIISyntezBelok"