Metals Phytotoxicity Assessment and Phyto Maximum Allowable Concentration

In this paper, the influence of metals (Cd, Pb, Cu, Co, Ni, Zn) on plants of spring barley (Hordeum vulgare L.) was investigated in polluted sod podzolic sandy loam on layered glacial sands and calcareous deep chernozem on loamy loess soils. We propose to highlight the metals’ phytotoxicity with help of the Phyto Maximum Allowable Concentration. The Phyto Maximum Allowable Concentration is a permissible level of metals for plants in polluted soil and represents the safe degree for plants in contaminated ecosystem. The Phyto Maximum Allowable Concentration gives the possibility to estimate and to forecast the danger of metals for plants as a biological object that plays a very important role in the life of ecosystem. This approach may be applied for another metals phytotoxicity assessment for other plants.


INTRODUCTION
Metals are significant environmental pollutants, and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional and environmental reasons (Nagajyoti et al. 2010, Tangahu et al. 2011, Mamatha et al. 2014).Metals, such as cadmium, copper, lead, nickel, cobalt and mercury are major environmental pollutants, particularly in areas with high anthropogenic pressure.Metal accumulation in soils and plants is of concern in agricultural production due to the adverse effects on food safety and marketability, crop growth due to phytotoxicity, and environmental health of soil organisms.The influence of plants and their metabolic activities affects the geological and biological redistribution of heavy metals through pollution of the air, water and soil (Nagajyoti et al. 2010, Gyuricza et al. 2010).
Anthropogenic metals contamination of ecosystems, as a result of the application of industrial, transport, agrarian and other technologies, causes a damage of the functioning of plants as an important component in the ecosystem (Bradl 2005 2017).However, the setting of a safe level of toxicant for the plant is also very important, because it can help to prevent and to control the negative effects of metals in the ecosystem (Ryzhenko et al. 2017a).
Today a methodology that would determine the safe concentration of metals directly for plants in the soil is absent.After all, the existing standards for the content of metals in environmental objects are sanitary-hygienic and focused just on human health (Lewis 1998, Smirnov et al. 2002, Warne and van Dam 2008).Determination of the metals safe level in the soil for plants can help to objectively assess state of the ecosystem and prevent the metals dangerous influence on plant (Ryzhenko et al. 2017b).The phyto maximum allowable concentration (PMAC) was suggested as a safe level of metal in the soil for plants.

MATERIALS AND METHODS
Spring barley (Hordeum vulgare L.) was selected as a model plant.Spring barley is one of the important cereals crop in Ukraine.Mean standard deviations, variance, and minimum, maximum, standard errors were calculated from four replicates.The experimental results were interpreted using standard statistical methods.
The soils of experimental pots were: sod podzolic sandy loam on layered glacial sands (Podzol soil) and calcareous deep chernozem on loamy loess (Chernozem soil).Sod podzolic soil has the following physic-chemical characteristics: pH salt -5.5; organic matter by Turin, Walkley-Black -0.87%, CEC -6.3 Cmol•kg.Chernozem soil has the following: pH salt -6.2, organic matter by Turin, Walkley-Black -2.89%, CEC -27.1 Cmol•kg.Control concentration of trace metals in soil (1 M HCl, mg•kg -1 ) was: Cd -0.1; Pb -0.3; Cu -0.92; Zn -2.4; Ni -1.1; Co -1.5 (sod podzolic); Cd -0.11; Pb -0.32; Cu -2.6; Zn -5.3; Ni -2.3; Co -2.5 (chernozem).The studied elements were extracted by 1 M HCl from the soils.The method of trace metals determination was thin layer chromatography (TLC).The method is based on the extraction of metal ions from solutions by diphenyldiithiocarbazone (ditizon).Complex compounds of metals (dithizonates) are formed in a certain range of pH.Further, the colored dithizonates of metals are identified by chromatography in a thin layer of the adsorbent with qualitative and quantitative determination.The method is officially recognized in Ukraine (Kavetsky et al. 2001).The thin layer chromatography for the determination of trace metals in soils and plants is not only very sensitive, but also a highly productive and inexpensive method that allows to effectively determine the six metals in the sample simultaneously. 1 M HCl was used as an extractor in the method of metal determination (TLC) for not only available, but also mobile and potentially mobile forms of metals in the soil.After all, it is known that the total form of the metal in the soil does not give a complete picture of the behavior of metals in the "soil-plant" system.Studied trace elements: Cd, Pb, Zn, Cu, Co, Ni were applied separately in the amount equal to the following concentration in the soils (Table 1).Table 1.Scheme of experiment used for the trace elements application.The investigation was conducted in green house conditions.Plants grew in plastic Mitcherlikh's pots.Soil preparation, pots filling, and trials were carried out in accordance with standard methodology (Dospekhov 1985, Medvedev et al. 1998, Gorodniy 2008).The metals were added to soil during soil preparation before filling the pots.Then, spring barley germinated seeds were planted into the pots and, at the stage of 3 leaves, the recommended population was established.
In this study, the algorithm of calculation of PMAC was proposed similar to the existing approach of calculation of maximum allowable toxic concentration (MATC) (equation 1) (Rand 1994).In the toxicology practice, the scheme to substance toxicity assessment using the lowest observed effect concentration (LOEC) and no observed effect concentration (NOEC) is quite effective and widely used (Smirnov et al. 2002, Warne and van Dam 2008, Guidance Document on… 2005, Globally Harmonized System… 2011).These indicators are also used to calculate the MATC on behalf of assessing the toxicity of substances in the aquatic environment.MATC is calculated by the following formula (Rand 1994): (1) Where: NOEC -no observed effect concentration LOEC -lowest observed effect concentration We propose to determine the PMAC by the following formula: (2) Where: C contr -background concentration (on the control variant of experiment -without additional metal input) PhLD 5 -phytotoxic 5% dose (PhLD 5 ) which caused the 5% reduction of initial weight (height, length of root, etc.) In my opinion, the 5% reduction of initial weight is the minimal effect, which is similar to the LOEC and shows the preliminary changes in the productivity of the plant population.Moreover, the level of significance of deviations, which are considered sufficient for ecological and biological research at the 5% level (p < 0.05) was chosen.The algorithm of obtaining the PhLD 5 was represented in previous papers (Ryzhenko et al. 2017b).

RESULTS AND DISCUSSION
Table 2 presents the values of PhLD 5 and PMAC for all investigated metals, as well as the background concentration in the soil (0-20 cm).PMAC was obtained with the help of equation 2. The PMAC for Cd in sod podzolic soil was calculated in this way: (3) The PMAC for Ni, Pb, Cu, Co, Zn in the two soils were calculated similarly.The lowest value of the PMAC had Cd, the highest value of the PMAC had Zn in the two studied soils.The chernozem soil had higher values of the PMAC than Sod podzolic soil.It could be explained by higher content of organic matter, granulometric composition of soil and other properties of chernozem soil.
According to the value of PMAC, the metals can be ranked in the following descending order: Zn > Сo > Cu > Ni > Pb > Cd.The PMAC could be used as an environmental standard that regulates the safe level of pollutants in the soil for plant.

CONCLUSIONS
As a result of this investigation, it was proposed to use the PMAC as a permissible level for plants in the soil in the polluted ecosystem.The algorithm of calculation of PMAC was based on the approach of the existing calculation of MATC.
The PMAC was obtained for Hordeum vulgare L. for all researched metals in the two soils (mg•kg -1 ; 1 N HCl): Cd -1.21; Cu -7.60; Co -9.77; Zn -30.77;Ni -7.40; Pb -7.48 (sod podzolic sandy loam on layered glacial sands), and Cd -1.46; Cu -13.10;Co -13.61;Zn -44.90;Ni -12.69;Pb -9.20 (calcareous deep chernozem on loamy loess).The PMAC gives the possibility to set the permissible level of metal in the soil for plant as a biological organism, but not from the point of view of hygienic regulation.The using of concept of PMAC may be suitable for receiving the permissible level of metals in different soils for other plants in polluted ecosystems.Phyto maximum allowable concentration gives the possibility to estimate the danger caused by metals directly for plants as a biological object that play a very important role in the ecosystem.
, Alloway 2010, Kabata-Pendias and Mukherjee 2007).Often, plants are the main accumulator of metals in the polluted ecosystem (Ryzhenko and Kavetsky 2017).At the same time, plants play an important role in the ecosystem as biomass producers and as biodiversity creators (Rombke and Moltmann 1996, Kabata-Pendias and Mukherjee 2007, Sardar et al. 2013).Usually, phytotoxicity is considered as a harmful influence of metal on plant growth and development (Kabata-Pendias and Mukherjee 2007, Nagajyoti et al. 2010, Satpathy et al. 2014, Gill 2014, Ryzhenko