Investigation of Bioaccumulation Capacity of Barley for Soil Lead and Chromium under Drought Stress Conditions

The use of sewage sludge, which is mixed locally with poultry waste and is available at a relatively low cost, improves the circulation of nutrients and organic matter in the soil, reduces the concentration of CO2 in the atmosphere, and increases the level of soil organic carbon. Fertilization with this method is of particular importance in soils of arid and semi-arid regions that face erosion and organic matter reduction. However, there are concerns about the presence of essential and unnecessary heavy metals such as Cd, Cr, Cu, Ni, Pb, and Zn that enter the environment from domestic, light industrial, commercial and municipal wastewater sources and can lead to soil contamination and eventually enters the food chain through absorption, transport, and accumulation in agricultural and non-agricultural products and has threatened human and animal health. Phytoremediation is the cleaning up of polluted terrestrial areas and aquatic sites from heavy metal and organic contaminants by green plants. An appropriate plant for phytoremediation should ideally have a high ability to translocate contaminants into the plant shoot. However, the toxicity of the remains of these plants has become a severe problem for human health. Iran is an arid and semi-arid country and many soils face the problem of using animal manure sources with sewage sludge and the possibility of contamination with heavy metals. Farmers cultivate the barley plant (Hordeum vulgare L.) in these areas widely, and it has a significant role in the food chain of livestock and humans. Therefore, in this study, we evaluated the barley plant in terms of lead and chromium accumulation by increasing drought levels in the field.

It was a two-year field experiment with three irrigation levels (irrigation per 100 (control), 75 and 50% of field capacity). The amount of chromium and lead in soil and plant samples was measured using atomic spectroscopy with flame mode after extraction by digestion in acid. We used bio-concentration coefficients including root bioaccumulation factor ( ), shoot bioaccumulation factor ( ) and translocation factor ( ) to measure the plants bio-accumulation capacity. A plant with a root bioaccumulation factor bigger than one and a bio-translocation factor of less than one is suitable for plant stabilization of elements. In contrast, a plant with a shoot bioaccumulation factor and bio-translocation factor of more than one and root bioaccumulation factor of less than one is suitable for plant extraction of elements from the soil.

After barley harvest, the average concentration of lead and chromium in soil decreased by 23% and 17% compared to before harvest. The results of the analysis of variance showed that the effect of experimental years was significant on the concentration of chromium in the soil and the aerial part of barley and shoot bioaccumulation and root bioaccumulation factor of the same elements in the barley (p<0.05). The effect of drought was significant on the shoot and root dry weight, chromium concentration in both shoots and roots, lead concentration in shoots, lead and chromium shoot bioaccumulation factor, chromium root bioaccumulation factor, and lead bio-translocation factor (p<0.01) and chromium bio-translocation factor (p<0.05) but the interaction effect of year and drought was not significant on any of these traits. In all cases, the concentration of elements in the roots was higher than the aerial part, and with increasing drought stress, the concentration of lead in the roots remained constant but increased in the aerial parts while the concentration of chromium decreased. As the amount of drought increased, the shoot bioaccumulation factor increased for the lead but decreased for the chromium. The root bioaccumulation factor of chromium also decreased while the translocation factor increased for both elements, but the increase was more pronounced for the lead. Lead shoot bioaccumulation factor decreased linearly with an increasing dry weight of aerial parts (β = -0.86), but chromium shoot bioaccumulation factor increased (β = 0.62). Root dry weight predicted chromium root bioaccumulation factor (β = 0.85). The total plant dry weight regression model could predict the lead translocation factor (β = -0.89) and chromium transfer factor (β = -0.67).

In this experiment, the studied bioconcentration coefficients were all less than one. So, barley is an avoidant plant when encountered with lead and chromium in the soil, and in increasing drought conditions in the field, it does not translocate these toxic metals to the food chain.