To ameliorate heavy metal stress in plants, the cost effective approach is phytoremediation. Phytoremediation use to fix, accumulate or degradation of contaminants from soil by using green plants. The term phytoremediation mean “plants are able to restore or cure”.
Heavy metal occurs naturally in biosphere as a result of volcanic eruption and weathering of rocks. It can also added into the environment by the burning of fossil fuel, mining and undeveloped sewerage system. Heavy metal cause stress in plants through a series of events i.e. leaf rolling, stomata resistance, leaf falling and increases the degree of deposition of suberin in root of the plant. It decreases the number and size of leaf and stomata and also the diameter of xylem.
Heavy metals also effects the root hydraulic conductivity. Functioning of plants (like biological and physiological) are affected by heavy metals. Plant show morphological and metabolic changes in response to metal stress. Heavy metals like Cd (cadmium), Cu (copper), Pb (lead) and Zn (zinc) are responsible for the polluted soil.
Nickel (Ni) and cobalt (Co) are used in metallurgical industry. Cd restrict plant growth and alter their physiological and biochemical processes like maintaining of water, nutrient uptake and electron transport. Cr prohibit the electron transport, reduce CO2, water potential increase transpiration rate and induced abnormalities in the chloroplast. Cr and Cd interfere with the functioning of micronutrients.
In oat plant tissue, observe that high amount of Cr in soil decrease nitrogen (N) contents and increase phosphorus (P) contents. P compete with Cr for surface site and iron (Fe), sulphur (S) and manganese (Mn) are competing Cr for transport binding. Cd reduces the availability of microbes and essential elements in soil.
Cd and Ni decreases the photosynthetic activity in plants. Cu, Pb and Zn stress cause hyperactivity of antioxidant enzyme in plants. Pants are classified into three categories according to their growth in metal enrich soil.
- Excluder; are those in which metal concentration across a wide range of soil maintained at low level in the shoot.
- Accumulators; are those which accumulates heavy metal in above ground plant parts.
- Indicators; are those in which external amount can be calculated respect to internal concentration.
High heavy metal concentration from shoot to root in hyperaccumulators. Non-hyper accumulators have high concentration in root rather than shoot.
Why phytoremediation
To ameliorate heavy metal stress in plants, the cost effective approach is phytoremediation. Phytoremediation use to fix, accumulate or degradation of contaminants from soil by using green plants. The term phytoremediation mean “plants are able to restore or cure”.
Mechanism involved in uptake, translocation and storage of micronutrients are same that are involved in heavy metal translocation and storage. For chemically polluted land vegetation play an increasingly important ecological and sanitary roles.
Proper management can also restore natural environment. Native plants are better for phytoremediation because they are better fitted in survival, growth and reproduction under higher metal stress. Plants are selected on the bases of their root depth, nature of soil and climate of that area.
Grasses are more preferable than other ones. The highest uptake of Cd is attained by Argemone mexicana stem followed by Calatropis procera. Phytoremediation of Cd from soil with the help of Brassica juncea along with Saccharum spp., Solanum spp., Lycopersicon spp. Helianthus annus, Brassica nigra and Lycopersicon esculentum are documented for effective management against Cu.
These plants accumulate Cu in their root and shoot. Pb is Phyto-remediated by sunflower (Helianthus annus). Rice is best suited for phytoextraction of Zn. Brassica juncea and Brassica carinata are tolerant to Ni. Panikum (Panicum antidotal), napier grass (Pennisetum purpureum), cotton (Gossypium hirsutum) and sunflower (Helianthus annus) are effective for Co and Cr removal from soil.
Processes of phytoremediation
There are four processes of phytoremediation for heavy metal contaminated soil. Phytoextraction, phytostabilisation, phyto-volatilization and phyto-degradation. Phytoextraction is maintained by using tolerant plant that accumulate metals transfer and stored in ground parts. Phyto-stabilisation is fixing heavy metals by plants through the adsorption, precipitation and reduction of root.
Phytovolatilization is absorbing the metal and transfer into gaseous state by chemical secreted by root. The positive phases of phytoremediation are more effective, environmental friendly and applicable to a wide range and pleasing method. Using this technique, we can ameliorate the heavy metal effected soils.
This article is jointly written by Binish khan, Muhammad Ishfaq, Shakeel Ahmad Anjum, and Saba Durrani. The authors are from Department of Agronomy, University of Agriculture, Faisalabad. Corresponding author Muhammad Ishfaq can be reached at ishfaq2727@gmail.com
