Introduction Higher plants develop several adjustments to almost all different environments. Due to their immobile structure, plants are constantly subjected to abiotic and biotic stress factors that negatively affect plant growth, development, and reproduction (Rejeb et al., 2014). The impact of unfavourable conditions on the ecosystem is increasing at an alarming rate due to the rapid increase in soil and water pollution and destruction of vegetation through the excessive release of wastes produced by human activities such as industrialization and urbanization (Nagajyoti et al., 2010; Mantri et al., 2012). Heavy metals are important environmental pollutants and their toxicity is of increasing importance due to ecological, evolutionary, nutritional, and environmental reasons. They have the greatest availability in soil and water ecosystems and present a relatively small proportion in the atmosphere as particulates or vapours. The extent of heavy metal toxicity in plants differs according to the plant species, metal specificity, concentration, chemical form, exposure type, and duration (Nagajyoti et al., 2010; Yadav, 2010). Mercury is known to be one of the most important toxic heavy metals in nature (Tchounwou et al., 2012). The source of natural mercury is from "hot spots" through natural processes such as hot springs or volcanic explosions. Although mercury is present in the earth's crust in low concentrations, human activities such as mining, burning of fossil fuels, release of industrial wastes into aquatic environments cause an increase in the mercury level in the atmosphere, soil, and water (Boening, 2000; Patra and Sharma, 2000; Lopes et al., 2013; Mahbub et al., 2017). Besides, the widespread use of mercury in chlorinealkali processes, dental amalgams, thermometers, and fungicides has contributed significant quantities of Hg into the aquatic and terrestrial ecosystems (Jarup, 2003). When mercury is released into the air, it can travel long distances and then be deposited into the water and ground.
