Modulation of Heavy Metal Resistance in Raphanus sativus L. by 5-ALA Involves the Chlorophyll and Reactive Oxygen Species Scavenging Mechanisms

Plants undergo different types of biotic and abiotic stresses during their life span. Excess of heavy metals (HMs) in soil is considered as one of the most challenging threats to modern agricultural developments and overall yield of crops. Increased level of metals in soil is a global concern because of their deleterious effects on plants growth and sustainability. Plants absorb these heavy metals from soil which further disrupts crop production and enhances the accumulation of metals in edible parts. Amongst various heavy metals, lead (Pb) is highly toxic which adversely affects the plant metabolism. Pb was widely practiced in carvings, ornamental substances, batteries, weapons and radioactive shields and also in car fuel with adequate metereological dissipation for several decades. In some anthropologically contaminated sites with high concentration of Pb including road site, polluted industrial area, flood sensitive areas (He et al., 2017, Egendorf et al., 2018). However, Pb translocated in the agroecosystem by means of contaminated soils and then into food chains (Ashraf et al., 2017). Pb exists in different oxidative forms which allows them to undergo different mechanisms of absorption and transformation in plants including biosorption, ion exchange, assimilations in plants and complex formation (Komarek et al., 2013). Pb migration depend upon the eco-physiology of soil such as oxido-reductive and alkylation/ dealkylation properties of soil (Kushwaha et al., 2018). After adsorbed at root surface, Pb ions translocate into the vacuoles through active efflux, organic/inorganic complexes formation and metal chelating stress proteins such as metallothioneins and phytochelatins (Gupta et al., 2013).