Lead sequestration, immobilization and xylem cavitation in melatonin-primed Amaranthus cruentus

Cultivation of Amaranthus cruentus along highways and local dumping sites in Nigeria has posed a threat to the health of humans and animals. This study aimed at evaluating the effects of melatonin in Lead (Pb) absorption by A. cruentus and its related anatomical and biochemical changes. Sterilized seeds of A. cruentus were primed in varying concentrations of melatonin (0 and 400 mu M to represent M0 and M400) for 24 h. Primed seeds were planted into sterilized soil and watered with different Pb concentrations (0, 10, and 20 mM to represent P0, P10, and P20) until maturity. Melatonin significantly (P < 0.05) reduced Pb accumulation in the root by 33% and Pb translocation to the leaves by 67%. Plants treated with 400 mu M of melatonin under high Pb treatment (20 mM) showed thickened root epidermis and 70% increase in root vascular bundles compared with its control (M0). However, an increased number of staggered root xylem was observed in all plants treated with melatonin which obstructed the movement Pb from the root to the shoot. Melatonin sequestered, localized and immobilized Pb at the root of A. cruentus, as reflected in the thickness of the root epidermis at P20. More so, at 400 mu M of melatonin, the size of root cuticle was increased and lipid peroxidation lowered in the Pb stressed plant. Also, 400 mu M of melatonin and Pb showed strong synergistic effects that caused the disappearance of trichomes, breakage of the concentric arrangement of the root xylem, which could lead to xylem cavitation. Consequently, increased hydrogen peroxide and reduced glutathione peroxidase were observed during Pb stress.