Lead (Pb) is second on the list of environmentally toxic heavy metals and represses plant growth at concentrations exceeding 30 ppm. Bioaccumulation of Pb during plant growth affects total chlorophyll content, fresh weight, root, and shoot length. Pb tends to accumulate more readily in plant roots than aboveground parts of the plant. However, traces of Pb will translocate to other regions of the plant, including the fruits and shoots.
Root cells detoxify heavy metals by making complexes with amino acids, organic acids, or vacuoles. These complexes help to broker the translocation of heavy metals, facilitating the protection of leaf tissues and metabolically dynamic photosynthetic cells from the harmful effects of heavy metal toxicity. Plants will employ phytochelatin synthase (PCS), which binds and relocate heavy metal ions to vacuoles. Plant exposure to heavy metal stress provokes the plant’s antioxidative systems to minimize the damage transiently in all aerobic organisms from molecular oxygen.
Reactive oxygen species (ROS) are natural byproducts of oxidative metabolism which cause damage to aerobic organisms. Photosynthetic organelles like chloroplast, mitochondria, and peroxisomes are the centers of ROS production. The toxic reactions from plant exposure to heavy metals include rapid production of ROS within the plant as the heavy metals disrupt electron transport activities of chloroplasts and mitochondria. This disruption causes membrane ion leaking as the redox status of the cells is damaged. Many ROS take on the role of critical signaling molecules that potentiate the activity of defense proteins–highly unstable and reactive molecules having a concise life oxidizing specific proteins, lipids, and nucleic acids leading to cell structure alteration and mutagenesis.
