Heavy Metal Toxicity: Physiological Implications of Metal Toxicity in Plants

When content of HMs, such as Cd, Cr, Ni, Pb or Zn, in soils is high, then they are taken up in excess and affect different physiological processes in plants. HMs affect growth of roots and shoots, changing their morphology and anatomy. However, at low doses HMs may stimulate plant growth, which is termed hormesis. At cellular level HMs cause changes in the configuration of the endoplasmic reticulum, higher vacuolization of cells, increase in the size of the cell nucleus, changes in the shape of the Golgi apparatus, as well as disruption of chloroplasts and mitochondria ultrastructure. HMs usually cause a decrease of chlorophyll content and significant inhibition of photosynthetic rate. The use of chlorophyll a fluorescence measurements to examine photosynthetic performance revealed that HMs affect negatively photosystems II and I, diminishing considerably such parameters as φP0. Excess of HM disturb also plant water relations. As a result strong reduction in transpiration rate (E), stomatal conductance (gs) and water use efficiency (WUE) is observed. Excess of reactive oxygen species (ROS) is produced in plants as a response to heavy metal stress. High amount of ROS cause lipid peroxidation, inactivation and/or direct damage to nucleic acids, modification of proteins and carbohydrates. In consequence, content of malondialdehyde (MDA) increases as well as activity of ROS-scavenging enzymes, such as catalase (CAT), superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and glutathione reductase (GR). Synthesis of phytochelatins is one of the plant responses to the presence of heavy metals (HM) in the environment. Phytochelatins (PCs) are synthesized from glutathione (GSH) by PC synthase (PCS) that is activated by HM ions. The role of PCs in HM stress relies on chelating metals, which are subsequently transported to the vacuole.