Plants dealing with heavy metals: bioindication potential, physiological responses and stress assessment techniques

The fast urbanization in the last decades have resulted in increasing contents of trace metals in the environment. In the last years, numerous studies investigated the damages, mechanisms of uptake and stresses caused by heavy metals (HMs) in plants, in order to use these organisms to detect, immobilize and even clean metals in polluted lands. Polygonum aviculare and Senecio vulgaris for instance, absorbed Ni from soils, and were found to be good indicators of this element. However, our results indicated that many different factors influence metal uptake in plants and pH seemed to be one of the most important. Our results on Stellaria media demonstrated that pH was positively correlated with Ni uptake in hydroponic culture, conversely was negatively correlated with Zn uptake. When high concentrations of metals enter the plants, toxicity symptoms like chlorosis and stunted growth accompanied by the production antioxidant substances are commonly observed. The production of antioxidant metabolites as well as the decrease in biomass and photosynthetic pigments are usually proportional to the metal stress and this fact was demonstrated using Polygonum aviculare and Senecio vulgaris as case study. Our results demonstrated that antioxidant molecules, photosynthetic pigment production and plant biomass were mostly correlated with metal content in plant shoots. On the other hand, the only presence of antioxidant metabolites, cannot give information neither about the type of metal that caused the stress nor regarding the concentration of the stressor. If HM effects on plants are mostly known to be deleterious, when present at very low concentrations, they could also be beneficial causing the so called hormesis effect. This effect characterized by increased root and shoot biomass, higher number of nodes in the Cd treated plants wad detected in P. annua and C. hirsuta at Cd concentrations between 0.5 to 1.5 µM.