Lead Toxicity in Plants.

This review looks critically at the relevance of lead (Pb2+) toxicity and proposed mechanisms of Pb2+ -induced stress in algae and higher plants. As a basis, the current main sources of Pb2+ contamination in the environment are presented, which include agriculture, industry, and road traffic. Further, bioavailability of lead is discussed as a basis for evaluating the environmental relevance of the many studies on lead toxicity that have been published in the past decades. These studies suggest three main mechanisms of toxicity of Pb2+: inhibition of photosynthesis, oxidative stress, and "genotoxicity" including DNA damage and defects in mitosis. Looking at the applied concentration ranges in these studies reveals that likely the defects in mitosis are the environmentally most relevant effects. In contrast, inhibition of photosynthetic light reactions is far less efficient with Pb2+ compared to other metal ions, so that for Pb2+ toxicity it seems environmentally not relevant. As a conclusion and outlook, a direction of future studies towards establishment of reliable concentration thresholds of the various toxic effects and their causal interconnection is suggested.