Metals can be phytotoxic because oftheir capacity to trigger some specific physiological mechanisms which may induce plant death. Excess of metals are able to interact with amínoacid functional groups,leading to protein misfunction, to induce an excessíve production offree-radicals, highly toxic due to their high oxidative capacity (lipoperoxidation as one of the most important expressions) and to alter the antioxidative defence system.
Nevertheless, some plants are able to tolerate the metal excess and live normally even under a metallic stress condition. This tolerance -mainly genetically regulated- to the metal stress is due to some physiological mechanisms, triggered by the metal stress, among them being the next ones:
- metal exclusion, due barriers that impede a metal excessive uptake and keep the intracell metal content within normal values; this may happen by a root-Ievel barrier and/or by a root-induced alkalinization of its very close surrounding,
- accumulation and internal detoxification, meaning the neutralisation of the metal excess activity by means of inducting a synthesis of antioxidative defences (which react with the excess of free-radicals), proline (an aminoacid accumulated in plants as a response to different stress conditions), organic acid (meaning metal transport from the protoplasm to the vacuole where it is accumulated in insoluble forms), metalotioneins (genetically synthesised aminoacids having cistein-rich regions, also presented in animals exposed to metal stress, which are able to complex metals) and phytoquelatins (enzimatically synthesised polipeptides), and
- metal excretion, mechanisms that allow the plants to eliminate the metal excess to the external environment. |
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