Resum

Lead (Pb) is a highly toxic metal even at low concentrations and is present in a great variety of soils both naturally and as a result of industrial activity. For this reason, knowing the actuation and tolerance or resistance mechanisms is of great importance to farming practices.

In order to advance in this field of knowledge a series of experiments have been set up. From the results obtained, it is important to highlight that this metal, as well as other heavy metals, presents a barrier to translocation towards aerial parts that may be attributed to multiple causes such as lead binding to mucilage, its retention in cellular walls, its accumulation in the root's apoplast and simplast or in both radicular transportation ways. As was observed by root tip dying (fluorescent dyes).

In our conditions of growth we have found that lead manifests itself in the length of the radicles even before the metal acts on the accumulation of dry matter. Its effects upon the length of the radicles could probably be a consequence of the effects upon cellular division in combination with effects on cellular wall extensibility. Many a dysfunction created by lead may be due to effects upon the integrity of the plasmatic membrane, as was observed by root tip dying (vital staining).

By determining the hydraulic parameters through pressure probe techniques we were able to show that the mechanisms that govern the hydric behavior of this organ are complex and respond to many factors and that they do not obey to action upon the aquaporins. Exposing the root to different divalent metals makes it more vulnerable to attack by other heavy metals, as has been shown by the effects added by exposing the roots to HgCl2

By using IRGA (infrared gas analyzer) techniques we have been able to show that lead at low concentrations may attack important physiological processes such as the photosynthetic activity of the plant, directly and or indirectly.

By analyzing the contents of Pb in the plant, through the exude of xylem and the nutritive solution we were able to verify that the chelating agent EDTA may effectively reduce the toxicity of lead, while at the same time finding a mechanisms of Pb detoxification through the formation of a Pb complexes probably exuded by the roots that was also present in the translocated xylem solution.

These results show that there are complex Pb detoxification mechanisms at different levels and in different plant physiological processes that need to be researched in greater depth the knowledge of which would be of great use in possible phytoremediation effects and in obtaining plant species capable of growing in Pb contaminated soils.

Key words:detoxification, lead, root, phytotoxicity, fluorescent dyes, translocation, exude of xylem, pressure probe, compartimentation, vital staining, calcium.

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