A necessary precaution must be taken, however: because part of the metals remain likely to move towards the biomass, this woody material must be treated in controlled combustion units, that is to say, equipped with specific filters. This is the recommendation of Aricia Evlard with regard to current legislation overseas because here in Belgium there are no regional regulations governing this subject.
The endophytes show themselves
Subsequently, in order to increase knowledge about the phytoremediation potential of the willow, Aricia Evlard concentrated on the mushrooms present around the roots. What role do these play in the fixation of metals? How exactly do the fungal cells act? Does their presence slow down the flow of metals to the plant (acting as a barrier) or, on the contrary, does their presence facilitate the flow of metals to the above-ground plant surfaces?
With her team, the doctoral student concentrated on the job of perfecting an in vitro protocol aimed at assessing the sensitivity of the fungal strains associated with the roots of alder and willow trees. First she made a collection of 91 fungal strains taken from the banks of the non-navigable watercourses of Wallonia. The objective: to test the tolerance of these strains in vitro by subjecting them to various concentrations of cadmium (1, 5, 10, 25 or 50 mg per liter of culture medium) this heavy metal was chosen due to its high level of toxicity. The first surprise: against all expectations, the majority of the strains showed themselves to be perfectly tolerant to the metal, even for a maximal dose of 50 mg. “I was expecting lethal effects. Yet this was certainly not the case”. Second surprise: “among the most resistant strains, I discovered endophytes, more particularly ‘Dark Septate Endophytes’, that is to say, mushrooms belonging to the sub-branch of ascomycetes and which live inside roots ; they are not parasites nor are they really symbiotic (2).
![Willow Mushrooms]( "Willow Mushrooms.")
It seems that tolerance to these high doses of cadmium can be explained by a phenomenon of absorption of melanin present in the walls of the mushroom. “We can reasonably suppose that this absorption phenomenon can also be observed for other metals, but this remains to be established. We have already reproduced this procedure with Zinc: the results are in the process of being interpreted. In any event, we can, at this stage, posit the theory that the role of this microflora in the control of metals could be important in the willow and alder trees. It would be interesting to test the most highly resistant strains by inoculating them with woody species used in phytoremediation”.
Finally, in the light of this work, what hopes can we place in phytoextraction and phytostabilisation? “It must be remembered that these two approaches are slow and are spread out over several years. They are only useful when used as a complement to traditional physico-chemical techniques when there is persistent residual pollution. In addition, these more radical techniques very often disturb the structure of the soil and leave it very fragile. Phytoremediation projects by means of the willow tree can help to compensate for the loss of natural resources and the damage caused by traditional techniques. It is in this sense that phytoremediation can be considered as a complementary and/or compensatory measure, described in the ‘soil’ legislation recently passed in Wallonia».
