Survival strategies of pomegranate and almond trees in a fluoride polluted area.

In the southern suburb of Sfax (Tunisia), almond and pomegranate trees grow in the vicinity of a phosphate fertilizer-producing factory. In order to reveal strategies adopted by these fruit species to live in restrictive conditions, we investigated the effect of fluoride pollution on the distribution of some elements during the growing season. The results show that calcium and magnesium play an important role in trapping fluoride and delaying the appearance of necroses. Sensitive fruit species, like the almond tree, tend to reduce (F-) toxicity by increasing leaf Mg content, following a decrease in leaf calcium. Since the Mg has multiple roles in the plant metabolism, the MgF affinity seems to be temporary and constitutes, in the long run, a limiting factor for the main physiological processes. Therefore, it appears that resistant species, like pomegranate, have the ability, during fluoride intoxication, not only to assimilate calcium ions but also to maintain Mg concentrations at an adequate level, thus ensuring the cell smooth running. When calcium is no more available, Mg is taken from the chlorophyll molecule as MgF2, and so damage may appear as marginal necroses. By comparing fluoride content of necrotic and healthy leaf slices, it appears that marginal necroses represent typical symptoms of fluoride pollution. By increasing their phosphorus concentration, sensitive fruit species tend to reduce damage intensity, whereas resistant ones tend to avoid F- toxicity.