Ecophysiological response to water stress and heavy metals uptake in Salix alba L. cuttings

Heavy metals accumulation in soils as a result of human activities is constantly increasing. At this regard, phytoremediation (i.e. the use of plants to remove pollutants from the environment or to render them harmless) is attractive as it offers site restoration, partial decontamination, maintenance of the biological activity and physical structure of soils, and is potentially cheap. In a short-term experiment, cuttings of Salix alba L. were treated with a mix of heavy metal chlorides, consisting of copper (Cu), zinc (Zn), cadmium (Cd) and lead (Pb), in order to determine the physiological response and the uptake capacity of this species under different conditions of water availability. In fact, the willow tree is a typical species of riparian zones, and could be subjected to variable water supply due to the hydrological regime of the river (constant flow or torrential regime). The amount of absorbed metals allocated in biomass of roots, stems and leaves was determined by atomic absorption spectroscopy (ASS). Translocation (TF) and bioaccumulation (BAF) factors were also determined. Under non limiting water conditions, a phytotoxic effect of heavy metals was evident on gas exchanges, though a reduction of net photosynthesis mostly caused by metal-induced stomatal limitations similar to those induced by water stress. Moreover, at the end of the experiment, chlorophyll “a” fluorescence measurements (JIP-Test) highlighted a reduction of the number of active reaction centres per leaf cross section in the metal-stressed plants; this was followed by increase of the efficiency of the electron transport processes that, according to the the Lichtenthaler model of plant response to stress, can be interpreted as a response mechanism aimed at sustaining detoxification and repair processes. The analysis of heavy metals concentration in roots, stems and leaves of plants subjected to different water supply showed TF values < 1 in all sets; moreover, water shortage limited metal uptake and translocation to leaves. In conclusion, although this species cannot be considered an iperaccumulator due to its low TF, it could be used to limit heavy metals dispersion in the environment, and to restore riparian zones at polluted sites.