Populus alba tolerates and efficiently removes caffeine and zinc excesses using an organ allocation strategy

Mixed inorganic and organic contaminations are one of the main challenges in phytoremediation, due to the higher complexity derived by pollutant interactions and the increase of phytotoxicity. The cultivation of fast-growing poplars for removing contaminants from water could be a low-cost and flexible choice. The main objective of this study was to evaluate the tolerance of a poplar species (Populus alba ‘Villafranca’ clone) to irrigation with water contaminated with zinc (Zn) and caffeine (CFN). Poplars were maintained in hydroponic and exposed to four different treatments (Control, CFN, Zn and Zn + CFN) over 7 days. Poplar showed a good tolerance to Zn and CFN treatments, without any symptom of phytotoxicity. However, the type of treatment affected the contaminant dynamics in the plant-water system and a pollutant partitioning was observed among organs, with a higher accumulation of Zn in root (472 ± 128.7 mg kg− 1 DW) and CFN in shoot (30 ± 4.5 µg g− 1 FW). Under mixed condition, the CFN uptake significantly increased in root (+ 40%) and stem (+ 28%) while the Zn concentration decreased in leaves (-19%). A focus on the potential role of natural resistance-associated macrophage proteins (NRAMPs) in divalent metal transport has been performed. A down-regulation of NRAMP1.3 was detected in roots of plants exposed to CFN treatment in relation to an increase of Mn concentration. Data confirmed the suitability of Populus alba for the remediation of multi contaminated water.