Leaves position in Populus alba Villafranca clone reveals a strategy towards cadmium uptake response

The aim of this study was to investigate Cd uptake of Populus alba Villafranca clone applying 0, 0.5 and 1 mg Kg-1 soil DW of Cd (Cdc, Cdl and Cdh respectively) for 6 weeks. Cd allocation in the leaves, as a function of their position along the stem, was studied before its toxicity may alter physiological, biochemical and molecular biology of plants. Significant growth stimulation of roots (2.5 and 2.2 times more than control at Cdl and Cdh, respectively) and basal leaves (+79% and +55% at Cdl and Cdh, respectively) was recorded when Cd was accumulated in a relatively higher concentration compared to other organs. In these experimental conditions, Villafranca clone can be considered a ‘‘root accumulator’’; in fact Translocation factor (Tf ) calculated at leaves and stem levels were below 1. Even if Cd distribution among leaves did not change in function of their position, only apical leaves of Villafranca clone didn’t show changes in AsA and GSH concentration after Cd exposure as indication of different mechanism of Cd scavenge ability. A different expression of the genes related to antioxidant machinery such as GST, GPX and cAPX was observed in apical leaves, indicating that a strict regulation of the antioxidant defence system is required by poplar plant under Cd. All these energy consuming mechanisms were not found in more aged median and basal leaves. This could be a strategy adopted by Villafranca clone to avoid Cd effect in apical leaves, allowing plants to continue their growth without damages.