Split-root investigation of the physiological response to heterogeneous elevated Zn exposure in poplar and willow

Essential and toxic ions are often heterogeneously distributed within a soil landscape. This fact may complicate phytoextraction efforts to remove toxic ions such as heavy metals from the soil as the roots of some plants instead may colonize more favorable microenvironments and avoid heavy metal accumulation. In this study a split-root hydroponic experiment was used to test whether root-level exposure to zinc (Zn) stress to none, half or all of the root systems of Populus deltoides Bart. ex Marshall x Populus nigra L. (poplar) and Salix matsudana L. (willow) plants alters root and shoot physiology. Cuttings of both species were split to create two root system halves that were exposed to either control (-Zn) or stress (+1 mM Zn) solutions for 42 days. Weekly measurements of gas exchange, photosystem fluorescence parameters and pigments showed that poplar was more sensitive to the half stress (-Zn/+Zn) treatment than willow, with lower gas exchange rates and total chlorophyll and carotenoid values relative to their respective controls (-Zn/-Zn). Half stress willow plants had increased root growth in the half without Zn, demonstrating a reallocation of carbon to compensate with no negative impact to shoot physiology as compared to poplar. Water consumption was also reduced in half stress willow but not in poplar, suggesting an increase in water-use efficiency in response to heterogeneous Zn exposure that was not derived from dramatic reductions in transpiration rate. These significant differences between these two species in response to patchy distribution of toxic Zn concentration may impact the efficiency of phytoremediation efforts and suggests additional criteria to consider in the pre-selection process.