Root potassium and hydrogen flux rates as potential indicators of plant response to zinc, copper and nickel stress

The practice of phytoremediation often requires that the plants used be exposed to a combined stress of multiple heavy metals. While the uptake and translocation abilities of roots is of primary importance, the direct effect of elevated concentrations of multiple heavy metals on the root physiology, including ion fluxes, is not often measured or compared between species in these studies. Four plant species (poplar, willow, hemp and alfalfa) that were selected for a long-term phytoremediation project were grown hydroponically in the presence and absence of a combined heavy metal stress (zinc, copper and nickel), resembling field conditions. Short- and long-term root potassium and hydrogen flux rates were measured with non-invasive ion selective electrodes. Anatomical observations of root and shoot tissues, as well as accumulations of heavy metals were made with transmission electron microscopy. Biomass, gas exchange parameters and pigment concentrations were all evaluated to assess whole-plant effects of the heavy metal treatment. Differences in the short-term induction of increased K+ efflux and decreased H+ influx from the roots among the four species were reflective of the long-term declines in photosynthetic capacity and growth observed in poplar and willow, but not in hemp. Tissue degradation patterns and increased K+ efflux in poplar, willow and alfalfa due to heavy metal stress are consistent with reports in the literature of an imbalance in ROS and efficient scavenging of hydroxyl radicals. Taken together, results indicate that ion flux measurements can predict heavy metal stress sensitivity and support their potential use for describing root-level responses to the combined contaminant conditions often observed in sites selected for phytoremediation.