Ultramorphological and physiological modifications induced by high zinc levels in Paulownia tomentosa

The efficacy of Paulownia tomentosa in the absorption and accumulation of Zn from contaminated soils has been recently described. However, no data are available regarding the modifications induced by high levels of Zn on the anatomy and physiology of this tree species. P. tomentosa were grown hydroponically at different Zn concentrations (100, 500, 1000, 2000, 3000, and 5000 uM). The plant growth and leaf gas exchange parameters (net CO2 assimilation and stomatal conductance) were significantly reduced at high Zn concentrations. Electron and confocal microscopy analysis showed differences in the cellular ultrastructure between control and treated (above 2000 uM) plants, which exhibited an accumulation of electron-dense materials. The major toxic effects of high Zn concentrations were related to damages to the cell functionality, i.e., the chloroplast ultrastructure, which negatively affected the photosynthetic performance, thus leading to a significant growth inhibition. P. tomentosa plants are able to limit Zninduced damages by activating effective mechanisms of Zn sequestration and accumulation of excess Zn in dedicated structures, such as petiole cell walls and root hairs, or by excluding part of the Zn in exudates located on the petiole surface.