Tissue localization and the physiological effects induced by an environmentally relevant mix of heavy metals in the liverwort Conocephalum conicum L. Dum

Tissue accumulation, ultrastructural alterations, oxidative stress, and effects on photosynthesis were assessed in the liverwort Conocephalum conicom exposed in vitro to heavy metals (HM) concentrations in three sites of the Savone River, representative of different anthropic impacts. The uptake and accumulation of HM in the thallus of the liverwort was first studied, and the biological effects in relation to the different accumulation sites of HM along thallus, ribs and wings, were then investigated, considering: bioaccumulation (by atomic absorption spectrometry), localization (by X-ray scanning electron microscopy microanalysis), ultrastructural damage of photosynthetic parenchyma (by transmission electron microscopy), oxidative stress (by ROS contents and antioxidant enzymes activities determination), photosynthesis (by chlorophyll fluorescence). The results showed the HM bioaccumulation in C. conicom was dependent by their concentrations in the contaminated water. As for spatial localization, HM preferentially accumulated in the nerve of gametophytes respect to the wings. With respect to tissue localization, HM were mainly found in the hyaline and in the photosynthetic parenchyma. Essential metals (Cu and Zn) were accumulated at higher concentrations with respect to non-essential metals (Pb and Cd). At the ultrastructural level, HM caused alterations of the fine structure of the cells, most evident along the nerve, inducing marked alterations of the chloroplast structure and therefore of the photosynthetic capacity. Based on the results of the presented study, C.conicum can be used as a marker to indicate heavy metal pollution in water natural resources.