Atmospheric Pb induced hormesis in the accumulator plant Tillandsia usneoides

While numerous studies reported hormesis in plants exposed to heavy metals, metals were commonly added in the growth substrate (e.g. soil or solution). The potential of heavy metals in the atmosphere to induce hormesis in plants, however, remains unknown. In this study, we exposed the widely-used accumulator plant Tillandsia usneoides to 10 atmospheric Pb concentrations (0-25.6 μg·m^-3) for 6 or 12 h. Three types of dose-response relationships between different response endpoints (biomarkers) and Pb concentrations were found for T. usneoides. The first was a monophasic dose response, in which the response increased linearly with increasing Pb concentrations, as seen for metallothionein (MT) content after a 6-h exposure. The second and dominating type was a biphasic-hormetic dose response, exhibited by malondialdehyde (MDA), superoxide anion radical (O₂^-), and superoxide dismutase (SOD) after 6 or 12 h of exposure and by glutathione (GSH) and MT content after 12 h of treatment. The third type was a triphasic dose response, as seen for leaf electric conductivity after 6 or 12 h of exposure and GSH after 6 h of exposure. This finding suggests that Pb inhibited the response of T. usneoides at very low concentrations, stimulated it at low-to-moderate concentrations, and inhibited it at higher concentrations. Our results demonstrate diverse adaptation mechanisms of plants to stress, in the framework of which alternating between up- and down-regulation of biomarkers is at play when responding to different levels of toxicants. The emergence of the triphasic dose response will further enhance the understanding of time-dependent hormesis.