Imaging the distribution of nutrient elements and the uptake of toxic metals in industrial hemp and white mustard with laser-induced breakdown spectroscopy

Abstract

Considering the quickly growing cannabis farming industry worldwide, it is important to do research on the ability of hemp to grow in heavy metal-contaminated soil. Hemp is a hyperaccumulator of heavy metals such as Cadmium (Cd) and Lead (Pb) and it can grow in soils with high concentrations of heavy metals. To gain more insight into the uptake and translocation of Cd and Pb in Cannabis sativa L., we exposed three-day old seedlings to solutions of varying concentrations of Cd and Pb for three days. Subsequently, we performed toxicology studies and analysed the localization and total uptake of heavy metals as well as essential and trace elements using Laser-Induced Breakdown Spectroscopy (LIBS) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). We subsequently conducted a toxicity test and LIBS bioimaging on the reference plant Sinapis alba L. A low concentration of 20 μM of CdCl2 and PbCl2 was toxic for S. alba, while for C. sativa a concentration of 500 μM of CdCl2 and 50 μM of PbCl2 decreased the growth of the plants. According to the 2D bioimaging LIBS maps, heavy metals in all concentrations accumulated mostly in roots, translocation to stem and leaves was not observed. These results were confirmed by ICP-OES. Additionally, we were able to do bioimaging of important elements (e.g., Ca, Cu, Mg and Sr) in the plants. Based on the data from LIBS and ICP-OES, we were able to create a calibration curve for Pb. There, the Pb intensities distributed over a number of pixels in LIBS maps were correlated to concentration values from the ICP-OES.