Heavy metals in cannabis: plant contamination and effects on cannabinoid production

Exposure of cannabis to heavy metals may interfere with metabolic pathways of cannabinoid biosynthesis and compromise its medical quality. Furthermore, inflorescence contamination with heavy metals presents a critical challenge for the production of safe pharmaceutical-grade cannabis, since it poses a significant health risk to consumers. The present study therefore evaluated the hypotheses that heavy-metal exposure affects cannabinoid production, leads to inflorescence-contamination, and compromises the cannabis plant function; and that the responses are genotype-dependent and heavy-metal dose-dependent.

To evaluate the hypotheses, we studied responses of four 'drug-type' medical cannabis cultivars to a cocktail of four heavy-metals (Cd, Pb, Ni, Co), in three concentrations each (0, 1, 5µM), and analyzed translocation and accumulation patterns of the heavy-metals in the plant organs, and the resulting impact on cannabinoid production and the plant's physiological integrity.

The results confirmed effects of the heavy metals on cannabinoid production, with a heavy-metal concentration threshold, and genotypic variability, thus confirming the hypotheses.

The roots accumulated the highest levels of heavy metals, demonstrating an avoidance strategy of exclusion from sensitive shoot organs; and the root-to-shoot translocation factor was Ni > Cd, Co > Pb demonstrating heavy-metal specificity. The accumulation patterns revealed that plant exposure to moderate-low heavy metal concentrations (5µM) poses health concerns, as the inflorescences' Cd and Ni concentrations were above the WHO-permitted threshold for medical plant consumption.