Systemic Phytotoxic Impact of as-Prepared Carbon Nanotubes in Long-Term Assays: A Case Study of Parodia ayopayana (Cactaceae)

To evaluate long-term phytotoxicity impact of as-prepared (or pristine) single-walled and multi-walled carbon nanotubes we designed two different experimental setups with the use of young seedlings of Parodia ayopayana, a small and slow-growing Cactaceae species: (a) soil-based experiment, which allows for a better match of the real environmental situation; and (b) in vitro experiment on the solidified culture medium in vertically-oriented Petri dishes, which gives an opportunity to monitor the development of both shoots and roots. After observations of plant growth for 22 (soil-based experiment) and 16 (in vitro experiment) weeks, we concluded that plant height, width and total root length were affected in both soil- and in vitro-grown plants, with a stronger effect in the latter case. Single-walled carbon nanotubes exhibited higher phytotoxicity as compared to multi-walled carbon nanotubes and nanodiannond (employed for comparison in in vitro-based assay), likely due to a high amount of impurities. The most interesting finding was the reduction in primary spine length and underdeveloped micrometer-sized secondary spines, which suggests a strong and systemic phytotoxic impact of carbon nanotubes (and to a lower degree of nanodiamond) on plant growth and development.