Assessment of the phytotoxicity of HNTs on two plants, Cynodon dactylon and Brassica rapa L†

Nanomaterials show promising potential as agrochemical carriers, but the safety of the nanoparticles toward crops should be assessed systematically. Here, Cynodon dactylon and Brassica rapa L were used as plant models to study the phytotoxicity of halloysite nanotubes (HNTs). The effects of HNTs on the growth for two different plants were compared using the leaf spraying method. The potentially toxic effects of HNTs were assessed using higher concentrations than actual applications (0.1, 1, 10, 50, 100, 200 mg mL⁻¹). A concentration of 50 mg mL⁻¹ of HNTs will inhibit the germination of Cynodon dactylon, but the concentration of HNTs could reach 200 mg mL⁻¹ to inhibit the germination of Brassica rapa L. During a short growth period, when the concentration of HNTs is 10 mg mL⁻¹, they have a significant inhibitory effect on Brassica rapa L. However, when the concentration of HNTs exceeds 50 mg mL⁻¹, they exhibit an inhibitory effect on Cynodon dactylon. The reason for the difference between the two plants may be that under the same conditions, the larger leaf area of Brassica rapa L (broad-leaved plant) can be exposed to more HNTs. HNTs covered and blocked the plant stomata, and they can enter the plant body through the stomata. However, the Cynodon dactylon stomata was not observed from SEM results. The varied degrees of cell damage lead to different plant growth states. Therefore, HNTs are more toxic to Brassica rapa L than to Cynodon dactylon. These results provide evidence for design of nutrient and pesticide delivery systems based on HNTs.