Long-term assessment of nano and bulk copper compound exposure in sugarcane (Saccharum officinarum)

As interest in the use of copper-based nanomaterials in agriculture continue to increase, research into their exposure effects must expand from short-term, high exposure studies to long-term studies at realistic concentrations. Long-term studies can better elucidate the implications of copper nanomaterial exposure by allowing plants to mature and adapt to higher copper concentrations. In this study, sugarcane plants were grown to maturity in large nursery pots using soils amended with one of the following treatments: Kocide 3000 (Cu(OH)₂), a nano-sized CuO (nCuO), a bulk-sized CuO (bCuO), copper metal nanoparticles (Cu NP), or CuCl₂ at 20, 40, and 60 mg kg^-1. After tissue harvesting, copper content in plant tissues, including pressed cane juice, were determined. Chlorophyll content and the activity of reactive oxygen species (ROS) related enzymes, in root tissues, were measured as an indicator of plant health. Elemental analysis revealed significant changes in root copper concentrations only upon application of the highest levels of Kocide 3000, nCuO, and Cu NP. However, translocation of copper to leaf tissues displayed consistent increases with added copper over controls. Plants treated with Kocide 3000 at 60 mg kg^-1 experienced a significant 31% decrease in cane juice yield; copper concentrations in the pressed juice of plants treated with: Kocide 3000 at 20 and 60 mg kg^-1, nCuO at 20 and 60 mg kg^-1, bCuO at 20 mg kg^-1, CuCl₂ at 40 mg kg^-1, and Cu NP increased by at least 58%. Chlorophyll content remained comparable to controls, and there was a significant 50 to 68% decrease in superoxide dismutase (SOD) activity in plants treated with nCuO, bCuO, Cu NP, and CuCl₂. The results indicate that sugarcane plants exposed to the selected copper-based treatments were not adversely affected.