Copper Oxide Nanoparticle and Copper (II) Ion Exposure in Oryza sativa Reveals Two Different Mechanisms of Toxicity

The effects of CuO NPs and bulk Cu at 0–1000 mg L−1 on the growth, photosynthesis and biochemical parameters were investigated in 30-day-old rice plants grown hydroponically. ICP-OES measurements showed that CuO NPs released ≤ 1 mg L−1 of Cu2+ ions compared with ≤ 81 mg L−1 by bulk Cu at their highest concentration. Both treatments showed that growth, photo-phosphorylation and carbon dioxide assimilation declined considerably. Bulk particles caused oxidative stress whereas NP showed no such effect. Electromicrographs showed that CuO NPs accumulated in chloroplasts resulting in destacking and distortions of thylakoid membranes while bulk Cu showed no such behaviour. Results suggest that NP affected the growth by accumulation in non-ionic form in chloroplasts causing damage to thylakoid membrane without oxidative damage, whereas the bulk Cu affected the growth by causing oxidative damage as a result of release of Cu2+ ions without affecting the ultrastructure of the chloroplasts.