Influence Mechanism Of Nano Titanium Dioxide On The Toxicity Effects Of Chinese Cabbage Under Cadmium Stress

Nano titanium dioxide?TiO₂?is one of the most widely used nanomaterials,which can enter into farmland ecological environment through several avenues such as sewage irrigation,waste landfill,fertilizing,and sludge application to farmland soils,these nanomaterials may alter the ecotoxicity of co-existing contaminants,such as Cd.As one of the most widely eaten and cultivated green leafy vegetables,Chinese cabbage is easy to absorb Cd from the soil and transfer it to edible parts.However,the influence of nano TiO₂ on the absorption,accumulation and toxicity of Cd in Chinese cabbage are still not fully understood.Therefore,this study investigated the absorption and transport of nano TiO₂ and Cd in Chinese cabbage?Brassica rapa var.glabra?and their joint toxicity through hydroponic experiments.The influence mechanism of nano TiO₂ on the toxic effect of Chinese cabbage under Cd stress was further discussed.The results showed that:?1?Nano TiO₂ can penetrate into the roots through the symplast pathway and accumulation,and transported upward to the blade;Nano TiO₂ was mainly distributed in the form of particle aggregates in the cell walls,cytoplasm,vacuoles and chloroplasts of the root and leaf cells of Chinese cabbage.?2?The root morphology of Chinese cabbage seedlings was abnormal,the root became shorter and thicker,the number of root tips and the root surface area were decreased significantly under the treatment of nano TiO₂,which lead to the significant decrease on the biomass of Chinese cabbage.The chloroplast structure was not significantly damaged under nano TiO₂ stress.When the treatment concentration of nano TiO₂ was 50 mg·L^-1,the chlorophyll content significantly increased,promoting the photosynthesis of Chinese cabbage.Nano TiO₂ induced the production of ROS and activated the antioxidant of SOD?CAT and POD and alleviated the damage of ROS on cell membrane.?3?When the concentration of Cd was 1 mg·L^-1 and 10 mg·L^-1,the bioaccumulation of Cd in Chinese cabbage was increased with the increase of nano TiO₂ concentration.Compared to Cd treatment alone,the content of Cd in the roots of Chinese cabbage significantly increased by 44.13%and 61.43%?P<0.05?,when coexisting with nano TiO₂ at 20 mg·L^-1 concentration.At lower Cd concentration(1 mg·L^-1),nano TiO₂ showed a significant promoting effect on SOD and POD activity in leaf tissues of Chinese cabbage,but had no significant effect on fresh weight,chlorophyll content and oxidative damage index of root system.At higher Cd concentration(10mg·L^-1),the activities of SOD,POD and CAT in leaves and root tissues of Chinese cabbage were significantly reduced when nano TiO₂ was 20 mg·L^-1,the cellular structure was severely damaged and the plant biomass and chlorophyll content of Chinese cabbage seedlings were significantly reduced.Therefore,nano TiO₂ increased the toxicity of Cd to Chinese cabbage.The results of the study revealed the mechanism of the biotoxic effect of Nano-TiO₂ on Cd,that is,nano-TiO₂,as the carrier of Cd,significantly promoted the accumulation of Cd in Chinese cabbage,induced membrane lipid peroxidation,destroyed the antioxidant defense system and cell ultrastructure of Chinese cabbage,and led to oxidative damage.Therefore,although the toxicity of nano-TiO₂ is relatively low,there is a risk of increasing the toxicity of coexisting pollutants to plants when coexisting with other pollutants.It can provide scientific basis for the country to establish the safety assessment and prevention system of nano materials,and provide risk prediction direction for ensuring the national food security and the healthy development of modern agricultural ecosystem.