Effects of foliar-spraying zinc and amino acid on cadmium uptake and accumulation in rice

Cadmium (Cd) pollution in paddy soil has posed a great threat to human health in recent years. The foliar resistance control technology can be expected to effectively realize the simultaneous remediation and production of the Cd-contaminated paddy soil, due to its economy, high efficiency, and low risk of secondary pollution. Among them, the foliar spraying of zinc (ZnSO₄) or amino acid can reduce the toxic effects of Cd elements on plant growth. This study aims to clarify the effects of foliar-spraying zinc and amino acids on the uptake and accumulation of Cd in rice leaves. A field experiment was also conducted in the paddy soil from Liuyang City, Hunan Province, China. A systematic investigation was then determined the antioxidant enzyme activities, subcellular distribution of Cd, as well as Cd uptake and accumulation in the rice at tillering and filling stages. Results showed that the foliar spraying of zinc and amino acid significantly increased the antioxidant enzyme activities and the proportion of Cd in the soluble components, whereas, the uptake and accumulation of Cd were reduced in the rice. The superoxide dismutase (SOD) activity and soluble protein content in the rice leaves that were treated with the foliar spraying of zinc and amino acid (A1X1 and A2X2) at tillering stage significantly (P<0.05) increased by 22.4%, 15.4% and 33.0%, 22.0%, respectively, compared with the control (CK). Meanwhile, the content of malondialdehyde (MDA) in the rice leaves at the tillering and filling stage significantly (P<0.05) decreased by 30.3%, 34.4%, and 18.9%, 37.8%, respectively. Compared with the CK, the proportion of Cd in the soluble components in the rice leaves under the A1X1 and A2X2 treatments significantly (P<0.05) increased by 65.6% and 87.3%, respectively, at the ripening stage. While the proportion of Cd in the cell wall components decreased by 32.0% and 43.6%, respectively. At the same time, the contents of Cd in the rice stems, leaves, husks, and brown rice under the A1X1 and A2X2 treatments significantly decreased by 58.3% and 94.2%, 74.5% and 81.0%, 72.7% and 86.6%, 71.1% and 73.1% (P<0.05), respectively, compared with the CK. The Cd bioaccumulation factor in brown rice decreased significantly (P<0.05) by 37.4%-70.9%, compared with the single spraying of amino acid or zinc. The foliar spraying of zinc and amino acids effectively prevented the uptake and accumulation of Cd in the rice. Therefore, the antioxidant enzyme in rice leaves was regulated to increase the fixation of Cd. The soluble components in cells were promoted to reduce the transport of the Cd from the leaves to the husks in the brown rice. This finding can provide a theoretical basis and technical support for the safe utilization of Cd-contaminated rice fields. However, the Cd content in the brown rice under the foliar-spraying amino acid and zinc was still higher than the food pollutant limit (0.2 mg/kg), according to the National Food Limit. Therefore, the foliar-spraying amino acid and zinc with some soil improvements can be expected to effectively inhibit the transport and accumulation of Cd in rice, in order to further optimize for the Cd in the brown rice less than the standard.