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Abstract Cadmium (Cd) pollution is a serious threat to food safety and human health. Minimizing Cd uptake and enhancing Cd tolerance in plants are vital to improve crop yield and reduce hazardous effects to humans. In this study, we designed three Cd concentration stress treatments (Cd1: 0.20 mg·kg−1, Cd2: 0.60 mg·kg−1, and Cd3: 1.60 mg·kg−1) and two foliar silicon (Si) treatments (CK: no spraying of any material, and Si: foliar Si spraying) to conduct pot experiments on soil Cd stress. The results showed that spraying Si on the leaves reduced the Cd content in brown rice by 4.79–42.14%. Si application increased net photosynthetic rate (Pn) by 1.77–4.08%, stomatal conductance (Gs) by 5.27–23.43%, transpiration rate (Tr) by 2.99–20.50% and intercellular carbon dioxide (CO2) concentration (Ci) by 6.55–8.84%. Foliar spraying of Si significantly increased the activities of superoxide dismutase (SOD) and peroxidase (POD) in rice leaves by 9.84–14.09% and 4.69–53.09%, respectively, and reduced the content of malondialdehyde (MDA) by 7.83–48.72%. In summary, foliar Si spraying protects the photosynthesis and antioxidant system of rice canopy leaves, and is an effective method to reduce the Cd content in brown rice.

To study the synergistic effects of water management and silicon (Si) foliar spraying on the uptake and transport of cadmium (Cd) in rice, we designed four treatments: conventional intermittent flooding + no Si foliar spraying (CK), continuous flooding throughout the growth stage + no Si foliar spraying (W), conventional intermittent flooding + Si foliar spraying (Si) and continuous flooding throughout the growth stage + Si foliar spraying (WSi). The results show that WSi treatment reduced the uptake and translocation of Cd by rice and significantly reduced the brown rice Cd content, with no effect on rice yield. Compared with CK, the Si treatment increased the net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of rice by 6.5–9.4%, 10.0–16.6% and 2.1–16.8%, respectively. The W treatment decreased these parameters by 20.5–27.9%, 8.6–26.8% and 13.3–23.3%, respectively, and the WSi treatment decreased them by 13.1–21.2%, 3.7–22.3% and 2.2–13.7%, respectively. The superoxide dismutase (SOD) and peroxidase (POD) activity decreased by 6.7–20.6% and 6.5–9.5%, respectively, following the W treatment. Following the Si treatment, SOD and POD activity increased by 10.2–41.1% and 9.3–25.1%, respectively, and following the WSi treatment, they increased by 6.5–18.1% and 2.6–22.4%, respectively. Si foliar spraying ameliorated the detrimental effects of continuous flooding throughout the growth stage on photosynthesis and antioxidant enzyme activity. We conclude that synergistic continuous flooding throughout the growth stage, combined with Si foliar spraying, can significantly block Cd uptake and translocation and is therefore an effective means of reducing the accumulation of Cd in brown rice.

The spatiotemporal heterogeneity of rice canopy leaves at different leaf positions is very important for non-destructive monitoring under Cadmium (Cd) stress, and is one of the key problems that need to be solved urgently in modern agriculture. This study aims to explore the temporal and spatial heterogeneity of chlorophyll content and fluorescence in rice canopy leaves under different Cd stress concentrations. The responses of the relative chlorophyll content (SPAD) and Chlorophyll fluorescence (ChlF) parameters to Cd stress in the tillering, heading, and filling stages were analyzed through pot experiments with three Cd concentrations: 0.20 mg·kg−1 (CK), 0.60 mg·kg−1 (Cd1), and 1.60 mg·kg−1 (Cd2). Results showed that the canopy leaf SPAD value increased with increasing Cd concentration. Compared with CK, the leaf SPAD value in the Cd1 and Cd2 treatments were 1.91–3.45% and 5.52–12.01% lower, respectively. The SPAD value of the lower leaves was higher in the tillering stage, while the SPAD value of the upper leaves was higher in the heading and filling stages. The non-photochemical quenching coefficient (NPQ) of the third leaf (D3), the initial fluorescence yield (Fo) of the second leaf (D2), and the maximum photochemical efficiency (Fv/Fm) of photosystem II (PSII) in the tillering stage; the NPQ, Fo, and Fv/Fm of the D3 in the heading stage; and the NPQ, Fo, Fv/Fm, and Fv/Fm of the D2 in the filling stage were more sensitive to Cd stress than the other leaves and ChlF parameters.