Effects of silicon on content, morphology and subcellular distribution of lead in tobacco under lead stress

To study the effect of silicon on tobacco growth under lead stress and the underlying mechanism, hydroponic experiments were conducted on K326 with exogenous lead in the form of lead nitratePb (NO₃) ₂ and exogenous silicon in the form of potassium silicate (K₂SiO₃). KNO₃ and HNO₃ were used to control the effects of increased pH and K⁺ of nutrient solutions caused by K₂SiO₃. In total, five treatments were set as following:control (CK), 100 μmol·L^-1 Pb²⁺ (Pb₁₀₀), 300 μmol·L^-1 Pb²⁺+ (Pb₃₀₀), 100 μmol·L^-1 Pb²⁺++1.0 mmol·L^-1 SiO₂ (Pb₁₀₀+Si) and 300 μmol·L^-1 Pb²⁺+1.0 mmol·L^-1 SiO₂ (Pb₃₀₀+Si). The results showed that comparing with the control, Pb₁₀₀ and Pb₃₀₀ treatments decreased the plant height and biomass of tobacco by 51.44%, 69.92% and 51.43%, 64.55%, respectively. Exogenous silicon could alleviate the harmfulness of lead stress on tobacco. Comparing with Pb₁₀₀ and Pb₃₀₀ treatments, the plant height and biomass of tobacco under Pb₁₀₀+Si and Pb₃₀₀+Si treatments increased by 69.23%, 35.99% and 84.94%, 76.75%, respectively; the differences were statistically significant. Silicon inhibited the migration of lead from root upwards and reduced the lead contents in stalk, lower leaves and upper leaves. Silicon decreased the proportion of ethanol-extractable lead in root and NaCl-extractable lead in leaf, but increased the proportion of HCl-extractable and residual lead both in root and leaf. Comparing with Pb₁₀₀ and Pb₃₀₀ treatments, the lead proportion in nuclei of roots under Pb₁₀₀+Si and Pb₃₀₀+ Si treatments decreased by 25.29% and 5.06%, respectively; the lead proportion in chloroplasts and mitochondria leaf decreased by 29.22%, 54.95% and 38.09%, 51.46%, respectively.