Abstract: To clarify the alleviation mechanism of silicon on Cd stress in Toona sinensis seedlings, we investigated the effects of silicon application with different concentrations (0, 0.5, 1.0 and 2.0 mmol·L^-1) on the growth index, osmotic balance, ion uptake, photosynthetic characteristics, and antioxidant capacity of Toona sinensis seedlings under Cd (200 μmol·L^-1) stress by hydroponics. The results showed that Cd stress significantly inhibited the growth of T. sinensis seedlings and reduced root vigor, ion content, photosynthetic pigment content, net photosynthetic rate (P_n), stomatal conductance (G_s), and transpiration rate (T_r), but significantly increased relative conductivity, intercellular CO₂ concentration (C_i), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) content. Under Cd stress, the application of Si significantly increased growth index, root vigor, Fe²⁺, Mn²⁺, Zn²⁺, Cu²⁺contents, chlorophyll and carotenoid contents, and gas exchange parameters (except C_i) of seedlings, while significantly decreased Cd²⁺ content, relative conductivity, C_i, H₂O₂ and MDA content. The physiological indicators showed a low-promoting and high-suppressing trend with increasing silicon concentrations. Our results indicated that silicon application under Cd stress could promote the growth of T. sinensis seedlings, ion content and photosynthetic efficiency, maintain cell membrane osmotic balance, and scavenge reactive oxygen species, thus alleviating the toxic effects of Cd stress on T. sinensis seedlings. The strongest alleviation effect was observed at a silicon concentration of 1.0 mmol·L^-1.

Key words: Toona sinensis, Cd stress, silicon, photosynthetic characteristics, ion content.