Silicon reduces long-term cadmium toxicities in potted garlic plants

A long-term experiment was conducted to investigate the alleviative effects of silicon (Si) on cadmium (Cd) toxicity in garlic plants grown in pots. Cd and Si were introduced into soil before sowing. Cd was added at a rate of 20 mg kg(-1) soil, and Si was applied at two rates: 50 mg SiO2 kg(-1) (Si1) and 500 mg SiO2 kg(-1) (Si-2). There were totally six treatments consisting of CT (control, no added Cd or Si), Si1, Si2, Cd, Cd + Si1, and Cd + Si2. The results showed that Si addition did not affect the growth of garlic plants under control conditions. Under Cd stress, the plant growth and PSII quantum efficiency were inhibited, and they were significantly improved in the presence of added Si. Added Si at Si1 level did not change the soil pH and Cd availability, while it increased Cd accumulation in both shoot and bulb, and improved Cd tolerance. Si added at Si2 level increased the soil pH and decreased Cd availability, and decreased Cd accumulation in different parts of the plant. Added Si had no effect on the activities of soil catalase, urease or invertase regardless of Cd presence. The results suggest that Si could increase Cd tolerance of garlic plants, and the tolerance increase was attributed to not only decreased Cd availability but also in planta detoxification mechanism. There is no evidence indicating that Si-mediated increase of Cd tolerance is related to improved soil microorganism environment as observed in biotic stress conditions.