Catalase and ascorbate peroxidase activities are not directly involveded in the silicon-mediated alleviation of ferrous iron toxicity in rice

Silicon (Si) is the second most abundant element in the soil and can alleviate several abiotic stresses in many plant species. However, the mechanisms involved in alleviating ferrous iron (Fe2+) toxicity by Si are still largely unknown, and no study has investigated the role of Si on the Fe2+-induced oxidative stress and antioxidant system in rice. Four cultivars of Asian and African rice (Oryza sativa L. and Oryza glaberrima Steud) were grown for 4 weeks under hydroponic conditions with or without Fe2+ (250 mg Fe2+ L-1) and with or without Si (250 mg SiO2 L-1). The plants that were treated with Fe2+ suffered Fe2+ toxicity, and Si helped to alleviate the toxicity symptoms. The bronzing index and the Fe concentration in the foliar tissue increased in the presence of Fe2+ but decreased significantly with the application of 250 mg SiO2 L-1. The concentration of malonyldialdehyde, that is commonly used as an indicator of oxidative stress, increased in the foliar tissue in the presence of 250 mg Fe2+ L-1 in the nutrient solution. The application of 250 mg SiO2 L-1 in the plant nutrient solution treated with Fe2+ considerably limited the increase of malonyldialdehyde. However, no significant effect of Si application on the activities of antioxidant enzymes (catalase and ascorbate peroxidase) and non-enzymatic antioxidants (total ascorbate, reduced ascorbate, oxidized ascorbate, and the ratio of the reduced to oxidized forms) was observed in the rice plants that were grown in the presence of Fe2+. These results suggest that Si does not act directly on the antioxidant defense system of rice but reduces the plant Fe2+ concentration, which reduces the oxidative stress.