Effect of silicon nutrition on some growth and physiological parameters of hydroponically-grown alfalfa (Medicago sativa) under toluene contamination stress

Silicon (Si) has not been considered as an essential nutrient for plant growth however, it has been shown that supplying crops with adequate Si can improve environmental stress tolerance. On the other hand, exposure to toluene, as a hazardous organic pollutant widely found in the water resources, is an environmental stress for plants growing around many industries. The aim of this nutrient solution culture experiment was to investigate the effect of Si nutrition on alleviating injuries induced by toluene contamination on alfalfa (Medicago sativa L.). The plant seedlings supplied or unsupplied with 1 mM silicon (in the form of Na2SiO3.5H2O) were exposed for 30 days to three levels of toluene (0, 225, 450 mg L-1) in the nutrient solution. The results showed that shoot and root fresh mass, plant height, and root length significantly decreased in the present of toluene contamination. Exposure to toluene significantly decreased relative water content of the leaves, leaf photosynthetic pigments concentration, and Fe and Zn concentration in the shoot and root. Toluene contamination led to elevated peroxidation of root cell membrane. In the present of toluene, addition of Si enhanced water use efficiency, membrane stability, chlorophyll concentration and photosynthetic activity of the leaves. These changes induced by Si may lead to higher accumulation of Fe and Zn in the plant shoot.