Nutritional effect of silicon on morphological characteristics and essential oils of basil (Ocimum basilicum L.) under cadmium toxicity

To evaluate the silicon effects on morphological characteristics as well as the percentage and composition of basil (Ocimum basilicum L.) essential oil under cadmium stress, a factorial greenhouse experiment was conducted in a completely randomized design. The treatments included cadmium metal (Cd) as the first factor at three levels of 0, 25, and 50 mg kg-1 soil and silicon (Si) as the second factor with concentrations of 0, 1, and 2 mM. The characteristics including root length, fresh and dry weight of roots, plant height, fresh and dry weight of stems, stem diameter, number of leaves, fresh weight of leaves, number of buds, number of sub-stems, and percentage and composition of essential oil were evaluated. The results showed that Cd had negative effects on all studied morphological characteristics and significantly decreased their values compared to the control. However, Si treatment improved all these traits and with increasing the concentration of this element, a significant increase in the values of these traits was observed. Application of Si in the concentration of 2 mM caused 63.8, 81, 61, and 61% increase in the number of sub-stems, root fresh weight, stem fresh weight, and number of buds compared to the control, respectively. The amounts of five major constituents of basil essential oil including 1,8-cineole, linalool, estragole, methyl eugenol, and eugenol were affected significantly with the Cd and Si treatments. However, the interaction of these two treatments showed a significant effect only on the three compounds 1,8-cineole, linalool, and methyl eugenol. The maximum amounts of 1,8-cineole (5.06%) and methyl eugenol (6.21%) were observed in the plants which simultaneously exposed to the highest concentrations of Cd and Si. The amount of linalool was also decreased with the presence of Cd and increased with the Si treatment. The Cd treatment increased the amount of estragole by 12.8% and decreased the amount of eugenol by 25.2%. The Si treatment also increased the amount of estragole by 23.9% and decreased the amount of eugenol by 18.3%.