Water stress have the potenial to enhance the biosynthesis of secondary metabolites from mesophytes. Nevertheless, whether water stress can exert similar effect on Silybum marianum (L.) Gaertn, a medicinally important plant with drought tolerance capabilities, is poorly investigated. The current study was conducted to assess how growth, yield and the whole plant biochemical profile in various plant organs of S. marianum is affected by drought stress. Results indicated that drought stress did not significantly affect shoots’ dry weight, though some other growth indices were decreased. Drought decreased number of flowering heads (capitula), whereas capitulum diameter, number of seeds/capitulum, seeds weight/capitulum and seed yield/ plant were not significantly affected. In all plant organs, drought stress enhanced the accumulation of total as well as individual phenols and flavonoids. The major flavonolignans that represent silymarin components were silybin A, silybin B, isosilybin B and isosilybin A. Drought stress significantly increased silymarin content in plant organs compared with respective ones in control plants, especially in the seeds. Seeds of drought-stressed plants contain about 91% higher silymarin content comared with seeds of control plants. Invariably, total phenolics and silymarin contents were higher in seeds than leaves and were also higher in mature compared with immature state. It could be concluded that imposing drought could be an effective strategy for enhancing silymarin content in S. marianum without negatively impacted on plant growth and development and that plant organs other than the seed contain considerable amounts of flavonolignans which justifies their utilization in silymarin production.
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