Impact of Plant Density and Mepiquat Chloride on Growth, Yield, and Silymarin Content of Silybum marianum Grown under Mediterranean Semi-Arid Conditions

Milk thistle (Silybum marianum (L.) Gaertn.) is a promising new crop in the Mediterranean region. Its seeds contain silymarin, a complex of flavonolignans, which is widely used in the pharmaceutical industry, mainly to produce dietary supplements. To meet the increasing demand for milk thistle, the production and productivity of milk thistle should also be optimized by employing adequate cultivation practices. In the present study, a two-year field experiment was conducted to assess the effects of plant density and a plant growth regulator on milk thistle crop growth, seed yield, and silymarin accumulation under Mediterranean semi-arid conditions. Our results showed that plant density had a significant impact on milk thistle crop growth and seed yield. The main crop characteristics, such as height, aboveground biomass, and seed yield were greatest when plant density was the highest. Increased plant density significantly reduced the silymarin content only in 2018. In contrast, mepiquat chloride (MC) treatment did not affect the following traits: plant biomass, relative chlorophyll content, silymarin content, and production. Nevertheless, mepiquat chloride reduced the plant height by 7.9%–14.8%, depending on the application rates and growth conditions. Moreover, the impact of climatic conditions on milk thistle production and quality was significant, since the lowest values of silymarin content and seed yield were recorded in the year with drought conditions during the period from March to May.