The Effect of Population Density on Growth, Yield, and Flavonolignan Content in Milk Thistle (Silybum marianum) A.R. Belitz and C.E. Sams Department of Plant Sciences The University of Tennessee Knoxville, TN 37996 USA Keywords: silymarin, phenolics, flavonoids, phenylpropanoids, HPLC, hepatoprotectant Abstract Milk thistle (Silybum marianum (L.) Gaertn.) is grown throughout the world for its hepatoprotectant flavonolignans, collectively known as silymarin. Silymarin is found primarily in the seeds. Milk thistle was grown in a controlled environment (16 h light at 1200 μmol m -2 s -1 and 23°C and 8 h dark at 16°C) for the determination of plant growth, seed yield, and flavonolignan content under various levels of population density stress. Total seed count (ranging from 0 to 293) and seed yield (from 0.0 to 4.3 g) decreased with the increasing plant density. Furthermore, the count and the yield of malformed or immature seeds also decreased with increasing density. Additionally, the number of blooms per plant, bloom diameter, and mature seed count and yield were negatively correlated to density. However, there was no significant effect of plant density observed on the flavonolignan content. INTRODUCTION Milk thistle is a medicinal plant whose use has been documented since ancient times as a treatment for liver and bile-related diseases, as well as for acute poisoning by Amanita mushroom (Kurkin, 2003; Fraschini, 2002). Current studies have explored use of milk thistle against various types of cancer, for cholesterol control, and for blood sugar control in those with type II diabetes (Gazak, 2007; Huseini et al., 2006). Many of these benefits are attributed to the phenolic content of the leaves and seeds of this plant, which, like many other secondary metabolites, are known to be affected by biotic and abiotic stressors (Beckman, 2000; Sudha, 2002). The seeds contain a group of hepatoprotectant phenolic compounds known as flavonolignans. Flavonolignans are formed from the coupling of a flavonoid, taxifolin (dihydroquercetin), and a phenylpropanoid lignan component, coniferyl alcohol. The primary bioactive flavonolignans include silybin A, silybin B, isosilybin A, isosilybin B, silychristin, and silydianin. These compounds, along with taxifolin, comprise the hepatoprotectant complex known as silymarin. Since these valuable medicinal compounds are found primarily in the seeds, factors regulating seed production in milk thistle crops are important. The relationship among blooms (or heads) and seed weight and count were described by Gabucci et al. (2002). The number of seeds per plant was positively correlated with the number of heads per plant. The weight of the seeds per plant was positively correlated with the number of heads per plant, the number of seeds per head, and the individual seed weight. In addition, the number of seeds per head increased with bloom diameter and decreased with the number of heads per plant (Gabucci et al., 2002). The objectives of this experiment were three-fold. The first was to examine the effect of population density on plant growth and seed yield of milk thistle. The second was to determine if population density stress affects the quality and quantity of silymarin in the seeds. The third objective of this research was to determine an optimum population density for seed and flavonolignan yield. MATERIALS AND METHODS Experimental Setup Plants were grown in 5 gallon (0.02 m 3 ) polygrow bags (Hydro-Gardens, 251 Proc. IS on Med. and Nutraceutical Plants Ed.: A.K. Yadav Acta Hort. 756, ISHS 2007