| Summary: | Four experiments were performed in a greenhouse or growth chamber to study the quantity and quality of flavonolignans in response to environmental stress in milk thistle (Silybum marianum L., Gaertn.). A preliminary experiment was conducted in spring 2004 to study effects of leaf harvests on growth, development, and flavonolignan content in milk thistle seeds. Taxifolin, a component of silymarin and precursor to flavonolignans, was significantly reduced due to leaf harvest treatments.
The weedy nature (sporadic germination) of Silybum made germinating enough plants for experimentation problematic. Initial germination studies to determine imbibition and pre-chilling requirements were inconclusive due to heavy fungal infections. A sterilization procedure was chosen to treat seeds before experimental use and flavonolignan analysis. Germination trials were also performed with seven other seed sources. Seeds harvested in Croatia showed the best germination and were used for subsequent experiments.
Flavonolignans in Silybum seeds are reported to vary depending on environmental conditions and genetic diversity within a population. In another experiment, total silymarin concentrations determined from eight seed sources from around the world ranged from 29.6 to 56.9 mg/gram of seed meal. Individual flavonolignans varied significantly in and among seed sources.
In the first stress experiment was, plants were grown in pine bark media in polyethylene bags. Plant densities from 1-24 plants/bag were established. Immature and total seed counts and yields decreased with increasing density. Number of blooms per plant, bloom diameter, and mature seed count and yield were negatively correlated to density. There was no significant effect of plant density on flavonolignan content.
In the second stress experiment, milk thistle plants were grown in perlite in polyethylene bags. Water treatments (200, 650, 1100, 1550, and 2000 mL/day including fertigation) were created using pressure-compensated emitters. The lowest watering rate significantly reduced stem height and bloom diameter. The highest water treatment showed the highest count of immature seeds. In primary blooms, the lowest water rate yielded the highest taxifolin concentration (0.89 mg/g). Flavonolignan content was not significantly affected in secondary blooms.
Silybum growth and development was affected by environmental stress. However, no significant effect on silymarin concentration or composition was established.
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