Blessed thistle agronomic and phytochemical response to nitrogen and plant density

Aerial parts of blessed thistle (Cnicus benedictus L.) contain high amounts of phytochemicals and would be improved through varying agronomic management practices such as N rate and plant density. The objective of this study was to investigate the effect of different N fertilizer rates and plant densities on agronomic performance, photosynthetic capacity, leaf area index, carbon to N ratio (C/N), cnicin content, total phenol and flavonoids, and antioxidant activity in blessed thistle. A two-year field experiment was conducted in a semi-arid region in Iran in 2013 and 2014. The experiment was arranged as a randomized complete block design with a factorial arrangement and three replicates. Treatments consisted of four N rates (0, 50, 100 and 150 kg N ha−1) combined with five plant densities (5, 10, 15, 20 and 25 plants m-2). Total chlorophyll and straw N contents as well as photosynthesis rate significantly increased by either 50 or 100 kg N ha−1. The significant reductions of straw N in 2013 and photosynthesis rate in both years were found with increasing plant density. Total phenol, flavonoid contents, C/N ratio and antioxidant activity were decreased as a result of increased N rate. The highest cnicin contents and yields were obtained from high plant densities under low N rates. This response may be attributed to N and plant density compensation effect. The enhancement in secondary metabolites and antioxidant activity were associated with high C/N ratio and low N contents, and accompanied by a decrease in chlorophyll content that showed a positive correlation with photosynthesis and biomass yield. The study demonstrates that increasing plant density (>15 plants m−2) with a low N rate (50 kg N ha−1) could be suitable for both biomass and phythochemichal production of blessed thistle. Furthermore, this study suggests that blessed thistle has the ability to achieve higher biomass and yield of cnicin in poor soils and unfavorable conditions.