The effects of sulfur and selenium on glucoraphanin and seleno-methylselenocysteine concentrations in broccoli (Brassica oleracea L. italica)

Abstract

Glucosinolates (GLSs) are sulfur-containing secondary metabolites produced by broccoli (Brassica oleracea subsp. italica) and other cruciferous vegetables. GSLs exist for use in plant defense, but are gaining research interest for their role in cancer prevention. Glucoraphanin (GR) is a particular glucosinolate found in broccoli that has great health benefit potential. Seleno-methylselenocysteine (SeMSC) is another compound unique to broccoli when it has been exposed to selenium during plant growth, and is also of interest for its chemopreventive potential. To understand the relationship between root fertilization of sulfur and selenium on GR and SeMSC concentrations in a production environment, we exposed a low-GR ("Green Magic") and a high-GR ("Beneforte") broccoli to sulfur (0 - 34 kg.ha��) and selenium (0 - 3.36 kg.ha��) fertilization treatments in the field. GR and SeMSC concentrations depended upon cultivar, treatment, and environmental factors."Beneforte" consistently delivered the highest GR concentration, and "Green Magic" the lowest, and "Beneforte" GR concentrations were less affected by the presence of Se treatments than "Green Magic". "Beneforte" also accumulated higher concentrations of SeMSC overall than "Green Magic". A colder, wetter spring in 2013 led to reduced sulfur uptake and lower concentrations of GR overall, while a warmer, drier climate during later Se applications increased Se uptake and subsequent SeMSC concentrations. Contrasting, a warmer, dryer spring the following year gave way to increased sulfur uptake and greater GR concentrations, and wetter, cooler conditions during Se applications negatively impacted SeMSC concentrations overall. To assess the efficacy of foliar Se fertilization as an alternative to root application, the same two varieties were grown in a greenhouse and were subjected to one of four sodium selenate treatments (0 - 93.74 mg Se.plant�). Again, Se significantly affected the concentration of GR in "Green Magic", but not in "Beneforte". Overall, a weak relationship between GR and SeMSC concentrations give a promising outlook to the ability to maximize GR and SeMSC for ultimate benefit upon consumption of broccoli. This is especially true in "Beneforte", where GR concentrations remain relatively stable in the presence of Se, but still allow Se uptake and SeMSC formation.