Glucosinolates and Carotenoids In Watercress Are Influenced by Selenium Fertilization

The perennial herb watercress (Nasturtium officinale R. Br.) is consumed raw as a salad green following production in natural water systems or greenhouse hydroponic culture.  The shoots of this herb contain high levels of secondary plant compounds which have beneficial impacts on human health.  One such class of compounds is the sulfur(S)-containing glucosinolates (GS).  Selenium (Se) is an essential micronutrient in human health which can have anti-carcinogenic properties when consumed at levels of 200 μg·day^-1, depending on age, sex and health status.  Selenium has been shown to substitute for S in plant biochemical pathways.  Watercress can accumulate plant secondary carotenoid compounds, such as β-carotene and lutein, which function to reduce certain cancers, cardiovascular and eye diseases, and enhance the immune system.  Therefore, the effect of Se on secondary compounds in watercress was investigated.  Seeds of watercress were germinated under greenhouse conditions at 22°C day/14°C night and transferred to nutrient solution culture in Normal, IL (lat. 40°30’N) and Knoxville, TN (lat. 35°57’N) in the fall of 2010 using a modified Hoagland’s solution containing Se treatment concentrations of 0, 0.125, 0.25, 0.50, 1.0, 2.0, and 4.0 mg Se·L^-1, delivered as sodium selenate (Na₂SeO₄).  Treatments were arranged in a randomized block design with 4 replications, containing 6 plants per replication.  Plants were harvested after 28 days in solution, weighed for biomass, lyophilized, and stored at -80°C prior to extractions and tissue analysis for GS and carotenoids using HPLC methodologies.  Elemental Se and S concentrations were measured in oven-dried shoot tissues using ICP-MS.  Shoot tissue fresh mass decreased linearly (P=0.08) in response to increasing Se treatments.  Total levels of GS increased, then decreased quadratically (P=0.002) in response to increasing levels of Se in the nutrient solution.  The GS compound gluconasturtiin also increased, then decreased quadratically (P=0.009) in response to increasing Se.  Gluconasturtiin is the GS compound that hydrolyses to form 2-phenethyl isothiocyanate, which has been shown to be chemopreventative.  There were linear increases in shoot tissue Se (P=0.05) and S (P=0.003) in response to increasing Se in solution.  Linear decreases were measured for total carotenoids (P=0.009), β-carotene (P=0.01), lutein (P=0.01), and total chlorophyll (P=0.009) in response to increasing Se treatments.  Results demonstrate that although Se supplementation can increase nutritionally important GS and Se tissue concentrations in watercress, carotenoids may be negatively affected.