Selenium Volatilization in Broccoli as Influenced by Sulfate Supply

Abstract

Summary The influence of sulfate supply on the rate of selenium volatilization in broccoli ( Brassica oleracea v. botrytis cv. Green Valiant) was investigated. Plants were cultured hydroponically in growth chambers. Sulfate was supplied at five different concentrations: 0.25, 0.5, 1.0, 5.0 and 10.0 mM in half-Hoagland's solution. All treatments received 20 µM Se as Na 2 ZSeO 4 . Measurements were made of the rate of Se volatilization per plant, Se and S concentrations in root, stem, and leaf blades, and of plant dry weight and leaf area. Each increase in sulfate level from 0.25 to 5 mM caused a progressive decrease in the daily rate of Se volatilization which decreased from 96.7 (at 0.25 mM) to 13.8 (at 10 mM) µg Se per square meter of leaf surface. The concentrations of Se in plant tissues (stem, leaf and root) responded differently to increased sulfate level than did Se volatilization rate: tissue Se concentrations did not change with increase in sulfate from 0.25 to 1 mM and decreased only at the higher sulfate levels, 5 and 10 mM. The step-wise decrease in Se volatilization with each increase in sulfate was however, correlated strongly with a step-wise decrease in the ratio of Se: S in plant tissues. These results suggest, that with increasing sulfate supply, sulfate in plant tissues increasingly competed with selenate for the enzymes of the S-assimilation pathway; this internal competition most likely led to a decreased production of selenoamino acids, especially selenomethionine (required for the production of the volatile form of Se, dimethylselenide), thereby reducing Se volatilization rate.