Selenium distribution in ryegrass and its antioxidant role as affected by sulfur fertilization

Selenium (Se) essentiality to plants has not been demonstrated although evidence indicates that it plays a significant role as antioxidant in higher plants. Research concerning to the uptake and allocation of Se in plant tissues is reported in numerous works. However, the effect of sulfur (S) on both the distribution and the antioxidant ability of Se in selenite-treated plant remains unclear. In this work the effect of S application (0-100 mg S kg(-1) soil) on shoot Se concentration of Lolium perenne cv. Aries was studied. Se distribution into different fractions of plants supplied with selenite (2 mg Se kg(-1) soil) and the state of the antioxidative system were determined. Results showed that shoot Se concentration decreased at least 33% by S application. Plants supplied with S registered the lowest GSH-Px activity and the highest lipid peroxidation. Most of Se was incorporated into the organic fraction of the plant tissue irrespective of the S treatment. However, a significant decrease of both the soluble protein and the amino acid fraction occurred, and the residual Se fraction seemed to increase at expense of the organic-Se soluble fraction. Although no essential selenoproteins have been clearly identified in vascular plants, the decrease of the soluble protein fraction and the different pattern of protein synthesized (SDS-PAGE analysis) may explain the observed reduction of the GSH-Px activity.