Phloem transport of sulfur in Ricinus

Abstract

Mature leaves of Ricinus communis fed with ³⁵SO ₄ ^2- in the light export labeled sulfate and reduced sulfur compounds by phloem transport. Only 1–2% of the absorbed radiosulfur is exported to the stem within 2–3 h, roughly 12% of ³⁵S recovered was in reduced form. The composition of phloem translocate moving down the stem toward the root was determined from phloem exudate: 20–40% of the ³⁵S moved in the form of organic sulfur compounds, however, the bulk of sulfur was transported as inorganic sulfate. The most important organic sulfur compound translocated was glutathione, carrying about 70% of the label present in the organic fraction. In addition, methionine and cysteine were involved in phloem sulfur transport and accounted for roughly 10%. Primarily, the reduced forms of both, glutathione and cysteine are prsent in the siever tubes.