Phloem transport of sulfur in Ricinus.

Mature leaves of Ricinus communis fed with (35)SO 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 (35)S 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 (35)S 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.