Long-distance transport of sulfur in Nicotiana tabacum.

Sulfur reduction in tobacco plants is a light-enhanced process that predominantly takes place in the leaves rather than the roots. The amount of sulfate reduced in mature leaves can exceed their own requirement and enables an export of reduced sulfur, both basipetal toward the roots as well as acropetal toward the growing parts of the stem. Evidence is presented that translocation of reduced sulfur toward the roots occurs in the phloem. TLC and paper chromatography reveal that glutathione is the main transport form of reduced sulfur in tobacco plants; 67-70% of reduced (35)S was confined to glutathione, 27-30% to methionine, and 2-8% to cysteine.