Sucrose in the free space of translocating maize leaf bundles.

Following exposure of portions of mature maize (Zea mays L.) leaf strips to (14)CO(2), xylem exudate from the leaf strips contained [(14)C]sucrose. Sucrose was the only sugar in the xylem exudate which was obtained from the cut surface of the leaf strips by reducing the external pressure. The sucrose found in the xylem exudate apparently was obtained from the free space of the vascular bundles, its concentration amounting up to 0.25%. When [(14)C]glucose or [(14)C]fructose was supplied in the dark to one end of a maize leaf strip, each was taken up by the xylem, and transported to the opposite end. Xylem exudate from such leaf strips contained (14)C-labeled sucrose in addition to the (14)C-labeled hexose. The results of this study support the view that sucrose is loaded into the companion cell-sieve tube complexes from the apoplast of the vascular bundles in the maize leaf.