Sugar Gradients and Translocation of Sucrose in Detached Blades of Sugarcane

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Abstract
Translocation of C14-labeled photosynthate takes place in detached blades, the sucrose following the veins to the midrib and accumulating at the base of the blade. Translocation does not require a sink but the amount translocated is increased by a sink. At 2000 ft-c major transport is basi-petal. Total darkness decreases translocation and generally reverses polarity in blades cut in the morning. Neither basipetal polarity in the light nor acropetal polarity in the dark is affected by the pull of transpiration, gravity, or position of the blade. The decrease in basipetal translocation in the dark is not prevented by supplying additional sucrose to the cut base and is, therefore, not a starvation phenomenon. Basipetal translocation in the dark increases in blades cut at successive times during the day and is initiated only in blades having a positive gradient in sucrose. Absorption of sucrose, maltose, glucose, mannose, or xylose (but not mannitol or NaCl in the same osmotic concentration as the sugars) by the cut apex of a morning blade reverses the polarity of translocation in the dark. Since afternoon blades in the dark, as well as morning blades in the light, can continue translocation against the gradient in sucrose, there must be a regulating factor other than continued photosynthesis. Darkening only the base or apex of a blade increases translocation thereto. Darkening the fed part curtails translocation to the base; darkening the fed part plus cutting off the apex almost completely inhibits translocation. Neither the water gradient nor the final sucrose gradient is that required by the conventional mass flow theory. The basipetal polarity of sucrose transport in the leaf constitutes an important part of the mechanism whereby the leaf exports its sugar into the stem. The strong basipetal polarity of sucrose translocation within the phloem of the blade depends upon light or a light-formed regulating factor other than continued photosynthesis of sucrose.