The influence of externally supplied sucrose on phloem transport in the maize leaf strip

Sucrose (2,5-1000 mmol l(-1)), labeled with [(14)C]sucrose, was taken up by the xylem when supplied to one end of a 30-cm-long leaf strip of Zea mays L. cv. Prior. The sugar was loaded into the phloem and transported to the opposite end, which was immersed in diluted Hoagland's nutrient solution. When the Hoagland's solution at the opposite end was replaced by unlabeled sucrose solution of the same molarity as the labeled one, the two solutions met near the middle of the leaf strip, as indicated by radioautographs. In the dark, translocation of (14)C-labeled assimilates was always directed away from the site of sucrose application, its distance depending on sugar concentration and translocation time. When sucrose was applied to both ends of the leaf strip, translocation of (14)C-labeled assimilates was directed toward the lower sugar concentration. In the light, transport of (14)-C-labeled assimilates can be directed (1) toward the morphological base of the leaf strip only (light effect), (2) toward the base and away from the site of sucrose application (light and sucrose effect), or (3) away from the site of sucrose application independent of the (basipetal or acropetal) direction (sucrose effect). The strength of a sink, represented by the darkened half of a leaf strip, can be reduced by applying sucrose (at least 25 mmol l(-1)) to the darkened end of the leaf strip. However, equimolar sucrose solutions applied to both ends do not affect the strength of the dark sink. Only above 75 mmol l(-1) sucrose was the sink effect of the darnened part of the leaf strip reduced. Presumably, increasing the sucrose concentration replenishes the leaf tissue more rapidly, and photosynthates from the illuminated part of the leaf strip are imported to a lesser extent by the dark sink.