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Articles

Interorgan Photosynthate Partitioning in Alfalfa ¹

Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, Minnesota 55108, United States Department of Agriculture, Agricultural Research Service, St. Paul, Minnesota 55108

The net partitioning of current photosynthate among vegetative organs of nodulated alfalfa (Medicago sativa L.) was investigated by determining radiolabel distribution from upper and lower source leaves, at different times of day, and during successively longer chase periods. Photosynthate was exported sooner and more completely by a fully expanded lower than by a fully expanded upper source leaf. The radiolabel pulse from the lower source leaf peaked in the main stem within 1.5 hours, in the crown and nodules after 3 hours, in the unexpanded leaves and apex of the main stem after 6 hours, and in the shoots growing from leaf axils on the main stem after 24 hours. The results suggest that the crown, apex, axillary shoots, and nodules both imported and mobilized photosynthate originating at a lower source leaf, while roots and shoots growing from the crown showed net accumulation. The pulse from the upper source leaf was initially rapidly exported by the main stem and imported by the root between 1.5 and 3 hours, but there was no net change of label content of these organs during the ensuing 21 hours. Rapidly growing organs had the highest concentrations, and the largest organs had the highest content of radiolabel. These results provide new information about the accumulation and circulation of photosynthate within the alfalfa plant.

¹ Supported in part by grant 59-2177-0-1-471-0 from the Competitive Research Grants Office of the United States Department of Agriculture, and by a University of Minnesota Graduate School Doctoral Dissertation Fellowship to HTC. Cooperative investigation of United States Department of Agriculture-Agriculture Research Service and the Minnesota Agriculture Experimental Station (Scientific Journal series 14285).

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