PARTITIONING OF C-14-LABELED PHOTOSYNTHATE TO DEVELOPING NODULES AND ROOTS OF SOYBEAN (GLYCINE-MAX)

A split-root growth system was used to study photosynthate partitionin g to developing nodules and roots of soybean (Glycine max L., Merr.). Opposite sides of the root systems were inoculated with Bradyrhizobium japonicum at 8 and 12 d after planting (early/delayed inoculation tre atment) or, alternatively, only one side was inoculated 8 d after plan ting (early/uninoculated treatment). Plants were incubated with (CO2)- C-14 at 24-h intervals from early inoculation until the onset of N-2 f ixation (acetylene reduction). After staining with Eriochrome black, r oot and nodule meristematic structures were excised under a dissecting microscope and their radioactivity determined by scintillation counti ng. The specific radioactivity of nodule structures increased with nod ule development, and was as much as 4 times higher in early nodules th an in roots and nodules on half-roots receiving delayed inoculation. B y the time that N-2 fixation could be measured in the first mature nod ules, the early inoculated half-root contained over 70% of the radioac tivity recovered from the entire root systems of both early/delayed an d early/uninoculated treatments. These results suggest that developing nodules create a strong sink for photosynthate, and that nodules and roots compete for current photosynthate. Early initiated nodules might develop at the expense of late initiated nodules, as well as at the e xpense of the roots themselves.