Allocation of photosynthates to root nodules in soybean plants was analyzed non-invasively using
11CO
2 and positron-emitting tracer imaging system(PETIS). In particular, distribution of
11C-photosynthates in the underground part of a hypernodulation mutant that forms a lot of small nodules on the root and shows reduced growth was characterized by comparison to wild-type. Both in the mutant and wild-type.
11C-photosynthates were transported to the root base within about 20 min after feeding of
11CO
2 and to the root tips within one hour. Most of
11C-photosynthates in the underground part were localized to the root base where packed nodules are found. It was shown that larger amount of
11C-photosynthates was transported into the nodules on the root base than into those on distal regions, both per nodule and per volume of nodule. This suggested that the basal nodules may have higher activity for nitrogen fixation both in the mutant and wild-type, and such position of a nodule may be a dominant determining factor for the activity. Moreover, there was no difference between the mutant and wild-type in the amount of
11C-photosynthates accumulated into the nodules per volume of nodule, both in basal and distal regions. These results suggested that the reduced activity of nitrogen fixation in the mutant might be generally caused by the increased proportion of the distal nodules which are poorly fed with photosynthates, but not just by the increased total number of nodules. This is the first report of photosynthate translocation into the individual nodules in an intact soybean with real-time observation.
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