13N-nitrate uptake sites and rhizobium-infectible region in a single root of common bean and soybean

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Abstract
The positron emitting tracer-imaging system (PETIS) was used to determine whether it was possible to obtain on image of 13N-distribution in common bean in which a single root was fed with a liquid medium containing nitrate at different concentrations with different 13N specific activities. The distribution of the images of the 13N atoms in the root could be obtained over a wide range of nitrate concentrations and 13N specific activities in the medium. As for nitrate stress on leguminous root nodulation, the positional relationship between the nitrate uptake sites and root hair just elongating area, where rhizobia capably initiate their infection, was studied in common bean and soybean. PETIS gave direct evidence that single roots of both common bean and soybean showed one or two dense 13N-distribution areas after 2 min pulse-feeding of 13N03 -. These areas remained stable over 30 min, and the first dense site, which was common in all the examined roots, extended over ca. 1 cm above the root apex. Microscopic observation revealed that this area covered both sites of rhizobium infection and of early nodule development in a common bean and soybean single root.