Translocation of13N and11C between Nodulated Roots and Leaves in Alfalfa Seedlings

Abstract

The positron-emitting isotopes of nitrogen and carbon, ¹³N and ¹¹C, in the form of ¹³N₂, $N13O3−$ , $N13H4+$ and ¹¹CO₂ have been used successively on the same plant to investigate tracer movement between root nodules and shoots in young alfalfa seedlings. The mass flow profiles of ¹¹C photosynthate appear similar in shape in both stem and root but differ in speed; the speed of movement is significantly slower in roots than in shoots (0·3 cm min⁻¹ compared with 0·5 cm min⁻¹) apparently in the phloem. Root nodules are sinks for the ¹¹C label. ¹³N fixation occurred in <3 min in root nodules and was followed by export of ¹³N compounds in less than 30 min after presentation in strong light. Fixation seems to be dependent on photosynthate supply. ¹³N export from the root nodules was highly irregular in profile and not a simple steady-state mass flow. The speed of movement of ¹³N fixation compounds, $N13O3−$ and $N13H4+$, suggests transport in the xylem, at rates of 6–12 cm min⁻¹ acropetally both in root and shoot.