Translocation of 13N and 11C between Nodulated Roots and Leaves in Alfalfa Seedlings

The positron-emitting isotopes of nitrogen and carbon, ¹³N and¹¹C, in the form of ¹³N₂, , and ¹¹CO₂ have been used successively on the same plant to investigatetracer movement between root nodules and shoots in young alfalfaseedlings. The mass flow profiles of ¹¹C photosynthate appearsimilar in shape in both stem and root but differ in speed;the speed of movement is significantly slower in roots thanin shoots (0?3 cm min^–1 compared with 0?5 cm min^–1)apparently in the phloem. Root nodules are sinks for the ¹¹Clabel. ¹³N fixation occurred in <3 min in root nodules andwas followed by export of ¹³N compounds in less than 30 minafter presentation in strong light. Fixation seems to be dependenton photosynthate supply. ¹³N export from the root nodules washighly irregular in profile and not a simple steady-state massflow. The speed of movement of ¹³N fixation compounds, and , suggests transport in the xylem, at rates of 6–12 cm min^–1 acropetally bothin root and shoot. Key words: Alfalfa seedlings, Translocation, Roots, Leaves, Carbon isotope ¹¹C, Nitrogen isotope ¹³N