Phloem transport of recently‐fixed nitrogen in the Gunnera–Nostoc symbiosis

summary Nitrogen fixation and translocation were examined in the symbiosis between the angiosperm Gunnera monoica and the cyanobacterium Nostoc. Nitrogenase activity was higher in mature than in apical regions of G. monoica plants. Ten and 12 h pulse‐chase labelling experiments with 15 N 2 showed that nitrogen fixed in mature regions was rapidly translocated throughout the plant, particularly to the apex. Pulse‐chase labelling coupled with steam‐girdling demonstrated the importance of phloem in the transport of recently fixed nitrogen from mature to apical regions. Steam‐gird ling of a length of stolon between a 15 N 2 labelled mature node and apical region inhibited 15 N movement past the point of treatment. This provided evidence that recently fixed nitrogen was translated along the stolon primarily by the phloem. Further steam‐girdling experiments showed that phloem also had a major role in translocating recently fixed nitrogen to the leaves. Such primary translocation is exceptional within symbiotic systems but is not inconsistent with the vascular arrangement and location of the symbiosis. As carbon is also imported into the symbiosis the export of nitrogen implies bidirectional phloem transport.