PHLOEM TRANSPORT OF RECENTLY-FIXED NITROGEN IN THE GUNNERA-NOSTOC SYMBIOSIS

Nitrogen fixation and translocation were examined in the symbiosis bet ween the angiosperm Gunnera monoica and the cyanobacterium Nostoc. Nit rogenase activity was higher in mature than in apical regions of G. mo noica plants. Ten and 12h pulse-chase labelling experiments with N-15( 2) showed that nitrogen fixed in mature regions was rapidly translocat ed throughout the plant, particularly to the apex. Pulse-chase labelli ng coupled with steam-girdling demonstrated the importance of phloem i n the transport of recently fixed nitrogen from mature to apical regio ns. Steam-girdling of a length of stolen between a N-15(2) labelled ma ture node and apical region inhibited N-15 movement past the point of treatment. This provided evidence that recently fixed nitrogen was tra nslated along the stolen primarily by the phloem. Further steam-girdli ng experiments showed that phloem also had a major role in translocati ng recently fixed nitrogen to the leaves. Such primary translocation i s exceptional within symbiotic systems but is not inconsistent with th e vascular arrangement and location of the symbiosis. As carbon is als o imported into the symbiosis the export of nitrogen implies bidirecti onal phloem transport.