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Abstract LINDBLAD, P. (Department of Physiological Botany, University ofUppsala, Box 540, S-75121 Uppsala, Sweden). Nitrogen and carbon metabolism in coralloid roots of cycads. Memoirs of the New York Botanical Garden 57: 104"113. 1990. Filamentous heterocystous cyanobacteria of the genus Nostoc are found in a zone between two cortex layers within cycad coralloid roots. The zone is traversed by elongated cycad cells, supposedly involved in the transfer of metabolites between the symbionts. Nitrogen fixation was maximal in the growing tip and declined towards basal parts of the root. In contrast, the heterocyst frequency gradually increased towards basal, older parts and formation of multiple heterocysts commenced simultaneously. The inverse relationship between nitrogen fixation and total heterocyst frequency indicates a high frequency of inactive heterocysts in the multiple complexes, especially since nitrogenase activity correlates well with the frequency of single heterocysts. As in free-living cyanobacteria, nitrogenase occurred in the heterocysts only. The activity and cellular/subcellular localization of the primary ammonia assimilating enzyme, glutamine synthetase, was comparable to that of free-living cyanobacteria. Moreover, glutamate synthase showed full in vitro activity, suggesting that the nitrogen metabolite transferred from the cyanobiont to the cycad may be a product of ammonia assimilation. The light harvesting phycobiliprotein phycoerythrin was present in the vegetative cells, although the cyanobionts occur in darkness. The absence of light was, however, manifested by no in vivo CO2-fixation. However, both ribulose 1,5-bisphosphate carboxylase and phosphoribulokinase activities were found in cell-free extracts.