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Planta

, Volume 176, Issue 4, pp 461–471 | Cite as

Pathways of assimilation and transfer of fixed nitrogen in coralloid roots of cycad-Nostoc symbioses

  • J. S. Pate
  • P. Lindblad
  • C. A. Atkins
Article

Abstract

Freshly detached coralloid roots of several cycad species were found to bleed spontaneously from xylem, permitting identification of products of nitrogen transfer from symbiotic organ to host. Structural features relevant to the export of fixed N were described for Macrozamia riedlei (Fisch. ex Gaud.) Gardn. the principal species studied. Citrulline (Cit), glutamine (Gln) and glutamic acid (Glu), the latter usually in a lesser amount, were the principal translocated solutes in Macrozamia (5 spp.), Encephalartos (4 spp.) and Lepidozamia (1 sp.), while Gln and a smaller amount of Glu, but no Cit were present in xylem sap of Bowenia (1 sp.),and Cycas (2 spp.). Time-course studies of 15N enrichment of the different tissue zones and the xylem sap of 15N2-pulse-fed coralloid roots of M. riedlei showed earlier 15N incorporation into Gln than into Cit, and a subsequent net decline in the 15N of Gln of the coralloid-root tissues, whereas Cit labeling continued to increase in inner cortex and stele and in the xylem sap. Hydrolysis of the 15N-labeled Cit and Gln consistently demonstrated much more intense labeling of the respective carbamyl and amide groups than of the other N-atoms. Coralloid roots of M. riedlei pulse-fed 14CO2 in darkness showed 14C labeling of aspartic acid (Asp) and Cit in all tissue zones and of Cit of xylem bleeding sap. Lateral roots and uninfected apogeotropic roots of M. riedlei and M. moorei also incorporated 14CO2 into Cit. The 14C of Cit was restricted to the carbamyl-C. Comparable 15N2 and CO2-feeding studies on corallid roots of Cycas revoluta showed Gln to be the dominant product of N2 fixation, with Asp and alanine as other major 14C-labeled amino compounds, but a total absence of Cit in labeled or unlabeled form.

Key words

Carbon dioxide fixation Citrulline Coralloid roots Cycads (nitrogen fixation) Nitrogen fixation Nitrogen transport Nostoc 

Abbreviations

Ala

alanine

Asp

aspartic acid

Cit

citrulline

Gln

glutamine

Glu

glutamic acid

Orn

ornithine

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Copyright information

© Springer-Verlag 1988

Authors and Affiliations

  • J. S. Pate
    • 1
  • P. Lindblad
    • 2
  • C. A. Atkins
    • 1
  1. 1.Botany DepartmentUniversity of Western AustraliaNedlandsAustralia
  2. 2.Department of Physiological BotanyUniversity of UppsalaUppsalaSweden

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