Symbiotic Nitrogen Fixation by Coralloid Roots of the Cycad Macrozamia riedlei: Physiological Characteristics and Ecological Significance

Abstract

'Coralloid' roots containing blue-green algae occur commonly on the upper root stocks of M. riedlei in natural habitat in Western Australia. Each coralloid mass persists for several seasons; replacement sets form at irregular intervals, especially after fire.

¹⁵N₂ and acetylene reduction assays demonstrate that coralloid roots fix nitrogen at physiologically significant rates.

C₂H₂ reduction rates by coralloid roots are higher in winter than in summer. Performance is positively correlated with rainfall; soil temperature appears to be of lesser importance. Diurnal fluctuations in nitrogenase activity occur.

Calibration using ¹⁵N₂ gives a molar ratio of C₂H₂ reduced : N₂ fixed of 5.8 : 1. The seasonal average of C₂H₂ reduction of 14.8 nmol per g fresh wt coralloid root per min is then equivalent to 37.6 g N per kg fresh wt per year, a fixation rate potentially capable of doubling coralloid root nitrogen once in every 8 weeks, and whole plant nitrogen every 8-11 years.

Returns of fixed nitrogen in two natural populations of Macrozamia are estimated by compounding measurements of biomass of host and symbiotic organs with the seasonal average for coralloid fixation rate. The values obtained (18.8 and 18.6 kg N ha^-1 year^-1) indicate that Macrozamia contributes significantly to the nitrogen economy of its ecosystem.