The occurrence of coralloid roots on the South African species of the Cycadales and their ability to fix nitrogen symbiotically

Root-based N2-fixing Symbioses: Legumes, Actinorhizal Plants, Parasponia sp. and Cycads

“…Grobbelaar et al (1986) also demonstrated that all 33 species of Encephalartos and one Stangeria tested, using the acetylene reduction technique as well as 15 N-enrichment, were capable of fixing N 2 . In the field, two Macrozamia riedlei stands growing naturally in Western Australia in Eucalyptus forests fix 19 kg N ha )1 yr )1 (Halliday and Pate, 1976) and about 8 kg N ha )1 yr )1 (Grove et al, 1980), respectively.…”

“…Species within the genera Macrozamia and Zamia are also common in dryer soils of low fertility in Australia, and often comprise an understorey vegetation of Eucalyptus forests (Grove et al, 1980). More than 35 species of the genus Encephalartos (endemic to Africa) constitute the large cycad flora in South Africa, while Stangeria is endemic to its coastal regions (Grobbelaar et al, 1986). Due to their highly decorative appearance, cycads are widely cultivated and used as ornamental plants in greenhouses and in private and public botanical gardens world-wide.…”

“…It has been speculated that the coralloid roots may have initially evolved as pneumatophores (apogeotropic roots facilitating an efficient gas exchange) that only later became cyanobacterial host-organs (Grobbelaar et al, 1986;Wittman et al, 1965). However, fossil evidence of symbiotic tissues invaded by cyanobacteria is still missing.…”

Cyanobacterial Associations

“…We now know that these are often, if not always, invaded by cyanobacteria in nature. For instance, Grobbelaar et al (1986) encountered coralloid roots with cyanobacteria in all 33 species of Encephalartos, which is indigenous to the Republic of South Africa. The coralloid roots, typically apogeotropic (although this has later been questioned; see Grobbelaar et al, 1986) and adapted for gas exchange, were first characterized as "tubercles" that were formed as a response to the presence of bacteria, other than cyanobacteria (McLuckie, 1922).…”

“…About 1.4-8.4 kg N per ha per year was fixed by M. riedlei growing in eucalyptus forests (Grove et al, 1980). The coralloid roots of all 33 Encephalartos species in natural populations in the Republic of South Africa were infected by cyanobacteria and fixed N 2 as shown by both C 2 H 2 and 15 N 2 assays (Grobbelaar et al, 1986).…”