Abstract:

Macadamia integrifolia originated from Australia where it grows in the rainforests of the eastern coastal areas of the Northern Rivers district of New South Wales and South-East Queensland. The Macadamia tree belongs to Proteaceae. The Macadamia was in fact the only native Australian plant ever developed to a commercial food crop. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with the roots of approximately 90% of terrestrial plant species. These fungi are an important component of ecosystems, and the diversity of the AMF could largely influence plant biodiversity, ecosystem variability and productivity.
We examined the role of M. integrifolia root exudates in stimulating the germination and growth of AMF. In September 2003, 100 g of M. integrifolia roots were sampled, dried and crushed. The roots were then extracted for two hours in a 70% methanol solution, stirred in 80 ℃ water and filtered. The extract was diluted to 20%, 40%, 60%, 80% and 100% with methanol. The AMF cultures were sterilized, and the spores of Gornas mosseae and Gigaspora margarita were inoculated and cultured at 25 ℃ in the dark for 20 days. The number of spores that germinated was counted and the length of the mycelium measured.
When the concentration of the extract was diluted by 60% methanol, spore germination of Gornas mosseae and Gigaspora margarita reached maximum levels of 81.7% and 76.0%, respectively. At higher extract dilution levels, spore germination decreased, and at 100% dilution, spore germination was lower than 65%. The mycelium reached maximum lengths when the extract was diluted by 60%. The length of Gornas mosseae was 31.2 mm and Gigaspora margarita was 28.0 mm. Mycelium growth declined when the extract dilution increased, especially at 100% dilution. At lower dilutions of 20%, the growth of Gornas mosseae appeared to be stimulated but no effect on Gigaspora margarita growth was observed. At the zero percent dilution, the promotion of mycelium growth was not observed. The concentration of flavones in the extract was the greatest in the 60% methanol dilution (19.26%) but was lower at lower dilutions and only 2.56% in the zero percent dilution. Flavone concentrations also decreased at higher dilutions and the content of flavones was lower than 80% at 100% dilution. There were significant positive correlations (correlation coefficient > 0.95) between the concentration of flavones in the extract and spore germination and mycelium growth of AMF.

Key words: Macadamia integrifolia, Methanol eluates, Arbuscular mycorrhizal fungi (AMF), Spores germination, Hyphal growth, In vitro culture