ABSTRACT
Casuarina equisetifolia is one of the ecologically and economically important tropical coastal trees nodulated by nitrogen-fixing actinomycete Frankia and forming symbiotic associations with both ecto- and endomycorrhizal fungi. The present study aims at the ultrastructural study of interactions between C. equisetifolia, Frankia, and mycorrhiza. C. equisetifolia seeds were sterilised and germinated under in vitro condition. The seedlings were transferred to conical flasks containing vermiculite and saw dust with Hoagland's solution. After 30 days, the inoculum of AM fungus--Glomusfasciculatum (A), ectomycorrhizal fungus-Pisolithus tinctorius (E) and actinorhizal Frankia (F) were inoculated individually and in various combinations, (A+E), (A+F), E+F) and (A+E+F). After 90 days, the experimental plant roots and nodules were harvested for assessment of growth characters of mycorrhizal and actinorhizal association by light and scanning electron microscope methods. C. equisetifolia roots were infected with arbuscles and vesicles of G. fasciculatum; P. tinctorius formed fungal sheath but no Hartig net. Large number of cortical cells were seen infected with Frankia, hyphae of Frankia were frequently seen penetrating from cell to cell directly through cell walls and Frankia occupied majority of the cell volume.