AERIAL NODULATION IN CASUARINA SPP - FIELD SURVEY AND PRELIMINARY EXPERIMENTAL-DATA

Following previous field studies on the aerial nodules (nodule-harboring protuberances) that develop spontaneously on the trunk of 40-50-year-old Casuarina cunninghamiana in La Reunion Island, an experimental study was carried out in a glasshouse and in vitro to mimic the formation of the aerial nodules. In the glasshouse, adventitious roots were produced on the stem of 1-year-old C. cunninghamiana and C. equisetifolia by keeping the stem moist and were inoculated with crushed aerial nodules from the field. Within 1 month, the adventitious roots of C. cunninghamiana nodulated profusely (ca. 30 nodules per stem), whereas those of C. equisetifolia did not nodulate at all. In vitro 90% of the axenically grown C. cunninghamiana plantlets formed 2.5 adventitious roots per stem under the best treatment, comprising addition of IBA. Both Casuarina spp. axenically grown in vitro formed aerial nodules when inoculated with a pure culture of Frankia ORS 021001, but only one third of the plants nodulated. The experimental data confirm the conclusion of the field study that several favorable factors must act together to induce the formation of aerial N2-fixing nodules on Casuarina. Further study is needed on the role of the rhytidomal microflora (including mycorrhizal fungi) in the induction of aerial nodules, and the ecology, physiology and genetics of the Frankia strain of La Reunion.