Abundance of Frankia from Gymnostoma spp. in the rhizosphere of Alphitonianeocaledonica, a non-nodulated Rhamnaceae endemic to New Caledonia

The capacity of different soils of New Caledonia to induce nodulation in Gy mnostoma poissonianum was studied. The soils were sampled under five Gymnos toma species, Alphitonia neocaledonica (a non-nodulated endemic Rhamnaceae) and Pinus caribea (an introduced species) growing in various ecological co nditions. Using G. poissonianum as trap-host, we observed a higher abundanc e of Frankia from Gymnostoma spp. in the rhizosphere of A. neocaledonica as compared with bare soils and P. caribea rhizosphere. The nodulating capaci ty of A. neocaledonica rhizosphere was almost similar to that of the five G ymnostoma species (symbiotic host) studied in the same stations. In compari son, bare soils or rhizosphere of Ft caribea had poor nodulating capacities . We isolated fourteen Frankia strains from nodules of G. poissonianum afte r baiting with the rhizospheric soils of five Gymnostoma and A. neocaledoni ca. Using the PCR/RFLP method, we confirmed the similarity with those alrea dy described. Frankia was abundant in the rhizosphere of A. neocaledonica i n all the sites studied. One explanation could be a positive tropism of Fra nkia towards species belonging to families having nodulated species, which is the case of A. neocaledonica endemic in New Caledonia. We can suppose th at the non-nodulated plants belonging to these families can excrete some ch emical substances able to attract Frankia and to induce its proliferation. (C) 2000 Editions scientifiques et medicales Elsevier SAS.