On tripartite Frankia–mycorrhizal associations in the Myricaceae

Abstract

Actinomycorrhizal symbiosis was studied in Comptonia peregrina (L.) Coult. and in Myrica gale L., both of the Myricaceae. Root nodules were common in all Comptonia plants in their natural habitats and in pot cultures under greenhouse conditions. Spontaneous actinorhizal infection under greenhouse conditions differed in two native forest soils. Spontaneous mycorrhizal infection in C. peregrina and in M. gale was found neither in natural habitats nor in plants growing in native forest soils under greenhouse conditions. Comptonia peregrina and M. gale inoculated with the actinomycete Frankia, with the vesicular–arbuscular mycorrhizal fungus Glomus intraradices, or with Frankia and G. intraradices together were infected by Frankia only. The nodulated plants were significantly larger compared with unnodulated plants, and their root systems showed acetylene reduction in a bioassay for nitrogenase activity. Uninfected Comptonia plants that grew in a forest soil under greenhouse conditions developed well when watered with a complete mineral nutrient solution or with a solution that lacked phosphorus, but degenerated when watered with a solution that lacked combined nitrogen or with deionized water. Comptonia peregrina and M. gale formed cluster roots that resembled proteoid roots in the Proteaceae. In Comptonia, cluster-root formation in sand culture was common in nodulated plants as well as in unnodulated plants. In M. gale, cluster-root formation was common in nodulated plants and infrequent in unnodulated plants.