Mycorrhizal Nodules and Growth of Podocarpus in Nitrogen-poor Soil

Abstract

MOST genera with nodulated roots are legumes, and the nodules form after infection by nitrogen-fixing bacteria. Thirteen other genera of dicotyledons may also develop root nodules, which form only after infection, and in which nitrogen fixation has been proved or inferred, but in these the endophytes seem to be actinomycetes^1,2. Among the gymnosperms, however, in Araucariaceae and Podocarpaceae, nodules develop without any stimulus from microorganisms³ although in nature they usually contain a coarse phycomycetous mycelium. This infection has been synthesized using Endogone spores⁴ and, like typical Endogone mycorrhizas, infected nodules sustain the growth of seedlings in phosphorus-deficient soils⁵. There is also a long standing belief that they fix nitrogen, supported recently by some experiments with 15 N (ref. 1). I have tested this hypothesis by attempting to grow seedlings of Podocarpus totara in a nitrogen-deficient soil.