Response of subterranean clover to dual inoculation with vesicular-arbuscular mycorrhizal fungi and a plant growth-promoting bacterium, Pseudomonas putida

Abstract

Plant growth and nodulation of subterranean clover ( Trifolium subterraneum L.) were studied in nonsterile soil inoculated with a plant growth-promoting rhizobacterium (PGPR) isolate of Pseudomonas putida and indigenous vesicular-arbuscular mycorrhizal (VAM) fungi. Although inoculation with the PGPR or VAM fungi increased plant growth after 12 weeks, a significant increase in root dry weight, compared to uninoculated controls, was observed only when both the PGPR and VAM fungi were present. Shoot dry weight of plants inoculated with PGPR and VAM was significantly greater than with the PGPR alone, VAM fungi alone or uninoculated controls. Nodulation was enhanced significantly by either the PGPR or VAM fungi alone (× 1.50-fold increase over controls at 12 weeks), but was significantly greater (× 2.03-fold increase over controls) when both the PGPR and VAM fungi were present. Inoculation with the PGPR increased colonization by VAM fungi from 7 to 23% of the root system infected at 6 weeks, but colonization levels by VAM fungi were similar (ca. 50%) at 12 weeks. Populations of the PGPR increased similarly in the rhizosphere of both mycorrhizal and nonmycorrhizal plants. Concentrations of Fe, Cu, Al, Zn, Co and Ni were considerably greater in the shoots of plants inoculated with the PGPR and VAM fungi than in plants inoculated with the PGPR or VAM fungi alone.