Synergistic Influence of an Arbuscular Mycorrhizal Fungus and a P Solubilizing Fungus on Growth and P Uptake of<i>Leucaena leucocephala</i>in an Oxisol

Abstract

An investigation was carried out to assess the role that P solubilizing micro-organisms play in the P nutrition of mycorrhizal and mycorrhiza-free Leucaena leucocephala (L am.). Soil microorganisms able to solubilize rock phosphate were isolated from the rhizosphere of L. leucocephala naturally growing on three different soils of Hawaii. The isolates were screened for their ability to solubilize rock phosphate in culture medium. The highest activity was observed with one of the fungal isolates, which was identified as Mortierella sp. It was multiplied and further evaluated with or without the mycorrhizal fungus Glomus aggregatum in a highly weathered soil for its effectiveness to enhance P uptake and growth of L. leucocephala. Phosphorus status of L. leucocephala pinnules monitored as a function of time revealed that plants colonized by both microorganisms had the highest P content followed by plants inoculated with the mycorrhizal fungus alone. Inoculation of soil with Mortierella sp. alone did not influence P content of plants measured at the time of harvest. However, Mortierella sp. increased the P content of mycorrhizal plants by 13% in the unfertilized soil and by 73% in the soil fertilized with rock phosphate. Shoot dry weight measurements showed that Mortierella sp. stimulated growth of nonmycorrhizal by 22%, while it stimulated the growth of myocorrhizal plants by 29% , regardless of P fertilization. The results suggest the existence of synergistic interaction between P solubilizing microorganisms and mycorrhizal fungi, although the degree of synergism was more pronounced in terms of P uptake than in terms of growth. Keywords: Glomus AggregatumDual InoculationMortierella SpMycorrhizaRock PhosphateTropical SoilFungal Isolates