"Interação microrganismos solubilizadores de fosfatos: fungos Ectomicorrízicos e o crescimento de Pinus taeda L."

Abstract

Increasing the production of reforestation areas is a real necessity in Brazil and worldwide. However, because of the restrictions imposed upon the use of new areas, research is focusing in improving the productivity of reforestated land. The low disponibility of soil P requires the application of soluble phosphates, with higher production costs. Besides, an important portion of the P applied may be converted into insoluble forms, less available to plants. Many soil microorganisms are active in P cycling, including phosphate solubilizing microorganisms (PSM) and mycorrhizal fungi. The PSM, most of them free-living soil fungi and bacteria, are able to transform soil P to products available to plants. Ectomycorrhizal fungi (ECMf) increase P-uptake by plants and its solubilization. Both types of microorganisms may also produce plant growth promoting substances. The application of these microorganisms in controlled inoculation is an alternative to traditional fertilization practices to augment reforestation productivity. Studies concerning the compatibility between these different microorganisms are necessary before their application as mixing inoculum in plant production systems. In this context, this study aims to determine the interactions between PSM and ECMf and their effect on growth and P-uptake by Pinus taeda L. seedlings. The first assay revealed that GEL and GMM are adequate media to double cultivating ECMf and PSM. Four ECMf and eight PSM were studied on solid and liquid media concerning their mutual effects and on phosphate solubilization. Most of the combinations between these two types of microorganisms showed neutral interaction. In GEL medium, PSM and ECMf (alone or in combination) were able to solubilize calcium phosphate. No correlation between P-solubilizing and pH of the medium was observed. Two ECMf presented higher P-solubilization potential than some PSM. Some ECMf-PSM combinations induced a higher quantity of solubilized P, and a synergetic effect was observed when the isolates UFSC-Cg02 (Cenococcum geophilum) + MSF-262 (Penicillium sp.) were cultivated togheter. In greenhouse, inoculation of P. taeda with PSM and ECMf, alone or in combination, promoted higher P-uptake and plant growth than the noninoculated control. There was a significant correlation between P-solubilized in GEL medium and P-uptake by plants. According to the results obtained by TLC, spectrophotometry and HPLC techniques, the isolates MSF-195, MSF-251 (Aspergillus spp.), UFSC-Cg02 (Cenococcum geophilum) and UFSC-Rh90 (Rhizopogon nigrescens) produced indol-3-acetic acid (IAA) in GEL medium supplied with L-tryptophan. No correlation was observed between IAA production and dry matter. In terms of P-solubilizing and P-uptake by plants, the best results were obtained with the combination of UFSC-Cg02 + MSF-262, UFSC-Cg02 + MSF239, UFSC-Cg02 + MSF-310A and UFSC-Sul55 + MSF-310A.