The combined effects of Pseudomonas fluorescens CECT 844 and the black truffle co-inoculation on Pinus nigra seedlings

The inoculation of forest seedlings with mycorrhizal fungi and rhizobacteria can improve the morphology and physiology of the seedlings and benefit the reforestation of Mediterranean areas and the reintroduction of mycorrhizal fungal inocula into these areas. Pinus nigra subsp. salzmannii,a forest component of the Mediterranean natural ecosystems, is currently used in the reforestation of Mediterranean regions. Its roots are able to form an ectomycorrhizal symbiosis with the Ascomycetes fungus Tuber melanosporum Vitt., the black truffle. The ecological, economic and social values of this ectomycorrhizal fungus is well known. Previously, we demonstrated that the inoculation of Pinus halepensis seedlings with Pseudomonas fluorescens CECT 844 rhizobacteria and the black truffle T. melanosporum improved the plant growth and N absorption of the seedlings. Furthermore, the addition of P. fluorescens CECT 844 doubled the rate of mycorrhization of T. melanosporum. In the present work, P. nigra seedlings were produced in a nursery under well-watered conditions. We studied the morphophysiological response of these seedlings to a combined T. melanosporum and/or a rhizobacteria P. fluorescens CECT 844 inoculation. Five months after inoculation, the growth parameters (seedling height, basal diameter, and shoot and root dry weight), mycorrhizal colonization, water parameters (osmotic potential at both full and zero turgor and modulus of elasticity), and the total contents and concentrations of N, P, and K in the seedlings roots and shoots were measured. The root growth potentials were subsequently estimated. The addition of P. fluorescens CECT 844 did not significantly improve the mycorrhizal colonization by T. melanosporum on P. nigra seedlings. Additionally, the P. fluorescens inoculation caused few significant improvements in the growth and water parameters. Moreover, apparently opposing effects were observed between the two inoculations regarding the seedlings P absorption. We discuss whether P. fluorescens CECT 844 could act as a Mycorrhizal Helper Bacterium (MHB) through different mechanisms depending on the environmental conditions.