Effects Of Cadmium And Nitrogen On The Nitrogen Utilizing And Secretion Of Organic Acids In Three Ectomycorrhizal Fungus

As the most needed nutrient element by plants, nitrogen plays critical roles in development and nutrition of the trees. In soils contaminated by cadmium, nitrogen absorption and utilization by trees may be restrained. Ectomycorrhizal fungi, which are associated with coniferous forests growing in nutrient-poor soils, can promote growth of host plants and improve their nutrition condition and tolerance to disease. Some ectomycorrhizal fungi have been shown to restrict heavy metal uptake and transport by binding the heavy metals onto fungal cell walls, by sequestration in vacuoles and/or by chelation by organic acids. Ectomycorrhizal association improves the ability of hosts to take up mineral nutrients in heavy metal polluted soil and thus ameliorates metal toxicity.A series of experiments were conducted with three fungal strains, namely Cg SIV, Pt 715 and So 014, to investigate the characteristics of ectomycorrhizal fungi for N utilization, organic acid secretion and Cd resistance. The fungal isolates were grown in Pachlewski liquid culture medium with variable N resources and Cd²⁺ contents (0 mgCd²⁺·L^-1(CK),0.097 mgCd²⁺·L^-1,0.485 mgCd²⁺·L^-1,0.970 mgCd²⁺·L^-1,2.910 mg Cd²⁺·L^-1, respectively) and analyzed for nitrogen, phosphorus and potassium concentration in mycelia, dynamic oxalic acids released in solution, and activities of enzymes involving nitrogen utilization.Nitrogen supplies influenced the secretion of organic acids significantly and fungal strains also varied significantly in organic acid exudation in response to Cd stress under various nitrogen supplies. Oxalic acid could be released by all EM fungi in solution with different Cd concentrations but none of the fungi isolates secreted formic acid. The secretion of acetate, citric acid, and succinate occurred according to the presence of concentrations of Cd in the solutions.Growth rate of Cg SIV and Bo 014 was maximized in NH₄⁺/-N medium. Biomass of Pt 715 was the maximum in NO₃^--N medium but its growth characteristics changed comparing with NH₄⁺-N, in which Pt 715 grew more stable and quickly started up. Mycelial concentrations of nitrogen, phosphorus and potassium were the highest in all of the tested strains while supplied Org-N. Oxalate efflux by Bo 014 and Cg SIV in NO₃^--N medium was significantly higher than supplying NH₄⁺-N and Org-N. Exudation of oxalic acid by Pt 715 was hardly influenced by nitrogen resources. The growth rate of Pt 715, which secreted relatively high concentration of oxalic acid under Cd²⁺ stress, was not influenced dramatically by Cd²⁺ concentration. Moreover, growth character and oxalic acid secretion of Pt 715 were not affected significantly by Cd²⁺ concentration. The results indicated that Pt 715 had strong resistance (or tolerance) to Cd²⁺. Cg SIV was sensitive to Cd²⁺. Although biomass of Bo 014 was higher than other strains, it was impacted significantly by Cd²⁺ and decreased dramatically while Cd²⁺ concentration increased. Oxalic acid released by Bo 014 and Cg SIV rose as Cd²⁺ concentration increased. Nitrogen content in mycelia of Pt 715 decreased in high concentration of Cd²⁺, at the same time its potassium content increased and its phosphorus content was not affected significantly. Contents of nitrogen, phosphorus and potassium in the mycelia of Cg SIV, similar to nitrogen content in mycelia of Bo014, increased along with the increasing of Cd²⁺ concentration. Contents of phosphorus and potassium kept relatively stable in Bo014 whatsoever.Nitrogen resources influenced activities of the enzymes involving nitrogen utilization. Generally, activities of nitrate reductase and protease in Pt 715 and Cg SIV were relatively high, with the exception of nitrate reductase activity of Pt 715 culturing in NO₃^--N. Chitinase activities were relatively high in Cg SIV and Bo 014. Furthermore, activities of the nitrogen utilizing enzymes in all of the 3 strains were significantly affected by Cd²⁺ concentration in the medium. Activities of nitrate reductase and protease in Cg SIV decreased, while activities of chitinase and urease increased significantly, when Cd²⁺ concentration increased. Activities of protease, nitrate reductase and chitinase in Pt 715 raised as Cd²⁺ concentration increased, while activity of nitrate reductase fell at certain Cd²⁺ concentration and activity of urease showed no significant variation. Activities of chitinase and nitrate reductase in Bo 014 increased along with Cd²⁺ concentration, whereas the activities of urease and protease kept relatively stable...