Tissue density and growth response of ectomycorrhizal fungi to nitrogen source and concentration

Amanita rubescens Pers., Lactarius affinis Pk., Leccinum aurantiacum (Fr.) S.F. Gray, Tylopilus felleus (Bull. ex Fe.) Karsten, and two isolates of Suillus intermedius (Smith & Thiers) Smith gi Thiers collected from an approximately 55-year-old Pinus resinosa Ait. plantation, and Pisolithus tinctorius (Pers.) Coker & Couch obtained from another source, were tested for their abilities to grow with protein as the primary source of nitrogen. Protein plates contained 63 mg l(-1) N as bovine serum albumen and 7 mg l(-1) N as arginine. Control plates contained only 7 mg l(-1) N as arginine. All isolates except Leccinum aurantiacum and one isolate of S. intermedius attained greater dry weight with protein as the primary source of N. Lactarius affinis, Leccinum aurantiacum, P. tinctorius, and both isolates of S. intermedius had higher tissue densities on protein medium. Amanita rubescens had lower tissue density. To determine if increase in tissue density was an effect of total N concentration or an effect of N source (protein versus arginine), we performed a second experiment in which arginine concentration was increased (7 mg l(-1) N versus 70 mg l(-1) N). The second experiment also included Cenococcum geophilum Fr. but excluded T. felleus. Higher tissue densities with increased nutrients were found in C. geophilum, Lactarius affinis, Leccinum aurantiacum, and both isolates of S. intermedius. Only A. rubescens and P. tinctorius did not have increased densities. The results suggest that these ectomycorrhizal fungi alter their growth forms according to N concentration. At low N concentrations, a growth form likely to promote exploitation of a large volume of medium for a given biomass is produced. At high concentrations, a growth form likely to promote exploitation of a rich source of N is produced. Whether ectomycorrhizal fungi growing in association with roots would act in a similar fashion is not known.