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 Su illus intermedius (Smith & Thiers) Smith gi Thiers collected from an approx imately 55-year-old Pinus resinosa Ait. plantation, and Pisolithus tinctori us (Pers.) Coker & Couch obtained from another source, were tested for thei r 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 isol ates except Leccinum aurantiacum and one isolate of S. intermedius attained greater dry weight with protein as the primary source of N. Lactarius affi nis, Leccinum aurantiacum, P. tinctorius, and both isolates of S. intermedi us had higher tissue densities on protein medium. Amanita rubescens had low er 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 inc reased (7 mg l(-1) N versus 70 mg l(-1) N). The second experiment also incl uded Cenococcum geophilum Fr. but excluded T. felleus. Higher tissue densit ies with increased nutrients were found in C. geophilum, Lactarius affinis, Leccinum aurantiacum, and both isolates of S. intermedius. Only A. rubesce ns and P. tinctorius did not have increased densities. The results suggest that these ectomycorrhizal fungi alter their growth forms according to N co ncentration. At low N concentrations, a growth form likely to promote explo itation of a large volume of medium for a given biomass is produced. At hig h concentrations, a growth form likely to promote exploitation of a rich so urce of N is produced. Whether ectomycorrhizal fungi growing in association with roots would act in a similar fashion is not known.