Pisolithus tinctorius forms long ectomycorrhizae and alters root development in seedlings of Pinus resinosa

Ectomycorrhizae were synthesized on Pinus resinosa seedlings with Pisolithus tinctorius under aseptic, controlled environmental conditions. Root systems were harvested at 4, 6, and 15 weeks after seed germination. All roots were classified as either long or short and were examined for ectomycorrhiza development. Mycorrhizae were classified as "long mycorrhizae" or "short mycorrhizae," depending upon the type of root where infection occurred.Long mycorrhizae predominated at the early harvests and remained prominent at 15 weeks. A Hartig net of long mycorrhizae formed a morphological continuum from thin to thick. Negative correlations were observed between long mycorrhiza growth rate and the extent of infection. This suggests that a threshold growth rate exists below which mycorrhiza formation occurs readily, but above which mycorrhiza formation is progressively diminished. Relative to the controls, P. tinctorius inoculation increased the ratio of second- and third-order short roots to long roots. The role of host and fungus in controlling lateral root development is discussed.