Interactions between a saprotroph and a mycorrhizal fungus of Sitka spruce in a model system
In microcosms, Picea sitchensis seedlings were grown aseptically in perlite + vermiculite with NH4-N or NH4NO3-N, with an ectomycorrhizal fungus, Lactarius rufus and/or a saprotrophic fungus Hygrophoropsis aurantiaca. Total and fluorescein diacetate (FDA) active hyphal length, mycorrhizal infection and plant dry weight were measured (8 to 16 weeks). In perlite with NH4-N, total hyphal length of extramatrical mycelium (EMM) produced by L. rufus was 693 m gdwt perlite or 47 m per mycorrhizal root-tip. This was reduced to 141 m gdwt in the presence of H. aurantiaca. Nutrient solution pH and plant growth were also reduced. In liquid culture, acidification of the medium (H+ equivalent) per NH4 ion uptake was ≤ 3.8 times greater for H. aurantiaca than L. rufus. Growth (dwt) of H. aurantiaca was inversely related to pH. Growth of L. rufus was unaffected by pH 2.7 to 5.0. In perlite + vermiculite with NH_4-N, L. rufus hyphal length was unaffected by the presence of H. aurantiaca. In this experiment, no mycorrhizas formed in treatments with NH_4NO_3-N and EMM production by L. rufus was greatly reduced compared to treatments with NH_4-N. In a different experiment with NH_4NO_3-N, total hyphal length of EMM for L. rufus grown alone was apparently greatly reduced, (compared with an experiment with NH_4-N) but was increased in the presence of H. aurantiaca. Plant dry weight was also increased in the presence of H. aurantiaca with perlite + vermiculite. Total hyphal length of H. aurantiaca was not affected by the presence of L. rufus in any experiment. FDA-active hyphal length of both fungi decreased or remained constant and was 1% to 3% of total hyphal length at a week 16. Percentage mycorrhizal root-tips was similar between treatments at week 16 but differences occurred at weeks 8, 10 and 12 which suggested an inverse relationship between % infection and pH.