GROWTH AND ASSIMILATION OF NH4- BY PAXILLUS-INVOLUTUS IN ASSOCIATION WITH BETULA-PENDULA AND PICEA-ABIES AS AFFECTED BY SUBSTRATE PH( AND NO3)

The influence of pH on the growth and assimilation of N-15-labelled am monium and nitrate was studied in intact ectomycorrhizal systems consi sting of Betula pendula Roth and Picea abies (L.) Karst. colonized wit h a common mycelium of Paxillus involutus (Batsch) Fr. The plants were grown together in Plexiglass observation chambers containing non-ster ile peat with three different pH values, 4.0, 5.1 and 6.1. The mycorrh izal mycelium was allowed to grow over a barrier into an area of peat from which plant roots were excluded. Labelled NH4NO3 was supplied, ei ther as (NH4NO3)-N-15 or as (NH4NO3)-N-15, exclusively to the fungal m ycelium. Shoots and roots were analyzed for N-15 in total nitrogen whi le the mycelium was analyzed for N-15 in NH4+, NO3- and free amino aci ds. The N-15 labelling pattern indicated that ammonium was immediately assimilated into amino acids, primarily glutamine, by the fungal myce lium at the uptake site. The amino acids were then translocated to the mycorrhizal roots. In contrast, nitrate-N was not assimilated in the mycelium but rather transferred to the mycorrhizal roots as nitrate. M ycelial uptake and transfer of N to the spruce and birch seedlings wer e significantly higher for NH4-N than for NO3-N. No firm conclusions a bout pH effects on the preferential uptake of ammonium and nitrate cou ld be drawn. However, pH had a pronounced effect on the mycelial growt h of P. involutus which was hampered severely at pH 6.1 and to a lesse r extent at pH 5.1.