SUMMARYThe influence of pH on the jjrowth and assimilation of ''N-labelled ammonium and nitrate was studied in intact ectomycorrhizal systems consistinj^ of Betula pendula Roth and Picea ahies (L.) Karst. colonized with a common mycelium of I'axillus imolutus (Batsch) Pr, The plants were grown together in Plexiglass observation chambers containing non-sterile peat with three diflVrent pH values. 4-0, 5-1 and 61. The mycorrhiza! m>celium was allowed to grow over a barrier into an area of peat from which plant roots were excluded. Laheiled XH^NOj was supplied. either as '"XH^NO., or as NH^'^NOg, exclusively to the fungal myceiium. Shoots and roots were analyzed for '"N in total nitrogen while the mycelium was analyzed for '•''N' in XH^"^, XO,^" and free amino acids. The '"''X labelling pattern indicated that ammonium was immediately assimilated into amino acids, primarily glutamine, by the fungal mycelium at the uptake site. The amino acids were then translocated to the mycorrhiza] roots. In contrast, nitrate-X was not assimilated in the mycelium but rather transferred to the mycorrhizal roots a.s nitrate, Mycelial uptake and transfer of X to the spruce and birch seedlings were significantly higher for XHj-X than for XO,-X. No firm conclusions about pH effects on the preferential uptake of ammonium and nitrate could be drawn. However. pH had a pronounced effect on the myceliai growth t>f P. ivrolutus which was hampered severely at p}i 6-1 and to a lesser extent at pli 51,

summary Growth rates of ectomycorrhizal fungi were measured in pure culture at pH 3–8 on MMN‐agar and sterilized peat, with and without nutrients added. Mycorrhizas were synthesized with Pinus sylvestris L. seedlings and the growth rate of the external mycelium was measured in peat at pH 3.8 and 7.3. The fungi were three isolates of Piloderma croceum Erikss. & Hjorts., two isolates of an unidentified (pink) mycorrhizal type and one isolate of another unidentified (white long) mycorrhizal type, all isolated from P. sylvestris roots. All three fungi showed much higher (2–5 times) growth rates when grown as symbionts than when grown in pure culture. The P. croceum isolates also grew as symbionts at pH 7.3, where no growth occurred in pure culture. These findings emphasize the danger of generalizing from data obtained in pure culture studies to explain what may happen when fungi grow in symbiosis with host plants. Generally, growth rates in pure culture were higher and the pH tolerance levels were wider on agar than on peat. The exceptions were one P. croceum isolate, which had the fastest pure culture growth rate on peat with MMN, and the pink isolates, which grew at higher pH on peat than on agar. There were no significant differences between growth on peat with and without MMN.

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SUMMARYSeedlings of Pinus sylvestris L. were grown in Plexiglas® observation chambers in limed (CaCOg, pH 5-0 and 5-9) and untreated (pH 4-1) peat. The seedlings were either colonized by the mycorrhizal fungus Paxillus involutus (Batsch: Fr.) Fr. Or were non-mycorrhizal. After 18 wk in the observation chambers, "^^N-labelled organic nitrogen, as lyophilized and ground mycelium of Suillus variegatus (Swartz: Fr.) O. Kuntze, or ammonium, was added to the peat. The plants were harvested after an uptake period of 14 d.Irrespective of the nitrogen form added, liming decreased both the content and concentration of ^^N in nonmycorrhizal plants, and, to a lesser extent, those in mycorrhizal plants. In mycorrhizal plants the uptake of ^^N was not correlated with area colonized by the mycorrhizal mycelium. The amount of KCl-extractable ^"N in peat without plants and mycorrhizal fungi decreased with liming. It is proposed that liming induced chemical or microbial immobilization of the added ^^N. This is suggested to be the main reason for the decreased uptake of in lime treatments.

S U M M A R YUptake of P(''^P) and CaC^Ca) hy seedlings of Picea abies (L.) Karst and Betula pendula Roth, non-mycorrhizal or mycorrhizal with Paxillus involutus (Batsch: Fr) Fr. was studied. Seedlings were grown in unamended peat (pH 4-0) or in peat limed (CaO) to pH S-1 or 6-1. A double-labelled (^-P and ^°Ca) complete nutrient solution was added to the peat 7 wk after planting. An 8 d period was allowed for uptake of the isotopes before the seedlings w^ere harvested.Mycorrhizal colonization clearly increased the uptake of P(^^P) in the unlimed substrate and in the substrate limed to a pH of 5-1. At the highest lime rate, the uptake of P(^-P) was greatly reduced in both mycorrhizal and non-mycorrhizal seedlings. The difference in P uptake between mycorrhizal and non-mycorrhizal seedlings was small at this lime rate. The mycorrhizal colonization of the roots was not affected by liming. However, it is possible that the mycelial growth into the substrate was inhibited. The reduction in uptake could thus be an effect of a lower availability of P in combination with a decreased fungal uptake surface at the highest lime rate.The mycorrhizal effect on uptake of Ca was much smaller than its effect on uptake of P. Mycorrhizal colonization increased the CaC'Ca) uptake in the unlimed treatment, where the Ca content in the substrate was very small. In the limed substrates the uptake of Ca was as high or higher in the non-mycorrhizal than in the mycorrhizal seedlings.