Growth and assimilation of NH₄⁺ and NO₃⁻ by Paxillus involutus in association with Betula pendula and Picea abies as affected by substrate pH*

Abstract

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,

Nitrogen metabolism of external hyphae of the arbuscular mycorrhizal fungus Glornus intraradices

“…The pool of free amino acids in the external AM hyphae expt 1 was within the range found in various ectomycorrhizal fungi (Martin et al, 1986;Finlay et al, 1988Finlay et al, , 1989Chalot et al, 1990;Ek et al, 1994). Cliquet & Stewart (1993) found similar concentrations of free amino acids in maize roots colonized by G. fasciculatum, and although their results are not directly comparable with ours, they support our findings.…”

Ectomycorrhizal fungal taxa differing in response to nitrogen deposition also differ in pure culture organic nitrogen use and natural abundance of nitrogen isotopes

“…In a field study using 15 N-enriched ammonium and nitrate, Buchmann et al (1996) reported that fungi (including ectomycorrhizal species) were the only ecosystem pool with higher labeling in 15 N-ammoniumtreated plots than in 15 N-nitrate-treated plots. Ek et al (1994) also reported evidence from culture studies using 15 N-labeled nitrate and ammonium, that nitrate was preferentially transferred to host plants by P. involutus. These studies suggest that if nitrate is not isotopically altered by passage through fungi, then any isotopic depletion in the soil nitrate pool would be seen in plants but not in fungi.…”

Nitrogen and phosphorus acquisition by the mycelium of the ectomycorrhizal fungus Paxillus involutus and its effect on host nutrition

“…Their capacity to transport N (Melin & Nilsson, 1952) and P (Melin & Nilsson, 1950) has long been demonstrated. More recently, transport of N (Finlay et al, 1988 ;Ek et al, 1994) and P (Finlay & Read, 1986 b ;Kammerbauer, Agerer & Sandermann, 1989 ;Timonen et al, 1996) to host plants by the extramatrical mycelium was measured in undisturbed plant-fungus systems. These experiments have in common that labelled elements ($#P, "&N) were fed to intact mycelium or cut hyphal strands for short periods.…”