As a result of increasing anthropogenic nitrogen deposition, N availability in many forest ecosystems, which are normally N-limited, has been enhanced. We discuss the impacts of this increased N availability on the ectomycorrhizal (ECM) symbiosis which is generally regarded as an adaptation to nutrient limited conditions. Nitrogen deposition can influence fruit-body formation by ECM fungi, the production and distribution of the extraradical mycelium in the soil and the formation of ectomycorrhizas.Available data from long-term N deposition studies indicate that the most prominent effects might be discernible above-ground (i.e. on the formation of fruit bodies). ' Generalist ' species, forming a symbiosis with a wide range of tree species, seem to be less affected by increased N availability than ' specialist ' species, especially those living in symbiosis with conifers. However, the importance of below-ground investigations to determine the impacts of N deposition on the ECM symbiosis must not be underestimated. Culture experiments show an optimum N concentration for the formation of extraradical mycelium and mycorrhizas. Often, negative effects only become visible at comparatively high N concentrations, but the use of a few easily cultivated species of ECM fungi, which are adapted to higher N concentrations, undermines our ability to generalize.So far, N deposition experiments in the field have only shown minor changes in the below-ground mycorrhizal population, as estimated from the investigation of mycorrhizal root tips. However, effects on the ECM mycelium, which is the main fungal component in terms of nutrient uptake, cannot be excluded and need further consideration.Because the photoassimilate supply from the plant to the fungal partner is crucial for the maintenance of the ECM symbiosis, we discuss the possible physiological implications of increasing N inputs on the allocation of C to the fungus. Together with ultrastructural changes, physiological effects might precede obvious visible changes and might therefore be useful early indicators of negative impacts of increasing N inputs on the ECM symbiosis.

It is well established that ectomycorrhizal fungi can use amino acids as nitrogen and carbon sources, but data on the kinetic properties of amino acid uptake systems of ectomycorrhizal systems are scarce. Using 14 C-labelled compounds we have determined the kinetics of uptake of amino acids by excised ectomycorrhizal roots for a range of distinct mycorrhizal types from three tree species, beech, spruce, and pine. All mycorrhizal types examined took up amino acids via high-affinity transport systems (K M values ranging from 19 to 233 mmol m -3 ). A comparative analysis of kinetic parameters for uptake of amino acids and the ammonium analogue methylammonium showed that ectomycorrhizal roots have similar or even higher affinities (lower K M values) for the amino acids, indicating that absorption of these organic forms of nitrogen (N) can contribute significantly to total N uptake by ectomycorrhizal plants. Analysis of amino acid uptake by ectomycorrhizal roots collected along a European north/south gradient of increasing mineral N pollution from northern Sweden to south Germany revealed no obvious trend in the uptake capabilities for amino acids by ectomycorrhizal roots in relation to the location of the sampling site on this gradient. Rather, the fungal species forming a particular morphotype was the factor determining uptake kinetics. It can therefore be deduced that the species composition of the fungal community will contribute significantly to the functional diversity of a population of mycorrhizal roots.

S U M M A R YSucrolytie enyzme activities and concentrations of hexoses typically increase in plant/microbe symhioses. However, there is little information on ectomycorrhizal associations. We measured invertase activity and soluble sugar contents in roots of mycorrhizal and non-tnycorrhizal Norway spruce {Picea abies (L.) Karst.] seedlings. We used two ectomycorrhizal fungi, a basidiomycete [Amanita muscaria (L. ex Fr.) Hooker] and an ascomycete {Cenococcum geophilum Er.). Mycorrhizas and non-mycorrhizal short roots, as well as other parts ofthe root system, were investigated at different developmental stages by micro-analytical methods.Neither sucrose nor invertase could he detected in fungal mycelia. In vitro measurable invertase activity and the sucrose, glucose and fructose content were reduced in both types of myeorrhizas compared with the nonmycorrhizal short roots. Correction of data for the fungal component in the mycorrhizas, indicated that there were no differences in plant-specifie amounts of sucrose and aeid invertase activity in niycorrhizal and non-mycorrhizal roots. However, amounts of glucose and fructose, which are present in both partners, were clearly reduced in the myeorrhizas. As high fructose levels inhibit acid invertase, a reduction in the aniount of fructose in the symbiotic tissue could favour in vivo acid invertase activity. Our results indicate that the situation in eetomyeorrhizas may be different from those in other biotrophic interactions.

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Starch and sucrose metabolism of one‐ and two‐year‐old needles of Norway spruce (Picea abies [L.] Karst., about 30 years old) was investigated from three months before until three months after bud break at a natural site. We distinguish different metabolic states according to the extractable activities of enzymes (α‐amylase [EC 3.2.1.1], ADP‐glucose pyrophosphorylase [AGP, EC 2.7.7.27], D‐enzyme [EC 2.4.1.25], starch phosphorylase [STP. EC 2.4.1.1]), sucrose phosphate synthase [SPS, EC 2.4.1.14], sucrose syntbase [SS, EC 2.4.1.13]. acid invertase [AI, EC 3.2.1.261) and pool sizes of related metabolites (starch, glucose, fructose, sucrose, raffinose, stachyose, fructose 6‐phosphate [F6P], glucose 6‐phosphate [G6P], fructose 2,6‐bisphosphate [F26BP], and inorganic phosphate [P1]). The period ending with bud break was characterized by high rates of net photosynthesis, a pronounced decrease in the amount of soluble sugars, and a steep rise in starch (from the detection limit to approximately 600 nmol glycosyl units [mg dry weight]‐1). In parallel, the extractable activity of AGP increased, while D‐enzyme was on a relative high level when compared with the period after bud break. With respect to sucrose metabolism, F26BP, an inhibitor of sucrose synthesis, decreased from 1 to 0.4 pmol (mg dry weight)‐1. This was complemented by SPS activity, which was due to both increased protein levels shown by immunoblotting and activation under metabolite control (high levels of G6P and a low Pi/G6P ratio). This indicates a high capacity of synthesis of starch and sucrose in the period before bud break. These observations are in accordance with estimates of photosynthetic carbon gain, which indicate that in early spring large amounts of carbon from current photosynthesis are exported out of the needles. In addition, the content of nonstructural carbohydrates (expressed as hexoses) increased in the bark of the stem. This could also be a consequence of an enhanced carbon export from the needles. After the onset of bud break, starch concentration decreased in all tissues under investigation. In contrast, the level of total nonstructural carbohydrates in the outermost sapwood nearly doubled from bud break until the end of sampling. In the needles, net photosynthesis was reduced by about 75% and a decrease in SPS activity and protein level were found together with lower G6P concentration, and an increased Pi/G6P ratio. These results suggest that during that period sucrose synthesis was reduced in the older needles. In addition, under conditions of reduced photosynthesis, carbon demand of current year needles was in part ensured by the mobilization of starch in the older needles. Taken together our data show that before bud break carbon metabolism of mature leaves is related with the sink demands of storage organs. After bud break the accumulated assimilate pools in needles and stem, mainly the bark, are mobilized and support carbon supply to new tissues.