The spatial distribution of plant-available mineral nutrients in forest soils is often highly heterogeneous. To test the hypothesis that local nutrient enrichment of soil leads to increased root proliferation in the nutrient-rich soil zone, we studied the effects of nutrient enrichment on the growth and nutrient concentrations of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) roots. Three-year-old seedlings were grown for 9 months in split-root containers filled with nutrient-poor forest mineral soil, with one side supplemented with additional mineral nutrients. Root dry weight and root length in Scots pine and Norway spruce were increased in the nutrient-supplemented soil compared with the nonsupplemented side, whereas root growth in Douglas-fir was unaffected by nutrient enrichment. Of the three species examined, Norway spruce exhibited the highest root and shoot growth and the highest nutrient demand. Specific root length (m g(-1)) and the number of root tips per unit root length were not affected by local nutrient addition in any of the species. Despite increased root growth in Norway spruce and Scots pine in nutrient-supplemented soil, their root systems contained similar nutrient concentrations on both sides of the split-root container. Thus, coniferous trees may respond to local nutrient supply by increased root proliferation, but the response varies depending on the species, and may only occur when trees are nutrient deficient. As a response to local nutrient enrichment, increases in root dry matter or root length may be better indicators of pre-existing nutrient deficiencies in conifers than increases in root nutrient concentrations.

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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Based on the in vitro synthesis of ectomycorrhizas between spruce seedlings and different fungal partners we assessed biochemical parameters connected to the regulation of both formation and utilization of sucrose. Mycorrhization of 4-month-old seedlings resulted in an increase of the activity of sucrose phosphate synthase, a key enzyme in the cytosolic formation of sucrose in source leaves. In parallel, the concentration of fructose 2,6-bisphosphate (F26BP), a potent inhibitor of fructose 1,6-bisphosphatase (FBPase), was decreased in needles. Both changes indicate an increased capacity for sucrose formation in mycorrhizal seedlings. To examine the fate of sucrose in the root, we compared transport properties of protoplasts from mycorrhiza-forming fungi (Amanita muscaria, Ceriococcum geophilurn) with apoplastic sucrose metabolism. Protoplasts from A. muscaria preferentially took up glucose (K,, 1.25 mM; I / , , , , , , 18 prnol/(106 protoplasts . min)), while sucrose was not transported. This is in accordance with the restriction of invertase activity (sucrose hydrolysis) to the host tissue. A distinction of different zones of symbiotic interaction (analysis of 0.5-mm-wide sections from the tip towards the base of single Picea abies -A. muscaria mycorrhizas) showed a decrease of sucrose in the zones exhibiting the highest fungal proportion (ergosterol), which was paralleled by a concomitant increase in trehalose. In addition, these zones of symbiotic interaction were characterized by high levels of F26BP (up to 1.8 pmol/mg dry weight), which exceeded those resulting from a mere addition of the levels contained in the single partners. An assay of potentially F26BP stimulated enzymes (phosphofructokinase (PFK), PPi-dependent fructose-&phosphate phosphotransferase (PFP)) revealed that in fungal extracts PFK was stimulated by F26BP while in host tissue it was PFP. This partner-specific difference in stimulation by F26BP could play a pivotal role in creating the strong sink activity in mycorrhizal roots. Possible implications on the partner-specific regulation of glycolysis are discussed.