We used a novel digital autoradiographic technique that enabled, for the first time, simultaneous visualization and quantification of spatial and temporal changes in carbon allocation patterns in ectomycorrhizal mycelia. Mycorrhizal plants of Pinus sylvestris L. were grown in microcosms containing non-sterile peat. The time course and spatial distribution of carbon allocation by P. sylvestris to mycelia of its mycorrhizal partners, Paxillus involutus (Batsch) Fr. and Suillus bovinus (L.): Kuntze, were quantified following 14C pulse labeling of the plants. Litter patches were used to investigate the effects of nutrient resource quality on carbon allocation. The wood-decomposer fungus Phanerochaete velutina (D.C.: Pers.) Parmasto was introduced to evaluate competitive and territorial interactions between its mycelial cords and the mycelial system of S. bovinus. Growth of ectomycorrhizal mycelium was stimulated in the litter patches. Nearly 60% of the C transferred from host plant to external mycorrhizal mycelium (> 2 mm from root surfaces) was allocated to mycelium in the patches, which comprised only 12% of the soil area available for mycelial colonization. Mycelia in the litter patch most recently colonized by mycorrhizal mycelium received the largest investment of carbon, amounting to 27 to 50% of the total 14C in external mycorrhizal mycelium. The amount of C transfer to external mycelium of S. bovinus following pulse labeling was reduced from a maximum of 167 nmol in systems with no saprotroph to a maximum of 61 nmol in systems interacting with P. velutina. The 14C content of S. bovinus mycelium reached a maximum 24-36 h after labeling in control microcosms, but allocation did not reach a peak until 56 h after labeling, when S. bovinus interacted with mycelium of P. velutina. The mycelium of S. bovinus contained 9% of the total 14C in the plants (including mycorrhizae) at the end of the experiment, but this was reduced to 4% in the presence of P. velutina. The results demonstrate the dynamic manner in which mycorrhizal mycelia deploy C when foraging for nutrients. The inhibitory effect of the wood-decomposer fungus P. velutina on C allocation to external mycorrhizal mycelium has important implications for nutrient cycling in forest ecosystems.

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Sl'M MARY[lyntenoscvplius ericae utilizes protein as a sole source of nitrogen through tbe production of an extraccllular proteinase. A sensitive procedure for assay of tbe enzyme is described and is used to detern-iine tbe major characteristics of tbe enzyme. Tbe enzyme bas a sharp pi I optimum at 2-2 with little activity above pH 5-0. \'akics for K and I',,.,^ of tbe enzyme were determined.Pepstatin, a specific inhibitor of carboxyl (acid) proteinascs, almost completely inhibited activity, coiiHrming that the enzv-n-ie is of this type.Tbe cbaracteristics of tbe enzyme are con-ipared witb tbose described for otber n-iicrobial acid proteinascs and tbe results are tlisciissed m relation to pbvsiological ecologv-both of tbe fungus and of its bost plant.

SUMMARYThe effect of pH on the production and specific activity of tbe extracellular proteinase enzymes of two ecologically distinct ericoid mycorrhizal fungi is described. Tbe proteinase of Hymenoscyphus ericae (Read), Korf & Kernan, isolated from roots of Calluna vulgaris (L.) Hull growing in soil of pH 35, was compared with a similar enzyme from an endopbyte of tbe calcicolous alpine sbrub Rhodothamnus chamaecistus (L.) Reicbenb. growing in soil of pH 6-5. Tbe fungi were grown in liquid culture at pH values ranging from 3-0 to 8-0 witb pure protein, bovine serum albumin, as sole source of N.Botb fungi yielded an extracellular acid proteinase witb pH optimum for activity between 20 and 3-0. Tbe production and activity of tbese enzymes was strongly affected by pH of tbe culture medium. Maximum enzyme production during exponential growtb occurred in botb fungi at a culture pH of 40-50, wbereas bigber pH treatments severely inbibited enzyme production.Tbe acid proteinase of H. ericae was tolerant of extreme acidity and retained near-optimal activity in solutions of pH 2-0. In contrast, tbe activity of tbe enzyme from tbe Rhodothamnus endopbyte was almost completely inbibited at tbis pH. However, proteinase from tbe Rhodothamnus endopbyte retained activity at mucb bigber pH values tban did tbe proteinase from H. ericae. Unlike H. ericae, tbe isolated endopbyte of Rhodothamnus was able to grow and use protein as sole source of N at pH 7-0 and 8 0.Tbe effects of pH on enzyme production and upon growtb of tbe fungi are discussed in relation to tbe cbaracteristics of tbe environments of tbeir bost plants.

, 1985 ;Denny & Wilkins, 1987). In the case without its mycorrhizal endophyte, over a range of of metals such as iron, which are essential in small concentrations of either of two calcium salts, the quantities for normal plant function, but which mycorrhizal (M) plants captured significantly more become potentially toxic as their availability iron. Increases in iron uptake took place in all Ca mcreases, it is the regulation of supply across a wide treatments, and were of such magnitude as to range of concentrations which is likely to be the most indicate the need for further analysis of the physi-important aspect of uptake physiology. This is ology of iron uptake in the ericoid mycorrhizal particularly true in situations such as those in heathland soils where the chemical form and availThe production of an iron-specific hydroxamate ability of iron is likely to fluctuate with changing siderophore, which is likely to be involved in redox conditions. enhancement of iron capture, was recently observedIn this paper we investigate the response of the in cultures of the ericoid endophyte Hymenoscyphus ericoid mycorrhizal endophyte H. ericae to exposure mc«e (Read) Korf & Kernan (Schuler&Haselwand-to iron, and examine the role of the fungus m ter 1988) but the kinetics of iron uptake by the regulating the supply of the element to two of its host mycorrhiz'al fungus and by M plants have not been plants, Vaccinium macrocarpon Ait. and Calluna.