SUMMARYYoung mycorrhizal plants of Vaccinium macrocarpon could utilize amino acids as a nitrogen source as readily as ammonium whereas aseptically grown plants and plants inoculated with common soil saprophytic fungi were unable to utilize the organic nitrogen compounds. Seedlings of Festuca ovina grown as controls in the sterile condition and in the presence of the ericaceous endophyte also failed to utilize the amino compounds. The capacity of the ericaceous plant to exploit these compounds is therefore dependent upon mycorrhizal infection. In contrast to cultures of Mdrasmius oreades, the endophyte failed to grow on complex nitrogenous humic and fulvic acids extracted from soils under ericaceous plants. The capacity to utilize simple organic nitrogen compounds could be of importance for mycorrhizal ericaceous plants in mor-humus soils which are characterized by slow rates of nitrogen mineralization.

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Summary The major soluble carbohydrates of mycorrhizal and non‐mycorrhizal roots of the cultivated cranberry, Vaccinium macrocarpon Ait., have been identified and quantitatively analysed. Mycorrhizal roots contained two sugars, mannitol and trehalose, not found in uninfected roots. As these sugars are abundant in extracts of endophyte mycelium grown in liquid culture, they appear to be of fungal origin. Analysis of neutral sugars derived by acid hydrolysis of insoluble residues showed an accumulation of a mannose‐containing polymer in mycorrhizal roots. Mannose was a principal component of insoluble polymers in the endophyte. All soluble carbohydrates in the root, including mannitol and trehalose, became labelled after feeding 14CO2 to the shoots. Sucrose was the most strongly labelled sugar in both types of root. Mannose present in hydrolysates of mycorrhizal roots was more heavily labelled than that derived from uninfected roots but most 14C in insoluble fractions was found in glucose polymers. The hypothesis that the endophyte derives a supply of carbohydrate from its host plant is discussed.

SUMMARY Mycorrhizal and non‐mycorrhizal plants of Vaccinium macrocarpon were grown in a controlled environment for 12 weeks on sand supplied with five different concentrations of ammonium nitrogen in a logarithmically increasing series ranging from 1.0 to 56.0 parts/106. All plants received a full complement of other mineral elements essential to growth. Interactions between the effects of mycorrhizal infection and nitrogen concentration were studied through the calculation of various growth‐functions. With increasing concentrations of ammonium nitrogen, uninfected plants showed significant increases in whole‐plant dry weight, percentage nitrogen content and specific absorption rate for N accompanied by decreases in root/shoot ratio and the value of the allometric constant k (the ratio of the logarithmic growth‐rates of root and shoot). Mycorrhizal infection induced similar changes at intermediate levels of nitrogen supply but did not affect seedlings grown under the highest and lowest concentrations of nitrogen used. Differences due to infection were generally smaller than those between uninfected plants from successive nitrogen treatments. The implications of these results are discussed in relation to the significance of mycorrhiza in the nitrogen nutrition of ericaceous plants in their natural habitats.