The types of mycorrhizal structure seen in the Ericaceae and in the closely related families Pyrolaceae and Monotropaceae are reviewed briefly. Ericoid, arbutoid, and monotropoid mycorrhizas are characterized. The processes of infection and of establishment of the mature ericoid mycorrhizal association are discussed from the structural, ultrastructural, and functional viewpoints. Infection provides an enhancement of N and P supply to the host plant. The enhancement of N supply arises from an improved exploitation of "available" ammonium ions and from the capacity, provided exclusively by the mycorrhizal fungus, to utilize simple organic nitrogen compounds. The endophyte also provides increased phosphatase activity and access to what are otherwise only slightly available organic sources of phosphates. In addition to enhancement of host nutrition in circumstances of low nutrient availability, it is shown that ericoid mycorrhizal infection leads to restriction of uptake of metallic elements when these are present in high concentration. Infection thus provides resistance to heavy metal toxicity. The significance of these observations is discussed in relation to the capacity of plants with ericoid mycorrhizas to exploit marginal ecological situations in temperate, boreal, subarctic, and alpine regions throughout the world.
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