The structure of eucalypt mycorrhizas

Abstract

All Eucalyptus species examined are capable of forming ectotrophic mycorrhizas in association with suitable fungi. The phenomenon is widespread but the intensity of infection varies according to the environment in which they occur, mycorrhizas being most common in leached soils which accumulate organlc materlal in or above the surface layers. The general structure and behaviour of these infected roots is similar to that described for Fagus sylvatica. Eucalypt mycorrhizas are compared with ontogenetically equivalent uninfected rootlets, revealing differences in anatomical structure which are assigned to two categories. These are quantitative differences in which certain aspects of mycorrhizal structure differ from the typical structure found in uninfected roots yet approximate to certain extreme expressions of uninfected root anatomy, and qualitative differences where infected and uninfected roots possess quite distinct characteristics. In the former category, suppression of root hair formation and reduction of meristematic and root cap tissues in mycorrhizal axes appear to be generalized changes of the kind which result from slow growth through an unfavourable environment, so that it is not clear whether the invading fungus induces or merely perpetuates the state found in mycorrhizas. In the latter category, radial elongation of epidermal cells and pronounced thickening of cell walls in the inner cortex are shown to be specific changes induced only by fungal infection. Mycorrhizal morphology may be simulated by uninfected roots treated with high concentrations of naphthaleneacetic acid. Such treatment also induces some generalized anatomical changes similar to those found in mycorrhizas, but fails to bring about the specific changes characteristic of fungal infection. The importance of applying anatomical criteria to studies on morphogenic compounds is discussed.