Eucalypt mycorrhizas contain electron-opaque granules that are rich in P. The major element accompanying P is Ca, although small amounts of other elements such as K and Mg and possibly also S are sometimes present. Neither P nor Ca was found in equivalent amounts in other areas of the cells analysed. The P-rich granules occurred in vacuoles in the inner sheath and Hartig net of the two species studied,Eucalyptus pilularisandEucalyptus fastigata. Their location and staining indicated that they were the same as the metachromatic granules observed by light microscopy and that both contained polyphosphate. Other deposits in vacuoles did not contain detectable amounts of these elements and correlated transmission electron microscopy showed that several types of vacuolar deposit were present. Fixation conditions had little effect on the composition of the P-rich granules, which were the same in both species, but staining with lead salts apparently removed elements, in particular Ca. Other cellular materials, notably phenols, also stained with lead salts, even at acid pH, and there is a risk therefore of misidentifying vacuolar phenols as polyphosphate. The significance of the P-rich granules and their high Ca content is discussed in relation to P accumulation in the fungal sheath and transfer to the host.
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