The Production by Roots of Calluna Vulgaris of a Factor Inhibitory to Growth of some Mycorrhizal Fungi

It is well established that tree species, such as birch, Betula pendula, and spruce, Picea abies, frequently fail to develop in association with heather, Calluna vulgaris. The affected trees can remain in a state of 'check' for many years, exhibiting no pathological symptoms but making little measurable growth. This lack of growth is characteristic of both roots and shoots, and, in addition, the latter fail to form any mycorrhizal associations. The poor development of uninfected roots is, in part, due to the presence of phytotoxic metabolites in the Calluna raw humus (Robinson 1971), but compounds released by decaying litter cannot be implicated with the absence of mycorrhiza. Various attempts have been made to explain this latter phenomenon (Rayner & Neilson-Jones 1944; Brian, Hemming & McGowan 1945), but it was Handley (1963) who first put forward the hypothesis that the paucity of mycorrhizal associations was due to a fungitoxic factor produced by the root system of living C. vulgaris. This idea received a large measure of support from field observations, in that it was noticed that any factor which restricted the growth of C. vulgaris was conducive to the formation of mycorrhizas by the 'checked' trees. However, although it was demonstrated that water extracts of raw humus from beneath living C. vulgaris contained a factor inhibitory to the growth of certain mycorrhizal fungi, the exact origin of the factor was not established. It is the purpose of this paper to show that a fungitoxic factor with the same characteristics as that isolated by Handley is, in fact, produced by the living root system of heather. Evidence in favour of this view was obtained in two ways: firstly, by showing that decaying litter of C. vulgaris did not give rise to such a factor and, secondly, by demonstrating that under certain experimental conditions a material is produced by the root system of C. vulgaris which is toxic to some fungi. Evidence relating to the first of these two points has been presented elsewhere (Robinson 1971), and is not dealt with further.