Many interactions between plants and herbivores and plants and pathogens are known. Less well known is that plants affect the growth of other plants. Such a phenomenon, allelopathy (Greek allelos, "another"; pathos, "suffering"), cannot always be unequivocally demonstrated, and was often said to be responsible for reduced plant growth without good evidence for such an interpretation.

The classical example for allelopathy is the inhibitory effect on germination by leaves of the walnut trees (Juglans regia, J. nigra). The inhibition is caused by juglone (1-hydroxynaphtha-lene), which is formed from 5-glucosyl-l,4,5-tri-hydroxynaphthalene under the influence of bacteria. Another classical example is the inhibition of grasses in the garigue of southern California, in the presence of the shrubs Salvia leucophylla and Artemisia californica. They produce a broad spectrum of possibly toxic, volatile terpenoids which impair growth of grasses. However, recently, doubts have been expressed about this interpretation; it is now thought that rabbits shelter under the bushes and graze the grasses causing their low biomass production. In most cases known, compounds which act allelopathically are phenolics which are metabolised by soil microorganisms and are thus "activated".

A good example of intraspecific allelopathy is shown by several representatives of the genus

Kalanchoe, where root exudates from the mother plant inhibit growth of the daughter plants which are formed at the edge of the leaf (Fig. 1.10.13). The main components of the root exudate (in decreasing amounts) are gallic acid, caffeic acid, p-hydroxybenzoic acid, protocate-chuic acid and p-coumaric acid, again all phenolic substances (Bär et al. 2000).

It is not yet known how these phenolic compounds affect growth of competitors; inhibition of germination often plays an important role.

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