TEMPORAL VARIABILITY OF PHENOLICS AND BATATASIN-III IN EMPETRUM-HERMAPHRODITUM LEAVES OVER AN 8-YEAR PERIOD - INTERPRETATIONS OF ECOLOGICALFUNCTION

Although many plant species produce high levels of secondary metabolit es, comparatively little is known about the temporal variability of ti le production and concentrations of these compounds. either in terms o f quantitative or qualitative aspects. In the Swedish boreal forest th e dwarf-shrub species Empetrum hermaphroditum produces high levels of phenolics which are important agents of allelopathy, regulators of her bivory and determinants of plant litter decomposition. We performed qu antitative analyses of total phenolics and monitored the phytotoxic ac tivity (defined as the ability of the extracts to retard germination o f Populus tremula seeds) of aqueous leaf extracts from three age class es of leaves, collected from the field approximately every two weeks f or every growing season from 1988 to 1995. The concentrations of the d ihydrostilbene batatasin-III. an E. hermaphrodium metabolite with a do cumented phytotoxic effect, were determined in both extracts and entir e leaves for material collected in 1988 and 1994. We also studied leaf gland variation of first-year leaves in relation to phenolic concentr ation and phytotoxic activity. Large differences existed between sampl ing times within years, with first-year shoots producing high levels o f phenolics; these levels were maintained for second-year shoots but p henolic concentrations declined for third-year shoots, i.e. prior to l eaf senescence. Phytotoxic activity was low immediately after leaf eme rgence, and was not consistently correlated to total phenolic concentr ations of the leaves. However, more detailed analyses showed that much of the phytotoxic activity of E. hermaphrodium extracts is due to the production of batatasin-III, which reaches its maximum concentration not until September of the first year. We believe that batatasin-III i s critical in determining the phytotoxic effects of E. hermaphrodium a nd that this compound may have additional benefits for E. hermaphrodiu m other than deterring herbivory. Leaf glands were present on newly fo rmed leaves, and were produced continuously over the growing season. H owever, correlation analyses between the number of leaf glands and eit her the release of phenolics or phytotoxic activity did not reveal any significant relationships. There were also important differences in b oth leaf phenolic concentrations and phytotoxicity between years, alth ough we were unable to relate this to inter-year macroclimatic paramet ers collected From the same site. We conclude that temporal variabilit y of the production of phenolic compounds by E. hermaphrodium is consi derable and is almost certainly of importance in introducing a degree of temporal variability into the biotic interactions that E. hermaphro dium participates in.