Temporal variability of phenolics and batatasin-III in Empetrum hermaphroditum leaves over an eight-year period: interpretations of ecological function

Although many plant species produce high levels of secondary metabolites, comparatively little is known about the temporal variability of the production and concentrations of these compounds. either in terms of quantitative or qualitative aspects. In the Swedish boreal forest the dwarf-shrub species Empetrum hermaphroditum produces high levels of phenolics which are important agents of allelopathy, regulators of herbivory and determinants of plant litter decomposition. We performed quantitative analyses of total phenolics and monitored the phytotoxic activity (defined as the ability of the extracts to retard germination of Populus tremula seeds) of aqueous leaf extracts from three age classes of leaves, collected from the field approximately every two weeks for every growing season from 1988 to 1995. The concentrations of the dihydrostilbene batatasin-III. an E. hermaphrodium metabolite with a documented phytotoxic effect, were determined in both extracts and entire leaves for material collected in 1988 and 1994. We also studied leaf gland variation of first-year leaves in relation to phenolic concentration and phytotoxic activity. Large differences existed between sampling times within years, with first-year shoots producing high levels of phenolics these levels were maintained for second-year shoots but phenolic concentrations declined for third-year shoots, i.e. prior to leaf senescence. Phytotoxic activity was low immediately after leaf emergence, and was not consistently correlated to total phenolic concentrations 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 is critical in determining the phytotoxic effects of E. hermaphrodium and that this compound may have additional benefits for E. hermaphrodium other than deterring herbivory. Leaf glands were present on newly formed leaves, and were produced continuously over the growing season. However, correlation analyses between the number of leaf glands and either the release of phenolics or phytotoxic activity did not reveal any significant relationships. There were also important differences in both leaf phenolic concentrations and phytotoxicity between years, although we were unable to relate this to inter-year macroclimatic parameters collected From the same site. We conclude that temporal variability of the production of phenolic compounds by E. hermaphrodium is considerable and is almost certainly of importance in introducing a degree of temporal variability into the biotic interactions that E. hermaphrodium participates in.