Allelopathic potential of Psychotria leiocarpa, a dominant understorey species of subtropical forests

Psychotria leiocarpa is an understorey woody shrub native to the forests of southern Brazil, which occurs in groups of relatively high density. Leaves of field-grown plants contain approximately 2.5% of their dry weight as the N-glycosylated indolic alkaloid N,β-d-glucopyranosyl vincosamide (GPV). To find out if allelopathy could play a role in the distribution pattern of the species, several assays were performed. Aqueous extracts of dried powdered leaves at 4% (w/v) collected during the winter of 2004 inhibited the germination and/or early growth of three different test species. Lactuca sativa plantlets were affected when grown in soil containing dried P. leiocarpa leaves and on plates containing aqueous extracts of leaves of the same species. Partial chemical characterization of the extract was carried out and an allelopathic effect of the purified major leaf alkaloid GPV was not observed at the highest concentration tested of 10 ppm. Experiments with different organic extracts and sequential extractions were also performed. The data suggested that polar phenolic compounds or iridoids are responsible for the phytotoxic effect observed.