Allelopathy of pine litter: Delivery of allelopathic substances into forest floor

Red pine ( Pinus densiflora Sieb. et Zucc.) forest forms sparse forest floor where herbaceous plants do not grow well. Allelopathic chemical interaction was speculated to play an important role in the formation of the forest floor. However, only several phenolic acids and their related compounds have been isolated from the forest floor. Those compounds are universal in many plants and cannot distinguish such strong allelopathic activity of red pine from other plants. The present research investigates whether red pine litter contains some potential allelopathic substances other than phenolics. Aqueous methanol extracts of red pine litter inhibited the growth of cress ( Lepidium sativum ) and Digitaria sanguinalis L., and increasing the extract concentration increased the inhibition. The extract was then purified by several chromatographies and biological activities of all separated fractions were determined. Two main inhibitory substances were finally isolated and their chemical structures were determined by spectral data as 9α,13β- epidioxyabeit-8(14)en-18-oic acid and abscisic acid-β-Dglucopyranosyl ester. These compounds may be synthesized in pine plants and delivered into the forest floor by defoliation of the needles. Their inhibitory activity on the germination and the growth may play an important role in the formation of the sparse forest floors.