Allelopathic effects of lichen metabolite usnic acid on growth and physiological responses of Norway spruce and Scots pine seedlings

Abstract

Lichens are globally widespread organisms playing an important role in diverse ecosystems. They produce secondary metabolites, unique compounds, which play many important ecological and biological roles, including their effects on other plants, through allelopathy. Usnic acid is one of the most frequent secondary compounds in thalli of lichens forming the layer on the surface of soils, interacting with the seedlings of conifers in the boreal forests. The main aim of this study was to investigate the growth, ploidy level, reactive oxygen species (ROS) production and element content in the Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) seedlings cultivated for 14 days using substrates containing addition (10 mg per cultivation tube) of (+) usnic acid (UA). We also investigated UA root content in these plants.The root:shoot ratio (R:S) decreased in stressed pines by over 31%. The average root length diminished by 48% and the shoot length (to the cotyledon base) by 25%. For spruce, the R:S ratio decreased by more than 41%, the root length by 46% while the shoot length by only 9%. The UA treatment particularly increased the number of non-fully developed seedlings during the germination. The seed germination rate did not vary significantly when compared to control. No significant ploidy differences between control and treated seedlings were observed in neither of the species. Ploidy aberration in two P. abies seedlings was discovered. The amount of UA in the roots, including UA bound on their surfaces, in spruce varied from 3.6 to 325.5 μg g−1 DW and in pine roots from 15.6 to 252.3 μg g−1 DW. A significant decrease in total macroelement content in roots of both species was noted, particularly for P, K, Ca, Mg and S contents. Interestingly, the contents of stress markers, e.g. superoxide dismutase and peroxidase were not significantly changed when compared to controls.