Invasion Intensity Modulates the Allelopathic Impact of Solidago canadensis L. Leaves and Roots Against Lactuca sativa L. During Germination and Early Seedling Stage

The allelopathic potential of invasive plant species (IPS) is critical to their successful invasion. The allelopathic potential of IPS is mediated by allelochemicals synthesized in the above-ground part (mainly leaf residues) and the below-ground part (mainly root secretions). However, the progress in the combined above-ground and below-ground allelopathic potential of IPS with different invasion intensities is limited. This study aims to estimate the combined above-ground and below-ground allelopathic potential of the notorious IPS Solidago canadensis L. with different invasion intensities (i.e., light invasion and heavy invasion) against Lactuca sativa L. during germination and early seedling stage. The allelopathic potential of S. canadensis extracts, particularly leaf extract, is dependent on extract concentration and also on the invasion intensity of the plant. While the above-ground allelopathic potential of S. canadensis on L. sativa was marked, the below-ground allelopathic potential was almost non-significant; and the differential impact of the two organs was evident, particularly on seed germination. Furthermore, the combined above-ground and below-ground allelopathic potential of S. canadensis may be greater than that of either part alone. Therefore, seed germination was more affected than the subsequent seedling growth by the above-ground extract of S. canadensis in a concentration and invasion intensity-dependent pattern.