Studies of allelopathy tend to center on plant species which are invasive or those which exhibit peculiar patterns of dominance in natural situations. A simple screening protocol is to extract chemicals from pulverized tissue using water or other solvents. This approach cannot mimic the natural situation both in terms of toxin delivery, and probably the chemical form of the toxin. For example, delivery in rainfall, through root exudation, or as a decomposition product differ greatly. Moreover damage to cell walls and membranes bring the entire chemical contents of a suspected allelopath into contact with oxygen and microbes. Here we evaluate the potential allelopathic effects of two native, and two invasive species acting through soil modification.
Soil samples were taken beneath five long-established thickets of invasives Lespedeza cuneata and Lonicera maackii, and natives Asimina triloba, and Cornus drummondii. Samples were stored frozen until needed, extracted in water, centrifuged, and filtered through a 0.22 µM Millipore® filter to remove bacterial and fungal contamination. Extracts and a water control were applied to sterilized seeds of Elymus canadensis (native) and Lactuca sativa (cultivated lettuce) on a sterile sand substrate. Total germination was assessed.
Results/Conclusions
Lactuca exhibited significantly (P < 0.0001) higher germination rates than Elymus (84 vs. 64%) across all extract treatments. There were no significant differences between extracts and controls or interactions of extract and target species. Individual replicates of the study (5) showed significant differences (P = 0.0343), raising the question whether there were inconsistencies among replicates which may have led to negative results. However, the interaction of replicate and extract treatment was nonsignificant (P = 0.4539) and effect sizes for individual replicates are comparable to the nonsignificant species effect (β = 0.01 to 0.04). We conclude that soil is not a medium by which allelochemicals might be delivered by these species.