Using density-dependent growth to study the effects of allelopathy.

摘要

The study of allelopathy—chemical interference between plants—has been hindered by a lack of methods that can demonstrate that allelopathy is occurring in any given situation. One of the critical steps in studies of allelopathy remains the isolation and identification of suspected allelochemicals. Polyurethane foam plugs were tested for their ability to “trap” allelochemicals while allowing for undisturbed plant growth. The polyurethane foam plugs were an inexpensive tool for isolating the allelochemical alpha-terthienyl ($a$-T) from under marigold (Tagetes erecta and T. patula) plants in the laboratory as well as from soil in the field. Relative amounts of the allelochemical released by several marigold cultivars could be compared.; Especially in field studies, it is difficult to distinguish the effects of allelopathy from those due to resource competition. Recently, it has been shown that density-dependent plant growth in the presence of a phytotoxin differs from the predicted response without a toxin, in such a manner that plant size is reduced at low density (due to high phytotoxicity) and high density (due to intense resource competition).; The common marigold (T. erecta), the cultivar that released $a$-T in the greatest quantity, was used in field studies to determine if an allelopathic interaction could be demonstrated by observing plant growth of a sensitive species at different densities, with and without the presence of $a$-T. The common marigold was interplanted with the sensitive species Lycopersicon esculentum (tomato) and Chenopodium album (lambsquarter) in the target-neighbor design (in which the number of neighbor plants is varied around a singular target plant) in two separate studies to test this method in the field. Although some of the growth effects of the sensitive species could be explained by the presence of $a$-T, conclusive proof for an allelochemical interaction was not obtained.; In laboratory experiments, sensitive tomato plants were exposed to concentrations of 0 to 10 ppm of juglone (the allelochemical released by black walnut, Juglans nigra) hydroponically. A juglone concentration as low as 0.5 ppm caused significant growth reductions to the tomato plants. The juglone concentration decreased more rapidly under the higher density of tomato plants (one tomato target surrounded by 6 tomato neighbors) compared to the lowest density (one tomato target only) and individual plants thus received less of the toxin.; Tomato plants grown under black walnut in the target-neighbor design in the field, were very stunted at the lower as well as the higher densities. The juglone concentration measured in soil under black walnut was highest in the summer, when concentrations as high as 131 $m$g/g dry soil were measured, indicating that the concentration of the allelochemical in the soil was sufficient to cause significant growth reductions in sensitive species.