Allelopathic potential of the wet forest invader Japanese hops (Humulus japonicus)
Japanese hops (Humulus japonicus) is an introduced east Asian species whose current distribution extends over most of the eastern U.S.A. Hops grows rapidly under warm, sunny conditions on moist to moderately dry soil. In late summer, dense blankets of hops foliage envelop tree seedlings and taller plants, derailing lowland forest succession. Hops thickets tend to be devoid of native species even before blanketing coverage is achieved. Given that light levels are high and soils remain moist at this stage, we hypothesized that hops may produce chemicals inhibitory to other species. A laboratory assay was performed to evaluate the allelopathic potential of foliage extracts on the germination and growth of fescue (Festuca rubra), radish (Raphanus sativus), and lettuce (Lactuca sativa). Four types of extracts were prepared by (1) soaking intact leaves in deionized water, (2) macerating fresh and dried plant material in deionized water, and (3) macerating fresh and (4) dried plant material in a 1:1 methanol/deionized water. All extracts were standardized for solute concentration. Seeds were placed on heat-sterilized silica sand and were watered with 8mL of standardized extract or DI water for controls. The plates incubated on a light table with a 14/10 hour light/dark cycle and an average temperature of 23°C. Germination was monitored daily and the experiment was terminated after 3 consecutive days of no new germination.
There were significant effects on percent germination between extracts and between species (all p < 0.001), as well as significant delays in germination depending on species and extract type (all p < 0.001). In general, methanol:water extracts had the strongest negative effects on overall germination rate and time to germination. Hops extracts strongly altered growth pattern, particularly root vs. shoot allocation. These findings provide a rationale investigate whether allelopathy in Japanese hops has a role at the community level. A field experiment is a necessary next step before allelopathy can be considered ecologically relevant.